DudeLebbowski
Well-Known Member
I actually want a hermy. Feminized seeds are in high demand compared to bud.
hermies pop up within hours!
- Thread starter thewinghunter
- Start date
fabfun
New Member
very interesting read doc thanks for posting it +rep my man for bring the truth once again
This came from the exact same article posted by Mr. SIR.
Sexual Variants in Cannabis
Cannabis has been studied for many years because of its unusual sexuality. Besides the normal dioecious pattern, where each plant bears exclusively male or female flowers, it is not uncommon for some plants to have both male and female flowers. These are called hermaphrodites, or monoecious plants, or intersexes. Hermaphroditic plants form normal flowers of both sexes in a wide variety of arrangements, in both random and uniform distributions.
Natural Hermaphrodites
Some hermaphrodites seem to be genetically determined (protogenous). That is, they naturally form flowers of both sexes given normal growing conditions. Possibly genes carried on the autosomes (the chromosomes other than the sex chromosomes) modify the normal sexual expression. Monoecious varieties have been developed by hemp breeders in order to ensure uniform harvests.
It is also possible that these particular are polyploid, which means they have more than the usual two sets of chromosomes. This kind of hermaphrodite may have XXY (triploid), or XXYY or XXXY (tetraploid) sex chromosomes. However, no naturally occurring polyploids have ever been verified (by observation of the chromosomes) in any population of Cannabis. Polyploids have been induced in Cannabis by using mutagens, such as the alkaloid colchicine.
Whatever then genetic explanation may be, one or more of these natural hermaphrodites may randomly appear in any garden. They are sometimes faster-maturing, have larger leaves, and are larger in overall size than their unisexual siblings. They usually form flowers of both sexes uniformly in time and distribution, and in some unusual patterns. For example, from Mexican seed, we have seen a plant on which separate flowering cluster consisted of both female and male flowers: and upper section of female flowers had upraised stigmas, and a lower section of male flowers dangled beneath the female flowers. In other plants from Mexican seed, the growing tips throughout the plant have female flowers; male flowers sprout from the leaf axils along the main stem and branches. Plants from "Thai" seed sometimes form male and female flowers on separate branches. Branches with female flowers tend to predominate, but branches having mostly male flowers are located throughout the plant.
Abnormal Flowers, Intersexes, Reversals
Gender is set in the new plant at the time of fertilisation by its inheritance of either the X or the Y chromosome from the male (staminate) plant. With germination of the seed, the environment comes into play. Heritage sets the genetic program, but the environment can influence how the program runs. (Sexual expression in Cannabis is delicately balanced between the two.) The photoperiod, for example, controls the plant's sequence of development. Also, the plant's metabolism and life processes are dependent on growing conditions. When the environment does not allow a balance to be maintained, the normal genetic program may not be followed. This is mirrored by abnormal growth or sexual expression.
{Figure 78. Upper left: Abnormal flowers. Lower left: Male flowers on a female plant. Upper right: Sexes on separate branches. Lower right: Male flower in female bud (reversing).}
Abnormal Flower Abnormal sexual expression includes a whole range of possibilities. Individual flowers may form abnormally, and may contain varying degrees of both male and female flower parts. For instance, a male flower may bear a stigma; or an anther may protrude from the bracts of a female flower. Abnormally formed flowers are not often seen on healthy plants, although if one looks hard enough, a few may be found in most crops. When many of the flowers are abnormal, an improper photoperiod (coupled with poor health) is the most likely cause. Abnormal flowers sometimes form on marijuana grown out of season, such as with winter or spring crops grown under natural light.
Intersexes and Reversals Much more common than abnormally formed flowers is for the plant's sex to be confused. One may find an isolated male flower or two; or there may be many clusters of male flowers on an otherwise female plant, or vice versa. These plants are called intersexes (also hermaphrodites or monoecious plants). Intersexes due to environment causes differ from natural hermaphrodite in having random distributions and proportions of male and female flowers. In more extreme cases, a plant may completely reverse sex. For example, a female may flowers normally for several weeks, then put forth new, sparse growth, typical of the male, on which male flowers develop. The complete reversal from male flowering to female flowering also happens.
