Extensions to Mendel

Concept: Degrees of Dominance

6m

Concept: Plieotropy, Epistasis, and Polygenic Inheritence

7m

Problem: A species of bird is known to have black beaks, yellow beaks, or spotted beaks, with patches of black and yellow. There is a single gene locus that determines beak pigment, and the most likely mode of inheritance is _____________________?

2m

Problem: A species of fish have tails that vary in size. It is found that genes at 2 loci, as well as diet, influence tail size. This trait is an example of _________________________?

1m

Problem: A ratio of 9:3:4 is produced during a heterozygote dihybrid cross when one gene pair interacts to control the expression of the second gene pair. This interaction is called _________?

1m

Extensions to Mendel Additional Practice Problems

The coat color of Labrador retrievers is determined by two genes that follow the law of independent assortment. If two black Labs that are heterozygous for both genes (BbEe) mate, what type of interaction will result in which modified phenotypic ratio?

A. Pleiotropy will result in a ratio of 9 black : 7 yellow Labs

B. Pleiotropy will result in a ratio of 9 black : 3 brown : 4 yellow Labs

C. Epistasis will result in a ratio of 9 black : 3 brown : 3 yellow : 1 purple Lab

D. Epistasis will result in a ratio of 9 black : 3 brown : 4 yellow Labs

E. Incomplete dominance will result in a ratio of 9 black : 3 brown : 4 yellow Labs

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In people with sickle-cell disease, red blood cells break down, clump, and clog the blood vessels. The blood vessels and the broken cells accumulate in the spleen. Among other things, this leads to physical weakness, heart failure, joint pain, and brain damage. Such a suite of symptoms can be explained by _____. 

A. a bacterial infection interacting with the sickle-cell allele

B. the pleiotropic effects of the sickle-cell allele

C. the polygenic nature of sickle-cell disease

D. an epistatic interaction between the sickle-cell allele and a proteolytic enzyme gene

E. side effects of the drugs used to cure sickle-cell disease

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A woman with type O blood is expecting a child. Her husband is type A. Both the woman's father and her husband's father had type B blood. What is the probability that the child will have type B blood?

A. 0%

B. 100%

C. 25%

D. 75%

E. 50%

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If the probability of being blood type B is 1/4 and the probability of being blood type A is 1/8, what is the probability of being blood type A or B?

A. 1/2

B. 1/4

C. 1/8

D. 3/8

E. 9/16

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In pigs, curly tail (T) is dominant to straight tail (t). Another pair of genes is epistatis to the genes for tail shape, such that if a pig is homozygous recessive (ee) for the epistatic trait, then a pig has no tail at all. If you crossed two heterozygous pigs (TtEe), what fraction of the baby pigs would you expect to have curly tail? Straight tails? No tails. Show all of your work.

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A, B, ad O blood type in humans is controlled by a single gene with three alleles: IA (or A), IB (orB), and i (or O). Type O is the recessive trait. The i (O) allele is recessive to both IA (or A), and IB (orB). Which of the following could be possible genotypes of the parents ofa person with type O blood?

A. IAi (AO) and IAIA (AA)

B. IAi (AO) and IBi (BO)

C. IAIB (AB) and IAi (AO)

D. IAIB (AB) and ii (OO)

E. All of the above are possible

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Fruit color in a particular plant is controlled by a set of three QTLs (quantitative trait loci, or "polygenes") that work in an equal and additive manner. Each QTL has two alleles (i.e. A and a); each allele represented by a capital letter produces "one dose" of yellow pigment, while alleles represented with lower case letters do not produce any pigment at all. A trihybrid plant (A/a;B/b;C/c) is selfed. Assuming no effects of the environment, what proportion of the offspring will have thesame fruit color phenotype as the trihybrid parent?

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A woman has a daughter. There are three men whom she claims might have been the father of the child. The judge in the paternity court orders that all three men, the child, and the mother have blood tests. The results of blod typing are given below:

The mother claims that this proves that Man#3 must be the little girl's father. Is the mother correct? Why or why not?

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It was suspected that two babies had been switched in a hospital. Mr. and Mrs. Brown received baby #1 and Mr. and Mrs. Green received baby #2. Blood typing tests on the parents and the babies gave the information shown below:

Did both families get the right baby? Why?

