SEX-LINKED CHARACTERISITCS

In animals and in some plants, one pair of chromosomes is different in the two sexes. For example, in humans there are 22 similar pairs of chromosomes (autosomes) and the 23rd pair may be different (sex chromosomes). The sex chromosomes are similar in females and both are referred to as the "X" chromosomes. In the male, the sex chromosomes are unlike in shape; one resembles the "X" chromosome of the female and is called the "X" chromosome. The other male chromosome is much shorter and is called the "Y" chromosome. Many alleles are located in the "X" chromosome but very few are known to be on the "Y" chromosome. Genes located on the sex chromosomes are said to be sex-linked.

A common sex-linked human characteristic is red-green color deficiency. Red-green color deficiency is dependent upon a recessive allele we'll represent with "d;" normal vision is controlled by a dominant allele and will be represented with "D." Examine the table below to determine the sex and phenotype of each genotype. Use this key to answer questions 1-4.

GENOTYPE / SEX / PHENOTYPE
XDXD
XDXd
XdXd
XDY
XdY

Please do ALL of your work on binder paper. You MUST show your work for credit.

For all of the following crosses, list the possible genotypes and phenotypes of the offspring.

1. If the father is XDY and the mother is XDXD, what are the possible genotypes and phenotypes of the children?

2. If the father is XDY and the mother is XDXd, what are the possible genotypes and phenotypes of the children?

3. If the father is XDY and the mother is XdXd, what are the possible genotypes and phenotypes of the children?

4. If the father does not have the disease and the mother is a carrier

a)List the genotype of father, and the genotype of mother

b)What are the possible genotypes and phenotypes of the offspring?

5. Hemophilia A is caused by a sex-linked recessive allele in dogs (Xh)

a)Cross a hemophiliac male dog with a homozygous non-hemophiliac female dog. Give genotypes and phenotypes of the possible offspring.

b)If a female produced in the cross above (a) is mated with a normal male dog, what are the genotypes and phenotypes of the possible offspring?

c)Why are males more likely than females to express a sex-linked trait?

6. In humans, hemophilia is also a sex-linked trait. The blood in hemophiliacs does not clot properly. A man has hemophilia.

a)What is the chance that he will pass the hemophilia allele on to his children?

b)From what parent did he get the hemophilia allele?

7. A man with hemophilia marries a woman who carries the hemophilia alleles but has normal blood clotting.

a)Do the cross.

b) What percentage of their sons would you expect to have hemophilia?

c)What percentage of their daughters would you expect to have the disease?

8. An X-linked recessive gene (c) produces red-green colorblindness in humans.

a)A normal woman who’s father was color blind marries a color blind man. What percentage of their children will be colorblind? (Do the cross to find out)

b)A normal man who’s father was colorblind marries a normal woman who is a carrier for the color-blind allele. What percentage of their children will be colorblind? (Do the cross to find out)

Challenge Question (Yes, you do have to TRY this one anyway!)

9. In cats, one gene for coat color is X-linked. Male cats are either black or orange, depending on which allele they carry. Females are black, orange, or tortoise-shell (patches of black and orange), with the tortoise-shell phenotype resulting from the heterozygous genotype. Two cats mate and have kittens. Of the female kittens, ½ are tortoise-shell and ½ are orange. Of the male kittens, ½ are orange and ½ are black. What are the genotypes and phenotypes of the parent cats?