Genetics Practice Problems

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Problem #1:

Dog In dogs, there is an 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.


A deaf female is genotype dd. What kind(s) of gametes (eggs) can she produce?
and/or

If the hearing male is DD, what kind(s) of gametes (sperm) can he produce?
and/or

If he is Dd, what kind(s) of gametes can he produce?
and/or

If he is DD:



Put the female gametes in the correct locations

Put the male gametes in the correct locations

Put the genotypes of the offspring in
the correct locations

All (100%) of offspring can hear

If he is Dd:



Put the female gametes in the correct locations

Put the male gametes in the correct locations

Put the genotypes of the offspring in
the correct locations

½ of offspring can hear
½ of offspring are deaf

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.


If two hearing dogs were both Dd, what kind(s) of gametes (eggs/sperm) could each produce?
and/or

If they are bred with each other:



Put the female gametes in the correct locations

Put the male gametes in the correct locations

Put the genotypes of the offspring in the correct locations

We would expect ¾ of the puppies to be normal and ¼ deaf.


Problem #2:

Family 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?


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

If that’s her genotype, what kind(s) of gametes (eggs) can she produce?
and/or

If the man has brown eyes, but has a blue-eyed child what must his genotype be?
(if you don’t understand why, review #1)

If that’s his genotype, what kind(s) of gametes (sperm) can he produce?
and/or

If they have children:



Put the female gametes in the correct locations

Put the male gametes in the correct locations

Put the genotypes of the offspring in the correct locations

½ of their children would be expected to be heterozygous brown
and ½ homozygous blue.