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# Frequency of offspring for X linked dominant trait

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5. State the distribution (in terms of expected percentages) of eye color among the male and female offspring derived from each of the following crosses considering the following:

In the fruit fly, red eye color "R" is dominant over white eye color "r"; and the gene for eye color is on the X chromosome.

a. White-eyed female and red-eyed male.

b. Heterozygous red-eyed female and red-eyed male.

c. Homozygous red-eyed female and white-eyed male.

d. Heterozygous red-eyed female and white-eyed male.

e. White-eyed female and white-eyed male

#### Solution Preview

Since males only have one X chromosome the will always only have one eye color allele determining what color eyes they will have.

Females on the other hand have two X chromosomes so their eye color is determined by both alleles. In this case we are told red eye color 'R' is dominant and white eye color 'r' is recessive. So in females the genotypes RR, Rr, rR will be red eyes. Only the rr genotype will have white eyes.
In males R will obviously be red and r white.

In each of these crosses we will always have 50% males and 50% females. Half of the males (25% of total ...

#### Solution Summary

Calculates expected offspring's eye color frequency in crosses of fruit flies where eye color is on the X chromosome and red eyes R is dominant of white eyes r.

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## Genetics and Punnet Squares Problem Set

Single Alleles:
Flies can have red or white eyes. Red is dominant to white.
1) Write down letter designations for the alleles. Be sure to distinguish which trait is dominant and which recessive. For example, Mendel used P to designate the purple allele, and p to designate the white allele for pea-flower color.
2) What would the genotype of a female fly with red eyes be whose mother had white eyes?
3) What would the genotype of a male fly with white eyes whose mother had red eyes?
4) If you crossed the male fly from question three with the female fly from question two, what kind of gametes would be produced by the male fly?
5) If you crossed the male fly from question three with the female fly from question two, what kind of gametes would be produced by the female fly?
6) Cross the male from question 3 with the female from question two. Draw out your Punnett square here, and designate the genotypic frequency of the offspring, as well as the phenotypic frequency of the offspring.

Multiple Alleles:
Rabbits can have solid or spotted coats, and the base color can be either brown or black. Spotted coat color is dominant to solid, and black is dominant to brown.
1. Write down letter designations for the alleles. Be sure to distinguish between the two traits, and be sure to consider which alleles are dominant and recessive. For example, Mendel used P to designate the purple allele, and p to designate the white allele for pea-flower color.
2. What would be the genotype of a female spotted brown rabbit whose mother was solid black?
3. What would be the genotype of a male solid black rabbit whose parents were homozygous for the black allele?
4. If you cross these two rabbits, what kinds of gametes would be produced by the spotted brown rabbit?
5. What kinds of gametes would be produced by the solid black rabbit?
6. Draw out your Punnett square here for the cross between the male rabbit from question three and the female rabbit from question two. Designate the genotypic frequency of the offspring, as well as the phenotypic frequency of the offspring.
7. What are the phenotypes of the offspring? (% of each?)

Extensions of Mendelism
1. Sex determination in birds is different from that in humans. The sex chromosomes are called Z and W. Males have two of the same chromosome (ZZ) while females have two different chromosomes (ZW). There is a Z-linked allele in some birds that causes the death of the embryo when the normal dominant allele is not present. What would be the sex ratio in the living offspring of a cross between a male heterozygous for the lethal allele and a normal female?
a. Provide an allele designation for the lethal Z-linked trait:
b. What are the genotypes of the parents? Male ________ Female ________
c. What gametes would each form? Male __________ Female __________
d. Draw a Punnett square below and determine the sex ratios of living offspring.

2. Four babies were accidentally mixed up in the maternity ward. The blood types of the babies were known to be A, B, O, and AB. The blood types of the four sets of parents were determined. Figure out which baby belongs to each set of parents.

Parental Blood Types Possible Genotypes Which baby is theirs?
(phenotypes) for their baby
AB x O
A x O
A x AB
O x O

Practice Problems:
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.
(* 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.)
What is the man's genotype?
What are the genotypes of the children?
What are the phenotypes of the children? (and % showing each)
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?
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 test cross problem)
If that's his genotype, what kind(s) of gametes (sperm) can he produce?

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.
A deaf female is genotype dd. What kind(s) of gametes (eggs) can she produce?
If the hearing male is DD, what kind(s) of gametes (sperm) can he produce?
If he is Dd, what kind(s) of gametes can he produce?
If two hearing dogs were both Dd, what kind(s) of gametes (eggs/sperm) could each produce?

3) 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, draw the Punnett square that illustrates this. If the man dies and the woman remarries to a colorblind man, draw a Punnett square showing the type(s) of children could be expected from her second marriage. How many/what percentage of each could be expected?
A man with normal vision is XY. What kind(s) of gametes (sperm) can he produce?
Any woman with normal vision could be XX or XX'. Since this woman has a colorblind son (genotype X'Y), she has to be XX' (a carrier). What kind(s) of gametes (eggs) can she produce?
In order to be colorblind, her second husband must be X'Y (like her son). What kind(s) of gametes (sperm) can he produce?

Define the following terms:

1) Allele
2) Dominant gene
3) Recessive gene