In some gene interactions, the four Mendelian genotypic classes produce fewer than four observable phenotypes. Why?
That is because one gene masks the phenotypic effects of another
Ex: Take the sleek, short-haired coat of Labrador retrievers; it can be black, brown or yellow depending on the combinations of two independently assorting coat color genes
In the case of the Labrador coat colors, the dominant B allele of the first gene determines black, while the recessive bb homozygote is chocolate brown. With the second gene, the dominant E allele has no visible effect on black or chocolate coat color. What happens when there is a double dose of the recessive allele (ee)?
Effectively hides the effect of any combination of the black or chocolate alleles to yield yellow coats
A gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene
The allele doing the masking (e allele of the E gene) is ________ to the gene that is being masked (the _______ gene).
In our Labrador coat example, where homozygosity for a recessive e allele of the second gene is required to hide the effects of another gene, the masking phenomenon is called ________ ________. Why?
The recessive ee homozygous genotype is considered _______ to any allelic combo at the first gene
Because the allele causing the epistasis is recessive
What is the result of crossing pure-breading BB EE labs and one type of pure-breeding yellow retriever (bb ee) labs. Determine the coat color of the F1 generation and the possible coat colors of the F2 generation as well as the expected ratio (use a Punnett Square)
In the dihybrid cross of the Labradors, why are there only three phenotypic classes?
The two genotypic classes without a dominant E allele: the three B-ee and the one bb ee- combine to produce yellow phenotypes.
The telltale ratio of recessive epistasis in the F2 generation is:
9:3:4, with 4 representing a combination of 3 (B- ee) + 1 (bb ee).
Why can't you tell simply by looking, if the genotype of a yellow Labrador will be B- (black) or bb (chocolate) at the B locus
The ee genotype completely masks the influence of the other gene for coat color
Explain why, in rare instances, two parents who appear to have blood type O, and thus would be predicted to be genotype ii, produce a child who is either blood type A (genotype IAi) or blood type B (genotype IBi)
This phenomenon occurs because an extremely rare trait, called the Bombay phenotype (discovered in Bombay, India) resembles blood type O.
The Bombay phenotype actually arises from __________ for a mutant recessive allele (hh) of a second gene that ______ the effects of any ABO alleles that might be present
In construction of the red blood cell, surface molecules that determine blood type, type A individuals make an ______ that adds polysaccharide A onto a sugar polymer known as ________; type B individuals make an altered form of the ______ that adds polysaccharide B onto the sugar polymer _______. Type O individuals make ________ _______.
neither enzyme (A-adding nor B-adding)
With regard to their substance H, what is the result of type O individuals making neither A-adding nor B-adding enzyme?
They have an exposed substance H in the membranes of their red blood cells.
All people of A, B, or O phenotype carry at least one _______ _____-_____ H allele for the second gene and thus produce some _______.
dominant wild-type H allele
In the rare Bombay phenotype individuals with genotype hh for the second gene, they do not make ________ at all, so even if they make an ________ that would add A or B to this polysaccharide base, they have ________ to add it onto. The result is that they appear to be type ___
Due to circumstances resulting in Bombay phenotype individuals appearing to be type O, __________ for the recessive h allele of the H-substance gene ______ the effects of the ABO gene, making the hh genotype _______ to any combo of IA, IB, and i alleles
A person who carries IA, IB, or both IAand IBbut is also an hh homozygote for the H-substance gene may appear to be type O, but he or she will be able to pass along an ___ or ___ allele in sperm or egg.
The offspring receiving, let's say an IAallele for the ABO gene and a recessive h allele for the H-substance gene from its father plus an i allele and a dominant H allele from its mother would have blood type ___ (genotype ___, ___) even though neither of the parents is phenotype A or AB
blood type A (genotype IAi, Hh)
Many different types of biochemical pathways may produce any specific F2 phenotypic ratio. The ratio may suggest or limit possibilities, but it cannot by itself provide enough information to allow you to infer the ______ ______. Does this go both ways?
