Genetics Practice Problems

Monhybrid:

1. In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?

Man’s genotype is Bb; Children ‘s genotypes are Brown eyed Bb and blue eyed bb

(* Actually, the situation is complicated by the fact that there is more than one gene involved in eye color, but for this example, we’ll consider only this one gene.)

a. If blue is recessive, what must the woman’s genotype be? bb

b. If that’s her genotype, what kind(s) of gametes (eggs) can she produce? Only b type

c. If the man has brown eyes, but has a blue-eyed child what must his genotype be?(if you don’t understand why, review the testcross problem) Bb

d. If that’s his genotype, what kind(s) of gametes (sperm) can he produce? B and b

2. In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.

Dd x dd or DD x dd. In the first case the phenotypic ratio would be 1:1 and in the second case, 100% hearing dogs.

a. A deaf female is genotype dd. What kind(s) of gametes (eggs) can she produce? Only d type

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b. If the hearing male is DD, what kind(s) of gametes (sperm) can he produce? Only D type

c. If he is Dd, what kind(s) of gametes can he produce? D or d

If any of the puppies are deaf, the male is Dd;but, assuming a litter of at least about 4 to 5 puppies or more, if they can all hear, he is most likely DD.

d. If two hearing dogs were both Dd, what kind(s) of gametes (eggs/sperm) could each produce? D and d 3. A man and woman living in a tropical area where malaria is prevalent and health care is not accessible have seven children. The genotypes of these children are ss, Ss, SS, ss, Ss, Ss, and SS. (S normal hemoglobin; s sickle cell)

a. What must the genotype of both parents be? Ss x Ss(Hint: what would be needed to have those kinds of children?)

b. If that’s their genotype, what kind(s) of gametes (eggs/sperm) can each produce?S and s types

c. Which of their children would you expect to live to adulthood and reproduce?

ss Ss SS ss Ss Ss SS

d. Which of the children are at a greater risk for Malaria?

ss Ss SS ss Ss Ss SS

Dihybrid

4. In humans, there is a gene that controls formation (or lack thereof) of muscles in the tongue that allow people with those muscles to roll their tongues, while people who lack those muscles cannot roll their tongues. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste certain substances is also genetically controlled. For example, there is a substance called phenylthiocarbamate (PTC for short), which some

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people can taste (the dominant trait), while others cannot (the recessive trait). The biological supply companies actually sell a special kind of tissue paper impregnated with PTC so students studying genetics can try tasting it to see if they are tasters or non-tasters. To people who are tasters, the paper tastes very bitter, but to non-tasters, it just tastes like paper. Let’s let R represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t represent non-tasting.

a. Suppose a woman who is both a homozygous tongue-roller and a non-PTC-taster marries a man who is a heterozygous tongue-roller and is a PTC taster, and they have three children: a homozygous tongue-roller who is also a PTC taster, a heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller who is a non-taster. If these parents would have a bunch more children so that they had 12 in all, how many of those 12 would you expect to be non-tasters who are homozygous for tongue-rolling?

RRtt (female) x RrT? (male)

Children: RRT? ; RrT? ; Rrtt

These results tell us that Dad is RrTt since he has a child who is Rr tt.

The mating is established as: RRtt x RrTt

RtRT RRTtRt RRtt ¼ x 12 = 3 Non-tasters who are homozygous for tongue-rollingrT RrTtrt Rrtt

b. If the first child (the homozygous tongue-roller who is also a PTC taster) marries someone who is heterozygous for both traits, draw the Punnett square that predicts what their children will be.

RRTt x RrTt

RT RtRT RRTT RRTt Rollers Tasters……… 6/8 or 75%Rt RRTt RRtt Rollers non-tasters 2/8 or 25%rT RrTT RrTt Non-rollers Tasters………..zerort RrTt Rrtt Non-rollers non-tasters…….zero

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c. If the man is both Rr and Tt (How do we know that?), he would be RrTt and so could produce gametes with either R or r and either T or t (one allele for each gene). There are two choices for the first trait (R or r). No matter which of those go into a given sperm, there are still two choices for the second trait (T or t). Therefore, a total of 2 × 2 = 4 possible types of gametes (sperm) may be produced. What kind(s) of gametes (sperm) can he produce? RT, Rt, rT, and rt

Multiple Alleles and Codominance

5. Suppose a person with type A blood and a person with type B blood get married. What are the possible genotypes their children could have? Case 1AA x BB all kids will be ABCase 2Ai x BB AB and BiCase 3AA x Bi AB and AiCase 4Ai x Bi AB, Ai, Bi, and ii

Sex-Linked Genes:

6. In humans, the genes for colorblindness and hemophilia are both located on the X chromosome with no corresponding gene on the Y. These are both recessive alleles. If a man and a woman, both with normal vision, marry and have a colorblind son.

a. Draw the Punnett square that illustrates this. Xn Normal Xcb color-blind

Xn Y

Xn XnXn XnY

Xcb XnXcb XcbY (color-blind son)

b. If the man dies and the woman remarries to a colorblind man, draw a Punnett Square showing the type of children could be expected from her second marriage.

Xcb Y

Xn XnXcb XnY

Xcb XcbXcb XcbY

c. How many/what percentage of each could be expected?

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Boy color-blind ½ (When considering only boys)

Boy normal ½

Girl color-blind ½ (When considering only girls)

Girl normal ½

http://biology.clc.uc.edu/courses/bio105/geneprob.htm

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