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You have identified two genes that control fur color in dogs. If dogs have a dominant allele "B", they will be black, otherwise, they will be brown. If dogs have a dominant allele "I", they will be white, otherwise, their phenotype will be determined by gene B. You cross two double heterozygous dogs (BbIi x BbIi) to produce puppies. 12 of the puppies have white fur color, 3 have black fur color, and 1 has brown fur color. This is an example of:

A. Genes in independent pathways

B. Recessive epistasis

C. Dominant epistasis

D. Synthetic lethal

E. Complementation

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The lubber grasshopper is a very large grasshopper, and is black with red, orange, or yellow stripes. Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the homozygous rr genotype, and orange stripes from the heterozygous Rr genotype. 

How is stripe color inherited in lubber grasshoppers? You cross two grasshoppers with orange stripes and produce 16 offspring. How many of each phenotype will you have?

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In shorthorn cattle, coat color may be red (RR), white (rr), or roan (Rr). Roan is a phenotype expressed as a mixture of red and white hairs. What mode of inheritance is coat color in short horn cattle?

If you cross roan cows together to produce 12 calves. how many of each phenotype do you expect?

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What are the parental and recombinant gametes resulting from crossing AaBb x AaBb?

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What is/arethe possible blood type(s) (i.e. phenotypes) of the father ifa woman with type A has a daughter with type B blood?

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A black lab (BbEe) was mated to a chocolate lab (bbEe). What are the expected genotypic and phenotypic frequencies of the offspring? Provide percentages for your answers. 

Remember epistatic effects: B_ = black; bb = chocolate; E_ = will have color; ee = yellow

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In a possible paternity case a child's blood type was found to be type O. If the father in question has type A blood, would this exclude him as the possible father. Why or why not? What blood type would exclude him from paternity in this case?

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In swine, when a pure-breeding red is crossed to a pure-breeding white, the F1 are all red. However, the F2 shows 9/16 red, 1/16 white,and 6/16 are a new color, sandy. Thesandy phenotype is mots likely determined by

A. complementary alleles of two different genes.

B. a heterozygote of the alleles determines red and white.

C. recessive epistasis.

D. the presence of a dominant allele at either of two loci where red is determined by the dominant alleles of both loci.

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Flower colour in a plant species is determined by two genes, gene B and gene P. The gene products of these genes are enzymes that convert a white substrate to an intermediate blue substrate and then to the final purple product.

This series of conversions are shown in a simplified pathway below.

A. What is the phenotype of the following genotypes (white, blue, or purple)

     1. BbPp

     2. BBpp

     3. bbPP

     4. bbpp

B. If you cross two dihybrids together, what proportion of the progeny will be:

     1. white              2. blue              3. Purple

C. If you test cross a dihybrid, what proportion of the progeny will be:

     1. white              2. blue              3. Purple


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Mother and father with blood types A and B, respectively, have a child with blood type O (blood group inheritance is codominant). What fraction of their children is expected to have blood type A?

A. 0

B. 1/2

C. 1/4

D. 1

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Suppose two independently assorting genes are involved in the pathway that determies fruit color in squash. These genes interact with each other to produce the squash color seen in the grocery store.

At the first locus, the W allele codes for a dominant white phenotype, whereas the w allele codes for a colored squash. At the second locus, the allele Y codes for a dominant yellow phenotype, and the allele y codes for a recessive green phenotype. The phenotypes from the first locus will always mask the phenotype produced by the second locus if the dominant allele (W) is present at the first locus. This masking pattern isk nown as the dominant epistasis.

A dihybrid squash, WwYy, is selfed, and produces 128 offspring. How many offspring are expected to have each of phenotypes: white, yellow, orange?

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Sally (blood type A) has a daughter Peggy who has blood type B. Sally's genotype is:

A. A/A

B. A/O

C. B/B

D. B/O

E. O/O

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You cross two true breeding lines of petunia. One produces red flowers and the other white flowers. The F1 offspring all show pink flowers. You conclude from this that:

A. Red is dominant.

B. White is dominant.

C. Red is incompletely dominant.

D. Red is recessive.

E. None of the answers listed above are correct.

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What genotypes would be produced by crossing a grasshopper with red and yellow stripes (Rr) with a grasshopper with yellow stripes (rr)? Show all your work using a Punnet square and give the phenotypic ratio and genotypic ratio. (Be sure to write the genotypes and phenotypes along with the ratio)

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Your neighbor, a "crazy cat lady", has two white cats with the genotype WwBb. You learned in genetics that the W allele shows dominant epistasis over the B gene, and the B allele (black) is dominant over the b allele (brown). If your neighbor crosses her two white cats, what phenotypic ratio would you expect in the litter? 