All other things being equal, the potency of intersexes and reversed plants is usually less than that of normal plants. If there are reversals or intersexes, both of the sexes will usually be affected. Female plants that reverse to male flowering show the biggest decline. Not only is the grass less potent, but the amount of marijuana harvested from male flowers is negligible compared to the amount of marijuana that can be harvested from a normal female. Plants that change from male to female flowering usually increase their potency, because of the growth of female flower bracts with their higher concentration of resin. Female flowers on male plants seldom form as thickly or vigorously as on a normal female. Between the loss in potency and the loss in yield because of females changing to males, a crop from such plants is usually inferior, in both yield and potency, to one from normal plants.
Environmental Effects
Many environmental factors can cause intersexes and sexual reversals. These include photoperiod, low light intensity, applications of ultraviolet light, low temperatures, mutilation or severe pruning, nutrient imbalances or deficiencies, senescence (old age), and applications of various chemicals (see bibliography on sex determination).
The photoperiod (or time of planting using natural light) is the most important factor to consider for normal flowering. In 1931, J. Schaffner (105) showed that the percentage of hemp plants that had confused sexual characteristics depended on the time of year they were planted. Normal flowering (less than five percent of the plants are intersexes) occurred when the seeds were sown in May, June, or July, the months when the photoperiod is longest and light intensity is strongest. When planted sooner or later in the year, the percentage of intersexuals increased steadily, until about 90 percent of the plants were intersexual when planted during November or early December.
Marijuana plants need more time to develop than hemp plants at latitudes in the United States. Considering potency, size, and normal flowering, the best time to sow for the summer crop is during the month of April. Farmers in the south could start the plants as late as June and still expect fully developed plants.
If artificial light is used, the length of the photoperiod can influence sexual expression. Normal flowering, with about equal numbers of male and female plants, seems to occur when the photoperiod is from 15 to 17 hours of light for a period of three to five months. The photoperiod is then shortened to 12 hours to induce flowering. With longer photoperiods, from 18 to 24 hours a day, the ratio of males to females changes, depending on whether flowering is induced earlier or later in the plant's life. When the plants are grown with long photoperiods for six months or more, usually there are at least 10 percent more male then female plants. When flowering is induced within three months of age, more females develop. Actually, the "extra" males or females are reversed plants, but the reversals occur before the plants flower in their natural genders.
Some plants will flower normally without a cutting of the photoperiod. But more often, females will not form thick buds unless the light cycle is cut to a period of 12 hours duration. Don't make the light cycle any shorter than 12 hours, unless the females have not shown flowers after three weeks of 12-hour days. Then cut the light cycle to 11 hours. Flowers should appear in about one week.
Anytime the light cycle is cut to less than 11 hours, some intersexes or reversed plant usually develop. This fact leads to a procedure for increasing the numbers of female flowers indoors. The crops can be grown for three months under a long photoperiod (18 or more hours of light). The light cycle is then cut to 10 hours. Although the harvest is young (about five months) there will be many more female flower buds than with normal flowering. More plants will develop female flowers initially, and male plants usually reverse to females after a few weeks of flowering.
Of the other environmental factors that can affect sexual expression in Cannabis, none are as predictable as the photoperiod. Factors such as nutrients or pruning affect the plant's overall health and metabolism, and can be dealt with by two general thoughts. First, good growing conditions lead to healthy plants and normal flowering: female and male plants occur in about equal numbers, with few (if any) intersexes or reversed plants. Poor growing conditions lead to reduced health and vigour, and oftentimes to confused sex in the adult plant. Second, the age of the plants seems to influence reversals. Male plants often show female flowers when the plant is young (vigorous) during flowering. Females seven or more months old (weaker) often develop male flowers after flowering normally for a few weeks.
Anytime the plant's normal growth pattern is disrupted, normal flowering may be affected. For instance, plant propagated from cuttings sometimes reverse sex, as do those grown for more than one season.
-------------------------------------------------------------------------------------------------------------------------------------
Now I'm no genius but I can read and it would seem that the rest of the article goes on to directly contradict Mr. SIR's assertion that males are males, females are females and no amount of stress will change that. This IS information which is absolutely related to the thread. If that makes me a troll, then so be it.
doc111
Well-Known Member
You're welcome my friend. I'll see if I have any other copy and pastes on hermies I can throw up here in this thread
LAX Skunky BwS
Well-Known Member
lmaoo .. all this cuz of hermie plants.. 
