Deck 4: Modification of Mendelian Ratios

Question 1

A mutant gene that produces brown eyes (bw)is located on chromosome 2 of Drosophila melanogaster,whereas a mutant gene producing bright red eyes,scarlet (st),is located on chromosome 3.Phenotypically,wild-type flies (with dull red eyes),whose mothers had brown eyes and whose fathers had scarlet eyes,were mated.The 800 offspring possessed the following phenotypes: wild type (dull red),white,scarlet (bright red),and brown.Most of the 800 offspring had wild-type eyes,whereas those with white eyes were the least frequent.
(a)Using standard symbolism,diagram the cross from the P generation (brown-eyed mothers X scarlet-eyed fathers)and the F? generation.Be certain to provide the alleles of the mutant genes.
(b)From the information presented above,how many white-eyed flies would you expect in the F? generation?

Answer 1

(a)P: bw/bw; st?/??? X bw?/???; ??/??
F?: bw?/??; ???/?? X bw?/??; ???/??
(b)50

Question 2

The trait of medium-sized leaves in iris is determined by the genetic condition PP'.Plants with large leaves are PP,whereas plants with small leaves are P'P'.A cross is made between two plants each with medium-sized leaves.If they produce 80 seedlings,what would be the expected phenotypes,and in what numbers would they be expected? What is the term for this allelic relationship?

Answer 2

20 (large leaves),40 (medium leaves),20 (small leaves); incomplete dominance

Question 3

With incomplete dominance,a likely ratio resulting from a monohybrid cross would be ________.

A)3:3
B)1:2:2:4
C)1:2:1
D)9:3:3:1
E)3:1

Answer 3

C

Question 4

A condition in which one gene pair masks the expression of a nonallelic gene pair is called ________.

A)codominance
B)epistasis
C)dominance
D)recessiveness
E)additive alleles

Question 5

Mice have a set of multiple alleles of a gene for coat color.Four of those alleles are as follows:
C = full color (wild)
c?? = chinchilla
c? = dilution
c = albino
Given that the gene locus is not sex-linked and that each allele is dominant to those lower in the list,diagram the crosses indicated below and give the phenotypic ratios expected from each.
(a)wild (heterozygous for dilution)X chinchilla (heterozygous for albino)
(b)chinchilla (heterozygous for albino)X albino

Question 6

Typical ratios resulting from epistatic interactions in dihybrid crosses would be ________.

A)9:3:3:1,1:2:1
B)1:1:1:1,1:4:6:4:1
C)9:3:4,9:7
D)1:2:2:4:1:2:1:2:1,1:2:1
E)3:1,1:1

Question 7

What is meant by the term epistasis? Distinguish between epistasis and dominance.Do not use examples in answering this question.

Question 8

With which of the following would hemizygosity most likely be associated?

A)codominance
B)incomplete dominance
C)trihybrid crosses
D)X-linked inheritance
E)sex-limited inheritance

Question 9

Because of the mechanism of sex determination,males of many species can be neither homozygous nor heterozygous.Such males are said to be ________.

A)dominant
B)hemizygous
C)recessive
D)complementary
E)None of the answers listed is correct.

Question 10

Assume that a dihybrid cross (AaBb X AaBb)is made in which the gene loci are autosomal,independently assorting,and incompletely dominant.What phenotypic ratio would you expect from such a cross? Just provide the ratio,not the phenotypes.

Question 11

A mutation in a gene often results in a reduction of the product of that gene.The term for this type of mutation is ________.

A)codominance
B)incomplete dominance
C)gain of function
D)multiple allelism
E)loss of function or null (in the case of complete loss)

Question 12

In a mating between individuals with the genotypes I??? X I???,what percentage of the offspring are expected to have the O blood type?