No if you know the biochemical pathway, you can predict the phenotypic ratios among the progeny of a cross involving genes in the pathway
Epistasis can also be caused by a _______ ______
In summer squash, two genes influence the color of the fruit. With one gene, the dominant allele (A-) determines yellow, while homozygotes for the recessive allele (aa) are green. A second gene's dominant allele (B-) produces white, while bb fruit may be either yellow or green depending on the ________ of the first gene.
In the interaction between these two genes, the presence of B _____ the effects of either A- or aa, producing white fruit, and B- is thus _______ to any genotype of the A gene. The recessive b allele has _____ effect on fruit color determined by gene A
Epistasis in which the dominant allele of one gene hides the effects of another gene
In a cross between white F1 dihybrids (Aa Bb), the F2 phenotypic ratio is:
is 12 white : 3 yellow : 1 green
*the 12 includes two genotypic classes: 9 A- B- and 3 aa B-
T or F: Based on knowledge of similar phenomena in other plants (like the summer squash), a likely biochemical pathway underlying the 12:3:1 genotypic ratio is shown
False, the 12:3:1 phenotypic ratio is shown
White leghorns have a doubly dominant AA BB genotype for feather color; white wyandottes are homozygous recessive for both genes (aa bb). A cross between these two pure-breeding white strains produces an all white dihybrid (Aa Bb) F1 generation, but birds with color appear in the F2 generation. What is the ratio?
pg 62 13:3
State the biochemical explanation for dominant epistasis in the generation of chicken feather color. You may use the Fig 3.18b on pg 62 to help (4-story)
Allele A encodes enzyme A, which synthesizes pigment
Allele a encodes no enzyme
Pigment deposition in the feathers depends on protein b encoded by allele b, the normal (wild-type) allele of a second gene.
The mutant dominant allele B, however, encodes an abnormal version of the protein that prevents pigment deposition, even when the normal protein b is present (*thats why we must have bb for pigment deposition)
In maize, two genes, A and B, control leaf development. Normal broad leaves develop as long as the plant has either a dominant A allele or a dominant B allele (A- B-, A -bb, or aa B-). However, the leaves of plants that have neither dominant allele (aa bb) are skinny. Why?
They contain too few cells
Why is the F2 phenotypic ratio signifying redundant gene action 15:1
Because leaves are malformed only in the absence of both A and B (aa bb)
The proteins (A and B) encoded by the dominant alleles function in _______, ________ pathways that recruit _______ cells to become part of the leaf
Often, redundant genes specify nearly identical proteins that perform the same function. Why does the organism have two genes that do the same thing?
Redundant genes often arise by chance evolutionary processes that duplicate genes
It takes a dominant wild-type allele at each of ___ genes (that can possibly cause deafness) to produce normal hearing. Thus, deafness is a __________ trait (define)
heterogeneous trait: a mutation at any one of a number of genes can give rise to the same phenotype
It is not always possible to determine which of many different genes has mutated in a person who expresses a hetergeneous mutant phenotype. For example deafness, in which we usually cannot tell whether a particular deaf man or woman carry mutations at the same gene. What is one way to find out?
If they have children together. If they only have children can hear, the parents most likely carry mutations at two different genes, and the children carry one normal, wild-type allele for both of those genes
If all of their children are deaf, it is likely that both parents are homozygous for a mutation on the same gene, and all of their children are also homozygous for this same mutation
Define complementation (1), and how does it affect mutation (2)?
Two different genes may give rise to the same phenotype
If mutations complement, this tells us that they are in different genes
If they fail to complement, the mutations are in the same gene
method for discovering whether two mutations are in the same or separate genes
If offsring receiving two recessive mutant alleles, one from each parent, express the mutant phenotype, ____________ does not occur. This is because the two mutation are _________ altering the same gene. Thus, the occurrence of complementation reveals genetic _________
Use 5 points to summarize how several variations on the theme of multifactorial traits can be identified
genes can interact to generate novel phenotypes
the dominant alleles of two interacting genes can both be necessary for the production of a particular normal phenotype
one gene's alleles can mask the effects of alleles at another gene
different genes may have redundant functions so that a dominant allele of either gene is sufficient for the production of a particular normal phenotype
mutant alleles at one of two or more different genes can result in the same phenotype
All the points mentioned on identifying multifactorial traits result in the merging of two or more of Mendel's four genotypic classes into one phenotypic class except?