LAX Skunky BwS
Well-Known Member
u get stable genetics u wont get hermies .. no matter how bad u stress them .. then again why would you stress your plant to the verg of them going hermie on you?
SIR SMOKER
New Member
Attachments
c.h.u.d.
Active Member
And what is breaking the 12/12? I really want to know what you know about that
fabfun
New Member
look forward to it i like to see all facts seems like their is some bad info being spread by some to people that dont know better glad there r members like you that take the time to prove your statements
You're welcome my friend. I'll see if I have any other copy and pastes on hermies I can throw up here in this thread
HomeGrown&Smoked
Active Member
Then how are feminized seeds made?
SIR SMOKER
New Member
100% correct (i just dont understand people who cant understand genetics).
THANK YOU.
SS.
Howard Stern
Well-Known Member
Females will hermi if you flower them too long and they are about to die. It is a last attempt to self polinate. I have read that in many sites, and you guys are right saying that strains are more likely to hermi but to say that they will never hermi if you get the right genics just isn't true.
HomeGrown&Smoked
Active Member
And I'll never understand those who can't wrap their heads around environmental factors.100% correct (i just dont understand people who cant understand genetics).
THANK YOU.
SS.
SIR SMOKER
New Member
OOOOOOOOOOOOHHHHHHHHHHHHHH feeling the jealousy are we ????????
See you 2 trolls later.
hahahahahahahahahaha.
leftreartire
Active Member
i just dont believe that. it is the nature of the plant to do everything it can to reproduce no matter the genetics. its the design of the plant by god to live forever. isnt it great weed will always be around
fabfun
New Member
excuse me? are u high wtf r u talking about. I dont recall mentioning you
OOOOOOOOOOOOHHHHHHHHHHHHHH feeling the jealousy are we ????????
See you 2 trolls later.
hahahahahahahahahaha.
SIR SMOKER
New Member
OK lets put it in simple terms. Plant genetics are the same as human genetics.
We have females (100%) females.
We have males (100%) males.
We have hermie/bi sexual people who are born with that genetic trait.
Its really that simple as nobody knows how the person will turn out as its controlled by the the genetics passed on to the child VIA the parents DNA.
Its just basic biology mixed with genetic traits that can NOT be changed and are passed from parents to children the same way plant genetics are.
HomeGrown&Smoked
Active Member
Look up "mutation"- basic biological principle, and with plants can be brought about with environmental stress.
leftreartire
Active Member
how can you compare humans to plants. we work together to co habitate the earth but our reproduction is not the same. a part of you cant fall off and continue to grow. iff all the females in the rest human race had died off we would end. if the plants were to they will self pollenate irself to reproduce. have you ever finished flowering your crop and had a seed in your bud knowing there was no way pollen got into your room. that is the plants natural genetic make up to continue on. clown fish will switch sex to make sure there is the correct ratio for them to survive. humans dont
bobbyhopefeild
Active Member
this guys pretty much on the money, plants arn't humans
doc111
Well-Known Member
Since we are having a riveting conversation about genetic mutations I figured I'd put up this gem here. I really enjoy science.
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A1876
8.1 Mutations: Types and Causes
The development and function of an organism is in large part controlled by genes. Mutations can lead to changes in the structure of an encoded protein or to a decrease or complete loss in its expression. Because a change in the DNA sequence affects all copies of the encoded protein, mutations can be particularly damaging to a cell or organism. In contrast, any alterations in the sequences of RNA or protein molecules that occur during their synthesis are less serious because many copies of each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
Mutations Are Recessive or Dominant
A fundamental genetic difference between organisms is whether their cells carry a single set of chromosomes or two copies of each chromosome. The former are referred to as haploid; the latter, as diploid. Many simple unicellular organisms are haploid, whereas complex multicellular organisms (e.g., fruit flies, mice, humans) are diploid.
Figure 8-1
For a recessive mutation to give rise to a mutant phenotype (more...)
Figure 8-1
.
