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Terms in this set (38)
A mutation in a gene often results in a reduction of the product of that gene. The term for this type of mutation is ________.
loss of function or null (in the case of complete loss)
"Gain of function" mutations are generally dominant since one copy in a diploid organism is sufficient to alter the normal phenotype.
Assume that a mutation occurs in the gene responsible for the production of hexosaminidase A, such that only about 50% of the enzyme activity is found in the heterozygote compared with a homozygous normal individual. If heterozygotes are phenotypically normal, we would say that the mutant allele is recessive to its normal allele.
With incomplete dominance, a likely ratio resulting from a monohybrid cross would be ________.
What is the difference between incomplete dominance and codominance?
In codominance, both phenotypes are expressed in heterozygotes. In incomplete dominance, the heterozygote shows a phenotype that is intermediate between the two homozygotes.
How many different phenotypes are possible in a one gene/three allele system that displays codominance to each other?
Six. With three alleles, a1, a2, a3, each unique combination of two alleles results in a distinct phenotype. Possible combinations are:
Which of the following is the most likely example of codominance?
A pure-breeding plant with red flowers is crossed to a pure-breeding plant with white flowers. All their progeny have flowers with some red patches and some white patches.
Heterozygotes express both alleles.
The color dilution gene in horses is an example of incomplete dominance.
The color of palomino horses is intermediate between those of their pure-breeding chestnut and pure-breeding cremello parents.
In a mating between individuals with the genotypes IAi × ii, what percentage of the offspring would be expected to have the O blood type?
In a mating between individuals with the genotypes IAIB × ii, what percentage of the offspring would be expected to have the O blood type?
Which of the following statements is always true when mutations occur in genes whose products are essential to an organism's survival?
A homozygote for a recessive lethal allele will not survive.
If the gene product is essential for survival, having two mutant alleles means the organism will not produce the gene product at all.
What term is used to express the idea that several genes exert influence over the same characteristic?
A condition in which one gene pair masks the expression of a nonallelic gene pair is called ________.
The following F2 results occur from a typical dihybrid cross:
Phenotype Genotype Ratio
purple A_B_ 9/16
white aaB_ 3/16
white A_bb 3/16
white aabb 1/16
If instead of a dihybrid cross, a double heterozygote (AaBb) is crossed with a fully recessive organism (aabb), what phenotypic ratio is expected in the offspring?
____ purple : ____ white
1 purple:3 white
Many of the color varieties of summer squash are determined by several interacting loci: AA or Aa gives white, aaBB or aaBb gives yellow, and aabb produces green.
If two fully heterozygous plants are crossed, what phenotypes will be seen in their offspring?
green, yellow and white squash
What will be the phenotypic ratio?
____ white : ___ yellow : ___ green
Crossing the forked and pale mutants
You continue your genetic analysis by crossing the forked and pale mutant lines with each other. The leaves of the F1 are light green (intermediate between pale and wild-type leaves) and forked. The F2 has six phenotypic classes:
You designate the forked mutant allele as F (wild type = f+ ) and the pale mutant allele as p (wild type = P).
Using these allele symbols, identify the genotypes of the six F2 classes by dragging one white label to each white target and one blue label to each blue target. Labels can be used once, more than once, or not at all.
a/b) PP, F_
c/d) Pp, F_
e/f) pp, F_
g/h) PP, f+f+
i/j) Pp, f+f+
k/l) pp, f+f+
In cases involving complete dominance, homozygous dominant individuals cannot be distinguished from heterozygous individuals by phenotype alone. Therefore, this phenotypic category contains two genotypes.
In cases involving incomplete dominance, each genotype has a distinct phenotype.
You continue your analysis by crossing the forked and twist lines. Your results are as follows:
Pure Lines: twist x forked
The forked mutation and the twist mutation are codominant alleles of the same locus.
The absence of wild-type progeny in the F2 of a cross indicates a monohybrid cross: The two lines crossed are mutant in the same locus. Any other explanation that involves two loci (for example, recombination or epistasis) fails because any cross with two loci will produce at least some F2 individuals with a wild-type genotype for both loci and therefore a wild-type phenotype.
In this case, the F1 expresses both mutant phenotypes, indicating that the forked and twist alleles are codominant.
You decide to conduct a genetic analysis of these mutant lines by crossing each with a pure-breeding wild-type line. The numbers in the F2 indicate the number of progeny in each phenotypic class.
Twist: the mutant allele is dominant to its corresponding wild type allele
Forked: the mutant allele is dominant to its corresponding wild type allele
Pale: the mutant allele is neither dominant nor recessive to its corresponding wild type allele
An allele is never intrinsically "dominant" or "recessive." Instead, these terms describe a relationship between two alleles. This relationship is evaluated by examining a heterozygote: The allele that determines the phenotype of the heterozygote is dominant to the other (recessive) allele.
Some heterozygotes have a phenotype that is intermediate between the phenotypes of the two homozygotes. This situation is the result of incomplete dominance: Neither allele is completely dominant to the other.
You decide to designate the twist allele as FT to distinguish it from the forked allele F.
Like incomplete dominance, codominance produces a distinct phenotype for each of the three genotypes in a monohybrid cross. Unlike incomplete dominance, however, codominance results in both alleles fully expressing their phenotype in the heterozygote.
With which of the following would hemizygosity most likely be associated?
The white-eye gene in Drosophila is recessive and sex-linked. Assume that a white-eyed female is mated to a wild-type male. What would be the phenotypes of the offspring?
wild type females; white-eyed males
The genes for zeste eyes and forked bristles are located on the X chromosome in Drosophila melanogaster. Both genes are recessive. A cross is made between a zeste-eyed female and a forked-bristled male. Assume no crossing over.
