The X and Y chromosomes are not exactly identical, and we should expect that there would be differences in how each of them carries genes. These differences are expressed in the unequal frequencies of phenotypes in the male and female offspring. If any phenotype is distributed unequally between male and female offspring, and those differences are due to X and Y chromosome differences, then we call those traits sex-linked. Actually, "sex-linked" means that the gene is carried on the X chromosome and not on the Y chromosome. We call these genes X-linked. It is easier to understand sex- linkage by looking at the sex chromosomes. Two genetic situations are illustrated below. The 23rd Pair of Chromosomes X-linked alleles X-linked alleles "Sex-linked" means 66 carried on the X chromOsome. Male-Making Genes Homologous Sections X Male Y X Female X First: There is a homologous section of the X and Y chromosomes that is the same, and there will be no differences in phenotype between male and female children. Second: Notice that the Y chromosome is very short. We would expect it to lack some of the genes that are carried on the X chromosome. There is an X-linked section on the X chromosome that carries genes that are missing from the Y chromosome. ?

QUESTION

1. How many copies of an X-linked gene does a male have? One

2. Will a male be able to give his X-linked genes to his daughter? Explain.

3. Will a male be able to give his X-linked genes to his son? Why or Why not?

4. How many copies of an X-linked gene does a female have?

5. A male child gets X-linked genes from which of his parents?

6. A female child gets X-linked genes from which of her parents?

7. If a father is carrying an X-linked allele, then how many of his sons will get that allele? How many of his daughters will get that allele?