For a recessive mutation to give rise to a mutant phenotype in a diploid organism, both alleles must carry the mutation
However, one copy of a dominant mutant allele leads to a mutant phenotype. Recessive mutations result in a loss of function, whereas dominant mutations often, but not always, result in a gain of function.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1).
Recessive mutations inactivate the affected gene and lead to a loss of function. For instance, recessive mutations may remove part of or all the gene from the chromosome, disrupt expression of the gene, or alter the structure of the encoded protein, thereby altering its function. Conversely, dominant mutations often lead to a gain of function. For example, dominant mutations may increase the activity of a given gene product, confer a new activity on the gene product, or lead to its inappropriate spatial and temporal expression. Dominant mutations, however, may be associated with a loss of function. In some cases, two copies of a gene are required for normal function, so that removing a single copy leads to mutant phenotype. Such genes are referred to as haplo-insufficient. In other cases, mutations in one allele may lead to a structural change in the protein that interferes with the function of the wild-type protein encoded by the other allele. These are referred to as dominant negative mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Inheritance Patterns of Recessive and Dominant Mutations Differ
Recessive and dominant mutations can be distinguished because they exhibit different patterns of inheritance. To understand why, we need to review the type of cell division that gives rise to gametes (sperm and egg cells in higher plants and animals). The body (somatic) cells of most multicellular organisms divide by mitosis (see Figure 1-10), whereas the germ cells that give rise to gametes undergo meiosis. Like body cells, premeiotic germ cells are diploid, containing two of each morphologic type of chromosome. Because the two members of each such pair of homologous chromosomes are descended from different parents, their genes are similar but not usually identical. Single-celled organisms (e.g., the yeast S. cerevisiae) that are diploid at some phase of their life cycle also undergo meiosis (see Figure 10-54).
Figure 8-2
Meiosis
Figure 8-2
.
Meiosis
A premeiotic germ cell has two copies of each chromosome (2n), one maternal and one paternal. Chromosomes are replicated during the S phase, giving a 4n chromosomal complement. During the first meiotic division, each replicated chromosome (actually two sister chromatids) aligns at the cell equator, paired with its homologous partner; this pairing off, referred to as synapsis, permits genetic recombination (discussed later). One homolog (both sister chromatids) of each morphologic type goes into one daughter cell, and the other homolog goes into the other cell. The resulting 2n cells undergo a second division without intervening DNA replication. During this second meiotic division, the sister chromatids of each morphologic type separate and these now independent chromosomes are randomly apportioned to the daughter cells. Thus, each diploid cell that undergoes meiosis produces four haploid cells, whereas each diploid cell that undergoes mitosis produces two diploid cells (see Figure 1-10).
Figure 8-2 depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Figure 8-3
Segregation patterns of dominant and recessive mutations (more...)
Figure 8-3
.
Segregation patterns of dominant and recessive mutations
Crosses between genotypically normal individuals (blue) and mutants (yellow) that are heterozygous for a dominant mutation (a) or homozygous for a recessive mutation (b) produce different ratios of normal and mutant phenotypes in the F1 generation. Although all the F1 progeny from a cross between a normal individual and an individual homozygous for a recessive mutation will have a normal phenotype, one-quarter of the progeny from the intercross between F1 progeny will have a mutant phenotype. Observation of segregation patterns like these led Gregor Mendel (1822 – 1884) to conclude that each gamete receives only one of the two parental alleles, a conclusion known as Mendel’s first law.
Now, let’s see what phenotypes are generated by mating of wild-type individuals with mutants carrying either a dominant or a recessive mutation. As shown in Figure 8-3a, half the gametes from an individual heterozygous for a dominant mutation in a particular gene will have the wild-type allele, and half will have the mutant allele. Since fertilization of female gametes by male gametes occurs randomly, half the first filial (F1) progeny resulting from the cross between a normal wild-type individual and a mutant individual carrying a single dominant allele will exhibit the mu-tant phenotype. In contrast, all the gametes produced by a mutant homozygous for a recessive mutation will carry the mutant allele. Thus, in a cross between a normal individual and one who is homozygous for a recessive mutation, none of the F1 progeny will exhibit the mutant phenotype (Figure 8-3b). However, one-fourth of the progeny from parents both heterozygous for a recessive mutation will show the mutant phenotype.