The first stating: genes can interact to generate novel phenotypes
For any type of gene interaction, the alleles of one or both genes may exhibit incomplete dominance or codominance, and these possibilities increase the potential for phenotypic ________. What does this picture depict?
Depicts how incomplete dominance at both genes in a dihybrid cross results not in a collapse of several genotypic classes into one but rather an expansion (each of the nine genotypes in the dihybrid cross F2 corresponds to a different phenotype)
According to the picture, allele A is _______ _______ to allele a and allele B is ________ _______ to allele b. For each gene, two alleles generate ______ phenotypes and the F2 progeny will have _____ phenotypes
32 = 9
How do geneticists know whether a particular trait is caused by the alleles of one gene or by two genes interacting in one of a number of possible combinations?
Breeding tests can usually resolve the issue
What percentage of those with a certain genotype will show the associated phenotype
ex: retinoblastoma (most malignant eye cancer) arises from a dominant mutation of one gene and only about 75% of people who carry the mutant allele develop the disease.
T or F: Retinoblastoma can affect one or both eyes
Penetrance can be ________, as in the traits that Mendel studied or ________, as in retinoblastoma
The amount of variation of intensity of a phenotype in a certain genotype
Expressivity can be ________, as in retinoblastoma, or _______ as in pea color
varying (one or both eyes affected)
unvarying (all yy peas are green)
3 factors that affect penetrance and expressivity
Describe the four possible scenarios with penetrance and expressivity
Not all genes that influence the appearance of a trait contribute ______ to the phenotype. Major genes have a large influence, while _______ genes have a more subtle secondary effect.
Modifier genes alter the ________ produced by the alleles of other genes. There is no formal distinction between major and modifier genes, instead, there is a _______ between the two, and the cutoff is _______
_________ is one element of the environment that can have a visible effect on phenotype. (example)
Ex: Coat color pattern of Siamese cats
Siamese cats are ________ for one of the multiple alleles of a gene that encodes an _______ that catalyzes the production of the dark pigment known as _______.
The form of the enzyme generated by the variant "Siamese" allele does not function at the cat's ______ ______ _____ temperature. It becomes active only at the ______ temperatures found in the cat's _______, where it promotes the production of melanin, which darkens the animal's ears, nose, paws and tail.
normal core body temp
The enzyme generated by the variant "Siamese" allele is thus ________ sensitive
Under normal environmental conditions in temperate climates, the Siamese phenotype doesn't vary much in _________ from one cat to another, but one can imagine the expression of a very different phenotype (with no _____ extremities) in equatorial deserts, where the ambient temperature is at or _____ normal body temp.
Temperature can also affect _________. In one type of experimentally bred fruit fly, some individuals develop and mulltiply normally at temps between 18C and 29C; but if the thermometer climbs beyond that cutoff for a short time, they become ________ ________; and if the temp remains high for more than a few hours, they _____
Whether a carrier of the retinoblastoma mutation develops the phenotype, and whether the cancer affects one or both eyes, depend on additional ______ events that occur ______.
To produce retinoblastoma, these events must alter the ______ ______ of the gene in specific body cells. This can be caused by routine exposure to _____ ____
Chance events provide the second "hit" (explain) necessary to turn a normal retinal cell into a cancerous one.
For instance, cosmic rays causing a mutation in the second copy off the retinoblastoma gene
By contributing to incomplete penetrance and variable expressivity, modifier genes, the environment, and chance give rise to phenotypic _______.
What is the difference between the probability of epistasis or recessive lethality and the probability of penetrance and the level of expressivity
The probability of penetrance and the level of expressivity cannot be derived from the original Mendelian principles of segregation and independent assortment; they are established empirically by observation and counting
aka Discrete trait: inherited trait that clearly exhibits an either/or status (Mendel's purple versus white flowers)
aka Quantitative trait: Inherited trait that exhibits many intermediate forms; determined by alleles of many different genes whose interaction with each other and the environment produces the phenotype (height across the human species)
The main difference in populations exhibiting discontinuous trait and those exhibiting continuous traits
Discontinuous traits are passed down in populations of little genetic diversity, usually caused by inbreading