Name ______Per. ______Date ______

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 / Female / Normal
XDXd / Female / Carrier for Colorblindness
XdXd / Female / Colorblind
XDY / Male / Normal
XdY / Male / Colorblind

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

Genotypes: ______

Phenotypes: ______

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

Genotypes: ______

Phenotypes: ______

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

Genotypes: ______

Phenotypes: ______

4. If the father does not have the disease and the mother is a carrier, what are the possible genotypes and phenotypes of the children?

Genotype of father: ______Genotype of mother: ______

Genotypes: ______

Phenotypes: ______

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

GENOTYPE / SEX / PHENOTYPE
XHXH / Female / Normal
XHXh / Female / Carrier for Hemophilia
XhXh / Female / Colorblind
XHY / Male / Normal
XhY / Male / Hemophiliac
  1. Cross a hemophiliac male dog with a homozygous non-hemophiliac female dog. Give genotypes and phenotypes of the possible offspring.

Genotypes: ______

Phenotypes: ______

  1. 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?

Genotypes: ______

Phenotypes: ______

  1. 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.

  1. What is the chance that he will pass the hemophilia allele on to his children? ______
  1. 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. Do the cross.

  1. What percentage of their sons would you expect to have hemophilia? ______
  1. What percentage of their daughters would you expect to have the disease? ______

8. 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?

GENOTYPE / SEX / PHENOTYPE
XBXB / Female / Black
XOXO / Female / Orange
XBXO / Female / Tortoise
XBY / Male / Black
XOY / Male / Orange

Genotypes of the parents: ______

Phenotypes of the parents: _____

9. An X-linked recessive gene (D) produces red-green colorblindness in humans. (use the key provided on page one of this handout)

  1. 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)
  1. 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)