Mutations Involve Large or Small DNA Alterations
Figure 8-4
Different types of mutations
Figure 8-4
.
Different types of mutations
(a) Point mutations, which involve alteration in a single base pair, and small deletions generally directly affect the function of only one gene. A wild-type peptide sequence and the mRNA and DNA encoding it are shown at the top. Altered nucleotides and amino acid residues are highlighted in green. Missense mutations lead to a change in a single amino acid in the encoded protein. In a nonsense mutation, a nucleotide base change leads to the formation of a stop codon (purple). This results in premature termination of translation, thereby generating a truncated protein. Frameshift mutations involve the addition or deletion of any number of nucleotides that is not a multiple of three, causing a change in the reading frame. Consequently, completely unrelated amino acid residues are incorporated into the protein prior to encountering a stop codon. (b) Chromosomal abnormalities involve alterations in large segments of DNA. Presumably these abnormalities arise owing to errors in the mechanisms for repairing double-strand breaks in DNA. Chromosomes (I or II) are shown as single thick lines with the regions involved in a particular abnormality highlighted in green or purple. Inversions occur when a break is rejoined to the correct chromosome but in an incorrect orientation; deletions, when a segment of DNA is lost; translocations, when breaks are rejoined to the wrong chromosomes; and insertions, when a segment from one chromosome is inserted into another chromosome.
A mutation involving a change in a single base pair, often called a point mutation, or a deletion of a few base pairs generally affects the function of a single gene (Figure 8-4a). Changes in a single base pair may produce one of three types of mutation:
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b).
Mutations Occur Spontaneously and Can Be Induced
Mutations arise spontaneously at low frequency owing to the chemical instability of purine and pyrimidine bases and to errors during DNA replication. Natural exposure of an organism to certain environmental factors, such as ultraviolet light and chemical carcinogens (e.g., aflatoxin B1), also can cause mutations.
Figure 8-5
One mechanism by which errors in DNA replication produce (more...)
Figure 8-5
.
One mechanism by which errors in DNA replication produce spontaneous mutations
The replication of only one strand is shown; the other strand is replicated normally, as shown at the top. A replication error may arise in regions of DNA containing tandemly repeated sequences (in this case, GTC) when a portion of the newly synthesized strand (light blue) loops out into a single-stranded form. This slippage displaces the newly synthesized strand back along the template strand (dark blue), with its 3′ end still paired with the template. As a result, the DNA-synthesizing enzymes copy a region of the template strand a second time, leading to an increase in length of nine nucleotides (yellow) in this example. A subsequent round of DNA replication results in the production of one normal duplex DNA molecule and one mutant duplex containing the additional nucleotides.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5 illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs.
In order to increase the frequency of mutation in experimental organisms, researchers often treat them with high doses of chemical mutagens or expose them to ionizing radiation. Mutations arising in response to such treatments are referred to as induced mutations. Generally, chemical mutagens induce point mutations, whereas ionizing radiation gives rise to large chromosomal abnormalities.
Figure 8-6
Induction of point mutations by ethylmethane sulfonate (more...)
Figure 8-6
.
Induction of point mutations by ethylmethane sulfonate (EMS), a commonly used mutagen
(a) EMS alkylates guanine at the oxygen on position 6 of the purine ring, forming O6-ethylguanine (Et-G), which base-pairs with thymine. (b) Two rounds of DNA replication of a strand containing Et-G yields a mutant DNA in which a G·C base pair is replaced with an A·T pair. Cells also have repair enzymes that can remove the ethyl group from Et-G (Chapter 12).
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12.