Question 13

In the mouse,gene A allows pigmentation to be deposited in the individual coat hairs; its allele a prevents such deposition of pigment,resulting in an albino.Gene B gives agouti (wild-type fur); its allele b gives black fur.
(a)Diagram the cross between a doubly heterozygous agouti mouse mated with a doubly homozygous recessive white mouse.
(b)What would be the expected phenotypic ratio in the progeny?

Question 14

The following coat colors are determined by alleles at one locus in horses:
palomino = golden coat with lighter mane and tail
cremello = almost white
chestnut = brown
The following table gives ratios obtained in matings of the above varieties:
Cross Parents Offspring
1 cremello X cremello all cremello
2 chestnut X chestnut all chestnut
3 cremello X chestnut all palomino
4 palomino x palomino 1/4 = chestnut
1/2 = palomino
1/4 = cremello
Assign gene symbols for the genetic control of coat color on the basis of these data.
Diagram the last two matings.

Question 15

The trait for medium-sized leaves in iris is determined by the genetic condition PP'.Plants with large leaves are PP,whereas plants with small leaves are P'P'.The trait for red flowers is controlled by the genes RR,pink by RR',and white by R'R'.A cross is made between two plants each with medium-sized leaves and pink flowers.If they produce 320 seedlings,what would be the expected phenotypes,and in what numbers would they be expected? Assume no linkage.

Question 16

How many different alleles can a gene have?

Question 17

Assume that a cross is made between two organisms that are both heterozygous for a gene that shows incomplete dominance.What phenotypic and genotypic ratios are expected in the offspring?

Question 18

Assume that a dihybrid cross is made in which the genes' loci are autosomal,independently assorting,and incompletely dominant.How many different phenotypes are expected in the offspring?

Question 19

The presence of more than two alternative forms of a given gene would be called ________.

A)multiple alleles
B)alternation of generations
C)codominance
D)incomplete dominance
E)hemizygosity

Question 20

In a mating between individuals with the genotypes I??? X I???,what percentage of the offspring are expected to have the O blood type?

Question 21

State a significant difference between X-linked and sex-influenced inheritance.

Question 22

If an X-linked disorder is lethal to the affected individual prior to the age at which one reaches reproductive maturation,the lethality will be expressed only in males.Why is this so?

Question 23

A cross was made between homozygous wild-type female Drosophila and yellow-bodied male Drosophila.All of the resulting offspring were phenotypically wild type.Offspring of the F? generation had the following phenotypes:
Sex Phenotype Number
male wild 96
male yellow 99
female wild 197
Based on this information:
(a)Is the mutant gene for yellow body behaving as a recessive or dominant gene?
(b)Is the yellow locus on an autosome or on the X chromosome?

Question 24

Two forms of hemophilia are determined by genes on the X chromosome in humans.Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man.What is the probability that their first son will have hemophilia?

Question 25

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?

Question 26

The enzyme glucose-6-phosphate dehydrogenase deficiency (G6PD)is inherited as a recessive gene on the X chromosome in humans.A phenotypically normal woman (whose father had G6PD)is married to a normal man.
(a)What fraction of their sons would be expected to have G6PD?
(b)If the husband had G6PD,would it make a difference in your answer in part (a)?

Question 27

Name three modes of inheritance that are influenced by the sex of individuals.

Question 28

Pattern baldness is determined by a single autosomal gene pair.When females are homozygous for this gene pair,can they show pattern baldness?

Question 29

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.Assume that two fully heterozygous plants are crossed.Give the phenotypes (with frequencies)of the offspring.

Question 30

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.
(a)If 200 offspring from this cross were obtained,present the expected number,sex,genotype,and phenotype in each class of the F?.
(b)If the F? offspring were crossed to produce 800 flies of an F? generation,present the expected number,sex,and phenotype in each class.Assume no crossing over.

Question 31

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.Crosses among heterozygotes give a 12:3:1 ratio.What type of gene interaction would account for these results?