Make sure to include the phenotypes in the ratio.

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In cattle, the allele producing red coat color (R) is incompletely dominant over the allele (r) for white coat color. Heterozygotes (Rr) are roan colored. Cattle homozygous for a different gene, "short spine" (aa), are deformed and die shortly after birth. Heterozygous carriers (Aa) are completely normal. If a series of matings is conducted between Aa roan animals, what will be the phenotypic ratio:

a) at birth?

b) several days after birth?

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If an intercross of dihybrids produces a 12:3:1 offspring phenotypic ratio, then the cross Aabb x AaBb should produce a ________ phenotypic ratio.

A. 9:3:3:1

B. 1:1:1

C. 2:1:1

D. 1:3

E. 6:1:1

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Eggplant fruit color may be dark purple, violet, or white. Crosses between purple and white eggplants always produce violet eggplant progeny. However, the crossing of these violet eggplants gives the following progeny: 110 violet, 54 purple, and 56 white eggplants. What is the genetic basis of fruit colors in eggplants?

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Coat color in mice is controlled by many genes, four of which we will consider. The gene C permits pigment deposition in the fur, whereas cc causes albinism. The gene B causes black pigmentation, whereas bb results in brown fur. The gene A causes deposition of a narrow band of yellow pigment near the tips of individual hairs, a condition called "black agouti" in black animals and "cinnamon" in brown animals; the genotype aa causes no yellow banding of hairs. The genotype dd causes a dilution of fur color (unless the fur color is already albino); the gene D has no effect on coloration at all. All of these genes are located on different autosomes. You cross a female mouse with genotype CcBbaaDd to a male mouse with genotype CcbbAaDd. The cross produces 10,000 offspring.

What color coat does the female parent have?
What color coat does the male parent have?
Complete the following table by indicating the number of mice with each different phenotype.

Remember there is no such thing as partial mouse so round number properly to the nearest whole number. 

List the different genotypes of dilute cinnamon offspring that could be produced in the above cross.

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Which the following Is an example of epistasis?

a. A chicken with a dominant allele at the R gene has a rose comb, and a chicken with a dominant alleles at the P gene has a pea comb. However, a chicken with dominant alleles at both R and P genes has a walnut comb

b. A person with two copies of the lA allele at the ABO gene has blood type A. and a person with two copies of the lB allele at the ABO gene has blood type B However, a person with the genotype lAIB has the blood type AB

c. A cat with two dominant alleles at the H gene has short hair, and a cat with two recessive alleles at the H gene has long hair However, a cat with a dominant allele and a recessive allele at the H gene has short hair

d. A fly with a dominant allele at the S gene has short body hairs and a fly with a dominant Q allele at the W gene has white eyes However a fly with dominant alleles at both the S gene and the W gene has short body hairs and white eyes

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The ABO blood group assorts independently of the Rhesus (Rh) blood group and the MN blood group. Two alleles, R, a dominant allele producing Rh+, and r, a recessive allele for Rh-, are found at the Rh locus, and codominant alleles M and N occur at the MN  locus. Each gene is autosomal.

Part A. A child with blood types O, Rh-, and M is born to a woman who has blood types A, Rh-, and MN and a man who has blood types B, Rh+, and M. Determine the genotypes of each parent. What is the mother's blood type?

Enter the alleles the following way: IA as I^A, IB as I^B. i as i. 

Use _ to indicate any allele is possible. 

List dominant alleles first then recessive alleles.

If listing both dominant alleles for ABO blood group, list IA first and then IB. Separate the three genes by a space. For example: I^Ai R_MM

The mother is __________.

Part B. What is the father's blood type? 

Enter the alleles the following way: IA as I^A, IB as I^B. i as i. 

Use _ to indicate any allele is possible. 

List dominant alleles first then recessive alleles.

If listing both dominant alleles for ABO blood group, list IA first and then IB. Separate the three genes by a space. For example: I^Ai R_MM

The father is __________.

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Why can an autosomal recessive disorder skip one or more generations in a family, but an autosomal dominant disorder cannot?


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Define maternal effect. 

In what cell type would you expect to find the protein product of a material effect gene?

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A man with type AB blood and a type B woman have a son with type B blood.

If the couple’s next child has type A blood, what is the chance that their third child is a type B daughter?

a. 1/4

b. 1/8

c. 1/2

d. 3/8

e. It cannot be determined for sure.