Some Human Diseases Are Caused by Spontaneous Mutations
Many common human diseases, often devastating in their effects, are due to mutations in single genes. Genetic diseases arise by spontaneous mutations in germ cells (egg and sperm), which are transmitted to future generations. For example, sickle-cell anemia, which affects 1 in 500 individuals of African descent, is caused by a single missense mutation at codon 6 of the β-globin gene; as a result of this mutation, the glutamic acid at position 6 in the normal protein is changed to a valine in the mutant protein. This alteration has a profound effect on hemoglobin, the oxygen-carrier protein of erythrocytes, which consists of two α-globin and two β-globin subunits (see Figure 3-11). The deoxygenated form of the mutant protein is insoluble in erythrocytes and forms crystalline arrays. The erythrocytes of affected individuals become rigid and their transit through capillaries is blocked, causing severe pain and tissue damage. Because the erythrocytes of heterozygous individuals are resistant to the parasite causing malaria, which is endemic in Africa, the mutant allele has been maintained. It is not that individuals of African descent are more likely than others to acquire a mutation causing the sickle-cell defect, but rather the mutation has been maintained in this population by interbreeding.
Figure 8-7
Role of spontaneous somatic mutation in retinoblastoma, (more...)
Figure 8-7
.
Role of spontaneous somatic mutation in retinoblastoma, a childhood disease marked by retinal tumors
Tumors arise from retinal cells that carry two mutant Rb− alleles. (a) In hereditary retinoblastoma, a child receives a normal Rb+ allele from one parent and a mutant Rb− allele from the other parent. A single mutagenic event in a heterozygous somatic retinal cell that inactivates the normal allele will result in a cell homozygous for two mutant Rb− alleles. (b) In sporadic retinoblastoma, a child receives two normal Rb+ alleles. Two separate somatic mutations, inactivating both alleles in a particular cell, are required to produce a homozygous Rb−/Rb− retinal cell.
Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13).
In a later section, we will see how normal copies of disease-related genes can be isolated and cloned.
SUMMARY
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8.1 Mutations: Types and Causes
The development and function of an organism is in large part controlled by genes. Mutations can lead to changes in the structure of an encoded protein or to a decrease or complete loss in its expression. Because a change in the DNA sequence affects all copies of the encoded protein, mutations can be particularly damaging to a cell or organism. In contrast, any alterations in the sequences of RNA or protein molecules that occur during their synthesis are less serious because many copies of each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
Mutations Are Recessive or Dominant
A fundamental genetic difference between organisms is whether their cells carry a single set of chromosomes or two copies of each chromosome. The former are referred to as haploid; the latter, as diploid. Many simple unicellular organisms are haploid, whereas complex multicellular organisms (e.g., fruit flies, mice, humans) are diploid.
Figure 8-1
For a recessive mutation to give rise to a mutant phenotype (more...)
Figure 8-1
.
For a recessive mutation to give rise to a mutant phenotype in a diploid organism, both alleles must carry the mutation
However, one copy of a dominant mutant allele leads to a mutant phenotype. Recessive mutations result in a loss of function, whereas dominant mutations often, but not always, result in a gain of function.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1
). Recessive mutations inactivate the affected gene and lead to a loss of function. For instance, recessive mutations may remove part of or all the gene from the chromosome, disrupt expression of the gene, or alter the structure of the encoded protein, thereby altering its function. Conversely, dominant mutations often lead to a gain of function. For example, dominant mutations may increase the activity of a given gene product, confer a new activity on the gene product, or lead to its inappropriate spatial and temporal expression. Dominant mutations, however, may be associated with a loss of function. In some cases, two copies of a gene are required for normal function, so that removing a single copy leads to mutant phenotype. Such genes are referred to as haplo-insufficient. In other cases, mutations in one allele may lead to a structural change in the protein that interferes with the function of the wild-type protein encoded by the other allele. These are referred to as dominant negative mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Inheritance Patterns of Recessive and Dominant Mutations Differ
Recessive and dominant mutations can be distinguished because they exhibit different patterns of inheritance. To understand why, we need to review the type of cell division that gives rise to gametes (sperm and egg cells in higher plants and animals). The body (somatic) cells of most multicellular organisms divide by mitosis (see Figure 1-10), whereas the germ cells that give rise to gametes undergo meiosis. Like body cells, premeiotic germ cells are diploid, containing two of each morphologic type of chromosome. Because the two members of each such pair of homologous chromosomes are descended from different parents, their genes are similar but not usually identical. Single-celled organisms (e.g., the yeast S. cerevisiae) that are diploid at some phase of their life cycle also undergo meiosis (see Figure 10-54).
Figure 8-2
Meiosis
Figure 8-2
.