Question 32

The following F? results occur from a typical dihybrid cross:
purple: A_B_ 9/16
white: aaB_ 3/16
white: A_bb 3/16
white: aabb 1/1
If a double heterozygote (AaBb)is crossed with a fully recessive organism (aabb),what phenotypic ratio is expected in the offspring?

Question 33

Assume that a dihybrid F? ratio,resulting from epistasis,was 9:3:4.If a double heterozygote is crossed with the fully recessive type,what phenotypic ratio is expected among the offspring?

Question 34

Regarding the ABO blood group system in humans,if an individual is genetically I??? and yet expresses the O blood type,which genotype is the individual likely to have?

Question 35

Two forms of hemophilia are determined by genes on the X chromosome in humans.Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man.What is the probability that their first daughter will have hemophilia?

Question 36

A particular cross gives a modified dihybrid ratio of 9:7.What phenotypic ratio would you expect in a testcross of the fully heterozygous F? crossed with the fully recessive type? Diagram the testcross using A,a,B,b as symbol sets.

Question 37

Below is a pedigree of a fairly common human hereditary trait in which the boxes represent males and the circles represent females.Shading symbolizes the abnormal phenotype.
Given that one gene pair is involved:
(a)Is the inheritance pattern X-linked or autosomal,recessive or dominant?
(b)Give the genotype of each individual in the pedigree.If more than one genotypic possibility exists,present all possible alternatives.

Question 38

Assume that a dihybrid F? ratio,resulting from epistasis,was 15:1.If a double heterozygote is crossed with the fully recessive type,what phenotypic ratio is expected among the offspring?

Question 39

Which types of ratios are likely to occur in crosses (F?)when one is dealing with two interacting,epistatic gene pairs?

Question 40

Comb shape in chickens represents one of the classic examples of gene interaction.Two gene pairs interact to influence the shape of the comb.The genes for rose comb (R)and pea comb (P)together produce walnut comb.The fully homozygous recessive condition (rrpp)produces the single comb.Assume that a rose-comb chicken is crossed with a walnut-comb chicken and the following offspring are produced: 17 walnut,16 rose,7 pea,6 single.
(a)What are the probable genotypes of the parents?
(b)Give the genotypes of each of the offspring classes.

Question 41

Penetrance specifically refers to the expression of lethal genes in heterozygotes.

Question 42

A 9:7 ratio indicates incomplete dominance.

Question 43

Expressivity is the term used to describe the balanced genetic output from a hemizygous condition.

Question 44

One result of X-linkage is a crisscross pattern of inheritance in which sons express recessive genes of their fathers and daughters express recessive genes of their mothers.

Question 45

The term expressivity defines the percentage of individuals who show at least some degree of expression of a mutant genotype.

Question 46

The ABO blood group locus in humans provides an example of epistasis.

Question 47

Provide an example of sex-influenced inheritance.

Question 48

With multiple alleles,there can be more than two genetic alternatives for a given locus.

Question 49

A typical epistatic ratio is 9:3:4.

Question 50

Pattern baldness and hen/cock feathering in fowl are examples of X-linked inheritance.

Question 51

What distinguishes sex-limited from sex-influenced inheritance?

Question 52

With both incomplete dominance and codominance,one expects heterozygous and homozygous classes to be phenotypically identical.

Question 53

A conditional mutant is one whose expression is influenced by some environmental condition.

Question 54

"Gain of function" mutations are generally dominant because one copy in a diploid organism is sufficient to alter the normal phenotype.

Question 55

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.

Question 56

Hemizygosity is the term used to describe the state of a gene that has no allele on the opposing chromosome.

Question 57

Alleles that are masked by an epistatic locus are said to be hypostatic to the genes at that locus.

Question 58

Genomic imprinting occurs when one allele converts another.

Question 59

Sex-limited inheritance is the same as sex-linked inheritance.

Question 60

Genomic anticipation refers to observations that a genetic disorder occurs at an earlier age in successive generations,whereas genetic imprinting occurs when gene expression varies depending on parental origin.