Q1) What phenotypes will be expected in the F1 offspring?
A1) wild type females; zeste-eyed males
Q2) If 200 F1 offspring were obtained, what would be the expected number of each?
____ wildtype females: ____ zeste males
Q3) If the F1 offspring were crossed to produce an F2 generation, what phenotypes would be seen in the offspring?
A3) wild type females; forked-bristled males; zeste-eyed females; zeste-eyed males
Q4) If 800 F2 offspring were obtained, what would be the expected number of each phenotype?
___ wild type female : ___ zeste females : ___ forked males : ____ zeste males
Sex-limited inheritance is the same as sex-linked inheritance.
A gene in which a male heterozygote has a different phenotype compared to a female heterozygote is referred to as __________.
Pattern baldness is expressed in heterozygous males, but not in heterozygous females.
Penetrance specifically refers to the expression of lethal genes in heterozygotes.
The term expressivity defines the percentage of individuals who show at least some degree of expression of a mutant genotype.
Epistasis is the interaction between genes such that one gene influences or interferes with the expression of another gene, leading to a specific phenotype. Epistatic genes can be dominant or recessive. Use your knowledge of epistasis to determine the genotypes of offspring in the following crosses.
Q1) In freshwater snails, pigment color is influenced by two genes. If two heterozygous pigmented freshwater snails were crossed and offspring were produced in a ratio of 9 pigmented snails to 7 albino snails, what are the genotypes of the offspring?
A1) pigmented snails: A_B_; albino snails: A_bb, aaB_, aabb
Q2) Is this an example of dominant or recessive epistasis?
A2) recessive epistasis
In recessive epistasis, two recessive alleles mask expression of an allele at a different locus. The 9:7 ratio observed in this example is actually a special case of epistasis called duplicative recessive epistasis, meaning the presence of either aa or bb was sufficient to mask expression of the other gene. In this case, if snails had two copies of either allele a or allele b, pigment production was suppressed, resulting in albino individuals.
In sheep, coat color is influenced by two genes. Gene A influences pigment production, while gene B produces black or brown pigment.
If two heterozygous white sheep were crossed and offspring were produced in a ratio of 12 white sheep to 3 black sheep to 1 brown sheep, what are the genotypes of the offspring?
white sheep: A_B_; A_bb
black sheep: aaB_
brown sheep: aabb
Is this an example of dominant or recessive epistasis?
This is an example of dominant epistasis, where one copy of an allele, in this case A, can mask the expression of another gene. Any sheep possessing the A allele (A_B_ and A_bb) had a white coat regardless of the gene present for pigment at the B locus.
In laborador retrievers, pigment color is influenced by two genes. Gene A determines the type of pigment produced and gene B affects whether the pigment gets deposited in the hair shaft.
If two heterozygous black laborator retrievers were crossed and offspring were produced in a ratio of 9 black dogs to 3 brown dogs to 4 yellow dogs, what are the genotypes of the offspring?
black dogs: A_B_
brown dogs: aaB_
yellow dogs: A_bb; aabb
Is this an example of dominant or recessive epistasis?
In foxes, two alleles of a single gene, P and p, may result in lethality (PP), platinum coat (Pp), or silver coat (pp).
Q1) What ratio is obtained when platinum foxes are interbred?
A1) 2/3 platinum, 1/3 silver
Q2) Is the P allele behaving dominantly or recessively in causing lethality?
Q3) Is the P allele behaving dominantly or recessively in causing platinum coat color?
In mice, a short-tailed mutant was discovered. When it was crossed to a normal long-tailed mouse, 4 offspring were short-tailed and 3 were long-tailed. Two short-tailed mice from the F1 generation were selected and crossed. They produced 6 short-tailed and 3 long-tailed mice. These genetic experiments were repeated three times with approximately the same results.
Q1) Indicate whether short tail would be dominant or recessive.
A1) Short is dominant to long.
Q2) What genetic ratios are illustrated? Hypothesize the mode of inheritance and diagram the crosses.
A2) Cross 1: Ss x ss = 1/2 Ss, 1/2 ss
Cross 2: Ss x Ss
1/2 Ss, 1/4 ss, 1/4 SS
List all possible genotypes for the A phenotype.
List all possible genotypes for the B phenotype.
List all possible genotypes for the AB phenotype.
List all possible genotypes for the O phenotype.
Is the mode of inheritance of the ABO blood types representative of dominance? of recessiveness? of codominance?
I^A and I^B are codominant, while being dominant to i.
Horses can be cremello (a light cream color), chestnut (a reddish brown color), or palomino (a golden color with white in the horse's tail and mane).
Of these phenotypes, only palominos never breed true. The following results have been observed:
cremello × palomino → 1/2 cremello 1/2 palomino
chestnut × palomino → 1/2 palomino 1/2 chestnut
palomino × palomino → 1/2 palomino 1/4 cremello 1/4 chestnut
Q1) From these results, determine the mode of inheritance
A1) incomplete dominance
Q2) Using the symbols Cch and Cc indicate which genotypes yield which phenotypes.
A2) chestnut: C^ch C^ch
cremello: C^c C^c
palomino: C^ch C^c
Q3) Predict the F1 result of many initial matings between cremello and chestnut horses.
A3) all palomino
Q4) Predict the F2 result of many initial matings between cremello and chestnut horses.
A4) 1/4 chestnut 1/2 palomino 1/4 cremello
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