Meiosis
A premeiotic germ cell has two copies of each chromosome (2n), one maternal and one paternal. Chromosomes are replicated during the S phase, giving a 4n chromosomal complement. During the first meiotic division, each replicated chromosome (actually two sister chromatids) aligns at the cell equator, paired with its homologous partner; this pairing off, referred to as synapsis, permits genetic recombination (discussed later). One homolog (both sister chromatids) of each morphologic type goes into one daughter cell, and the other homolog goes into the other cell. The resulting 2n cells undergo a second division without intervening DNA replication. During this second meiotic division, the sister chromatids of each morphologic type separate and these now independent chromosomes are randomly apportioned to the daughter cells. Thus, each diploid cell that undergoes meiosis produces four haploid cells, whereas each diploid cell that undergoes mitosis produces two diploid cells (see Figure 1-10).
Figure 8-2
depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog. Figure 8-3
Segregation patterns of dominant and recessive mutations (more...)
Figure 8-3
.
Segregation patterns of dominant and recessive mutations
Crosses between genotypically normal individuals (blue) and mutants (yellow) that are heterozygous for a dominant mutation (a) or homozygous for a recessive mutation (b) produce different ratios of normal and mutant phenotypes in the F1 generation. Although all the F1 progeny from a cross between a normal individual and an individual homozygous for a recessive mutation will have a normal phenotype, one-quarter of the progeny from the intercross between F1 progeny will have a mutant phenotype. Observation of segregation patterns like these led Gregor Mendel (1822 – 1884) to conclude that each gamete receives only one of the two parental alleles, a conclusion known as Mendel’s first law.
Now, let’s see what phenotypes are generated by mating of wild-type individuals with mutants carrying either a dominant or a recessive mutation. As shown in Figure 8-3a
, half the gametes from an individual heterozygous for a dominant mutation in a particular gene will have the wild-type allele, and half will have the mutant allele. Since fertilization of female gametes by male gametes occurs randomly, half the first filial (F1) progeny resulting from the cross between a normal wild-type individual and a mutant individual carrying a single dominant allele will exhibit the mu-tant phenotype. In contrast, all the gametes produced by a mutant homozygous for a recessive mutation will carry the mutant allele. Thus, in a cross between a normal individual and one who is homozygous for a recessive mutation, none of the F1 progeny will exhibit the mutant phenotype (Figure 8-3b). However, one-fourth of the progeny from parents both heterozygous for a recessive mutation will show the mutant phenotype. Mutations Involve Large or Small DNA Alterations
Figure 8-4
Different types of mutations
Figure 8-4
.
Different types of mutations
(a) Point mutations, which involve alteration in a single base pair, and small deletions generally directly affect the function of only one gene. A wild-type peptide sequence and the mRNA and DNA encoding it are shown at the top. Altered nucleotides and amino acid residues are highlighted in green. Missense mutations lead to a change in a single amino acid in the encoded protein. In a nonsense mutation, a nucleotide base change leads to the formation of a stop codon (purple). This results in premature termination of translation, thereby generating a truncated protein. Frameshift mutations involve the addition or deletion of any number of nucleotides that is not a multiple of three, causing a change in the reading frame. Consequently, completely unrelated amino acid residues are incorporated into the protein prior to encountering a stop codon. (b) Chromosomal abnormalities involve alterations in large segments of DNA. Presumably these abnormalities arise owing to errors in the mechanisms for repairing double-strand breaks in DNA. Chromosomes (I or II) are shown as single thick lines with the regions involved in a particular abnormality highlighted in green or purple. Inversions occur when a break is rejoined to the correct chromosome but in an incorrect orientation; deletions, when a segment of DNA is lost; translocations, when breaks are rejoined to the wrong chromosomes; and insertions, when a segment from one chromosome is inserted into another chromosome.