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A man with type AB blood and a type B woman have a son with type B blood.

What is the mother’s blood genotype?

a. I A I B

b. I B I B

c. I B i

d. ii

e. It cannot be determined for sure.

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A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively.

Which of the following is the probable genotype for the mother with respect to BOTH the A-B-O blood group AND the Rh trait?

a. IAiRR

b. IAIARr

c. IAirr

d. IAiRr

e. IAIARR

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A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively.

Which of the following is a possible phenotype for the father?

a. O negative

b. A negative

c. B positive

d. AB negative


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A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively.

Which of the following is a possible genotype for the mother? 

a. IAIB 

b. IAIA

c. ii

d. IAi

e. IBIB

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A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively.

Which of the following is a possible genotype for the son with respect to the A-B-O blood group? (IA= the "A" allele, as if the "I" had a superscript "A"; same for the "IB" allele)

a. ii

b. IBi

c. IAIA

d. IBIB

e. IBIA

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Radish flowers may be red, purple, or white. A cross between a red-flowered plant and a white-flowered plant yields all-purple offspring. The part of the radish we eat may be oval or long, with long being the dominant characteristic.

If true-breeding red long radishes are crossed with true breeding white oval radishes, the F1 will be expected to be which of the following?

a. White and long

b. Purple and oval

c. Red and oval

d. Purple and long

e. Red and long

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In domesticated cats, the following genetic patterns have been described as independently assorting. Normal ears (T) is dominant to tufted ears (t); curved whiskers (C) is dominant to straight whiskers (c); the presence of six toes (S) is dominant to five toes (s); and the gene for hair length is an X-linked codominant. The three phenotypes for hair length are long (XHXH), medium (XHXh), and short (XhXh); medium is the heterozygous condition. Sex determination in cats is the same as for humans (XX = female).

Given two parent cats:

Tt cc SS XHXh x Tt Cc Ss XHY

i) How many different gametes could be formed in the female cat with respect to these four traits?

ii) How many phenotypes are possible in the offspring from this mating?

iii) What fraction of the offspring will have tufted ears, curved whiskers, six toes, and long hair?

iv) In a litter of 10 kittens, six are male. How many of the males would be expected to have tufted ears, six toes, straight whiskers, and medium-length hair?

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Your research in hematology has led you to the discovery of a new type of inherited anemia, which exists in two forms- - mild and severe . The information you have gathered on the familial patterns for the disease is summarized below. From these data determine a mechanism for the inheritance of these traits that is consistent with all of the information.

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A dominant gene, A, causes yellow color in rats. The dominant allele of another independent gene, R, produces black coat color. When the two dominant genes occur together (A-R-), they interact to produce grey coat color. Rats of the double recessive genotype are cream-colored. If a grey male and a yellow female are mated and produce approximately 3/8 yellow, 3/8 grey, 1/8 cream, and 1/8 black, what were the genotypes of the parents?

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In rabbits, fur color is determined by a set of multiple alleles at one locus (gene) that have the following relationship:

C+ (agouti) is dominant to all other alleles

ch (himalayan) is dominant to c(albino)

cu (chinchilla) shows incomplete dominance with regard to c h and ca such that the genotypes cuch and cuca result in grey phenotypes

A) What breeding stock (parents) would you select if you wished all of the offspring to be chinchilla?

B) In one of the matings of rabbits, the litter contained 4 grey bunnies, 2 albino bunnies, and 2 himalayan bunnies. What were the genotypes of the parents?

C) In another mating, the litter contained 3 agouti bunnies and 3 grey bunnies. What were the genotypes of the parents of this litter?

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In the organism under consideration, r* acts like r allele, except when homozygous (r*r*). From the information given below, work out the phenotypic and genotypic ratios for each of the crosses.

Given: RR (red) x rr (white) → Rr (pink); and

rr* x rr* → 3/4 white, 1/4 dead zygotes

1. Rr x Rr

2. Rr x rr

3. Rr* x Rr*

4. Rr* x rr*

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A woman with type A blood has parents who are both type AB and a husband who is a type B. What is the probability that their first child will be a son with type O blood?

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In humans, the ABO blood groups are controlled by three alleles (only two of which occur in any one individual): the alleles for A and B type blood are co-dominant toward each other, and both are dominant to the allele for O type blood.

i) If a person with type AB blood marries someone with type O blood, what are the possible phenotypes of their offspring?

In the following, determine the genotypes of the parents:

ii) One parent has type A and the other has type B, but all four blood groups are represented in the children.

iii) Both parents have type A, but 3/4 of the children are A and 1/4 are O.

iv) One parent has type AB and the other has type B, but of the children 1/4 have type A, 1/4 have type AB, and 1/2
have type B.

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In cattle, the gene for hornless (H) is dominant to the gene for horned (h), the gene for black (B) is dominant to that of red (b), and the gene for white face (or Hereford spotting) (S) is dominant to that for solid color (s). A cow with the genotype BbHhSs is inseminated by a bull of the genotype bbhhSs. What is the probability of the calf's being:

i) a black, hornless cow with Hereford spotting

ii a red, horned bull with solid color

iii) a red, hornless bull with Hereford spotting

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Cat fur color is affected by two genes. The B gene codes for fur color. The dominant allele (B) produces black fur (phenotype) and the recessive allele (b’) produces a cinnamon phenotype (when homozygous). The second gene (D) codes for the density of brown or black pigment. The dominant allele (D) allows for full deposition of the black pigments and the recessive allele (d) allows for lesser deposition of pigment (when homozygous), as coded for by the B or b gene. Black cats appear blue with less dense deposition and cinnamon cats appear fawn. If a black cat (Bb’Dd) is crossed with a blue cat (Bb’dd).

For the description above, the D gene is said to be ________ relative to the B gene.

a) dominant

b) recessive

c) codominant

d) epistatic

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Cat fur color is affected by two genes. The B gene codes for fur color. The dominant allele (B) produces black fur (phenotype) and the recessive allele (b’) produces a cinnamon phenotype (when homozygous). The second gene (D) codes for the density of brown or black pigment. The dominant allele (D) allows for full deposition of the black pigments and the recessive allele (d) allows for lesser deposition of pigment (when homozygous), as coded for by the B or b gene. Black cats appear blue with less dense deposition and cinnamon cats appear fawn. If a black cat (Bb’Dd) is crossed with a blue cat (Bb’dd).

What ratio of cats are expected to be cinnamon? 

a. 1 of 8

b. 2 of 8

c. 3 of 8

d. 4 of 8

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A woman with type AB blood marries a man with type A blood.  Their first child has blood type B.  What is the probability that their next child will have type o blood?

a) ¼

b) ½

c) ¾

d) 1

e) 0

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The environment only rarely affects gene expression. 

a. True

b. False

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In the ABO blood type system, there are six possible genotypes, but only two possible phenotypes. 

a. True

b. False

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A man with type A blood and a woman with type B blood have a child with type O blood. What are the genotypes of the man and woman? 

a. IA i , and IB i

b. Both are i i

c. IA IA, and IB IB

d. IA i, and i i

e. i i, and IB i

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In an inherited form of high cholesterol, rr individuals have a blood cholesterol level greater than 500mg/dL, Rr individuals are near 300mg/dL, while normal (RR) cholesterol levels should be less than 200mg/dL. The R allele is ___ to the r allele. 

a. Codominant

b. Dominant

c. Recessive

d. Homozygous

e. Incompletely dominant

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If you cross two pea plants, one with green peas and the other with yellow peas. All of the offspring have yellow peas. You conclude: 

a. Green is dominant over yellow

b. Yellow and green are co-dominant

c. Yellow is incompletely dominant over green

d. Yellow is dominant over green

e. Color is random in pea plants

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Traits that depend on more than one gene are: 

a. Polygenic

b. Pleiotropic

c. Codominant

d. Recessive

e. Incomplete dominant traits

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If the heterozygous phenotype is intermediate between those of the two different homozygotes, this is called: 

a. Codominance

b. Independent assortment

c. Epistasis

d. Incomplete dominance

e. Polygenic

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In the ABO blood type system the IA allele is _____ with the IB allele. 

a. Dominant

b. Co-dominant

c. Recessive

d. Homologous

e. Heterologous

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In the ABO blood type system the alleles  IA, IB and i produce: 

a. Three phenotypes

b. Two phenotypes

c. One phenotype

d. Four phenotypes

e. Type A and type B blood types only

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A man and a woman, both with group AB blood, marry. What different genotypes and phenotypes can be expected among their children? What percentage of their children could have A type blood. What blood type or types could not appear in their children?

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A man with B type blood marries a woman with B type blood. Four of their children have B type blood and one has O type blood. What would be the genotype of the parents? What is the chance that their next child could have AB type blood?

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