A mutation involving a change in a single base pair, often called a point mutation, or a deletion of a few base pairs generally affects the function of a single gene (Figure 8-4a
). Changes in a single base pair may produce one of three types of mutation: - <LI id=A1883 class=reg>Missense mutation, which results in a protein in which one amino acid is substituted for another
<LI id=A1884 class=reg>Nonsense mutation, in which a stop codon replaces an amino acid codon, leading to premature termination of translation - Frameshift mutation, which causes a change in the reading frame, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b
). Mutations Occur Spontaneously and Can Be Induced
Mutations arise spontaneously at low frequency owing to the chemical instability of purine and pyrimidine bases and to errors during DNA replication. Natural exposure of an organism to certain environmental factors, such as ultraviolet light and chemical carcinogens (e.g., aflatoxin B1), also can cause mutations.
Figure 8-5
One mechanism by which errors in DNA replication produce (more...)
Figure 8-5
.
One mechanism by which errors in DNA replication produce spontaneous mutations
The replication of only one strand is shown; the other strand is replicated normally, as shown at the top. A replication error may arise in regions of DNA containing tandemly repeated sequences (in this case, GTC) when a portion of the newly synthesized strand (light blue) loops out into a single-stranded form. This slippage displaces the newly synthesized strand back along the template strand (dark blue), with its 3′ end still paired with the template. As a result, the DNA-synthesizing enzymes copy a region of the template strand a second time, leading to an increase in length of nine nucleotides (yellow) in this example. A subsequent round of DNA replication results in the production of one normal duplex DNA molecule and one mutant duplex containing the additional nucleotides.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5
illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs. In order to increase the frequency of mutation in experimental organisms, researchers often treat them with high doses of chemical mutagens or expose them to ionizing radiation. Mutations arising in response to such treatments are referred to as induced mutations. Generally, chemical mutagens induce point mutations, whereas ionizing radiation gives rise to large chromosomal abnormalities.
Figure 8-6
Induction of point mutations by ethylmethane sulfonate (more...)
Figure 8-6
.
Induction of point mutations by ethylmethane sulfonate (EMS), a commonly used mutagen
(a) EMS alkylates guanine at the oxygen on position 6 of the purine ring, forming O6-ethylguanine (Et-G), which base-pairs with thymine. (b) Two rounds of DNA replication of a strand containing Et-G yields a mutant DNA in which a G·C base pair is replaced with an A·T pair. Cells also have repair enzymes that can remove the ethyl group from Et-G (Chapter 12).
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a
). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12. Some Human Diseases Are Caused by Spontaneous Mutations
Figure 8-7
Role of spontaneous somatic mutation in retinoblastoma, (more...)
Figure 8-7
.
Role of spontaneous somatic mutation in retinoblastoma, a childhood disease marked by retinal tumors
Tumors arise from retinal cells that carry two mutant Rb− alleles. (a) In hereditary retinoblastoma, a child receives a normal Rb+ allele from one parent and a mutant Rb− allele from the other parent. A single mutagenic event in a heterozygous somatic retinal cell that inactivates the normal allele will result in a cell homozygous for two mutant Rb− alleles. (b) In sporadic retinoblastoma, a child receives two normal Rb+ alleles. Two separate somatic mutations, inactivating both alleles in a particular cell, are required to produce a homozygous Rb−/Rb− retinal cell.
Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a
). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13). In a later section, we will see how normal copies of disease-related genes can be isolated and cloned.
SUMMARY
- <LI id=A1893 class=reg> Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry only one copy.
<LI id=A1894 class=reg> Mutations are alterations in DNA sequences that result in changes in the structure of a gene. Both small and large DNA alterations can occur spontaneously. Treatment with ionizing radiation or various chemical agents increases the frequency of mutations.
<LI id=A1895 class=reg> Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
<LI id=A1896 class=reg> Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.
<LI id=A1897 class=reg>
 In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells (Figure 8-2). The members of each pair of homologous chromosomes segregate independently during meiosis, leading to the random reassortment of maternal and paternal alleles in the gametes.
 Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses (see Figure 8-3).
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- <LI class=nav-title>In this Page <LI class=nav-self-title>Molecular Cell Biology <LI class=navigation-parts>Mutations Are Recessive or Dominant <LI class=navigation-parts>Inheritance Patterns of Recessive and Dominant Mutations Differ <LI class=navigation-parts>Mutations Involve Large or Small DNA Alterations <LI class=navigation-parts>Mutations Occur Spontaneously and Can Be Induced <LI class=navigation-parts>Some Human Diseases Are Caused by Spontaneous Mutations
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