ANSWERS TO REVIEW QUESTIONS

1. Sex is expressed at the chromosomal level as inheriting XX or XY; at the gonadal level by developing ovaries or testes; at the phenotypic level by developing male or female internal and external structures; and at the gender identity level by feelings.

2. Genes in the pseudoautosomal region are the same as certain X-linked genes, but X-Y genes share only sequence similarities.

3. a. female b. female c. female

4. Absence of the SRY gene product causes the Mullerian ducts to develop into ovaries. The ovaries produce female hormones, which influence the development of external and internal reproductive structures.

5. Feelings of very young children; twin studies in which identical twins are more likely to both be homosexual than are fraternal twins; fruit fly behavior; genes on the X chromosome that segregate with homosexuality

6. A female homozygous dominant for an X-linked dominant allele would probably be so severely affected that she would not be alive, or healthy enough, to reproduce.

7. Sperm carrying the Y chromosome may weigh less and reach the oocyte faster than X-bearing sperm.

8. An X-linked trait appears usually in males and may affect structures or functions not distinct to one sex. A sex-limited trait affects a structure or function distinct to one sex. A sex-influenced trait is inherited as a recessive in one sex and dominant in the other.

9. Coat color in cats is X-linked. In females, one X chromosome in each cell is inactivated, and the pattern of a calico cat's coat depends on which cells express which coat color allele. A male cat, with only one coat color allele, would have to inherit an extra X chromosome to be tortoiseshell or calico.

10. Inactivation of the gene in some cells but not others results in a patchy phenotype.

11. Each cell in a female's body contains only one active X chromosome, which makes females genetically equivalent, in terms of X-linked genes, to males.

12. Mouse zygotes with two female pronuclei or two male pronuclei are abnormal. In humans, two male and one female genome in the same embryo yields placental tissue, while two female and one male genome yields a normal embryo with an abnormal placenta.

13. Genomic imprinting

ANSWERS TO APPLIED QUESTIONS

1. a. The causes of death by age and gender b. 97 c. Bangladesh

2. a. 1/2 b. 1/2 c. 1/2 3. The unevenness of the teeth of affected females may reflect expression of the defect in only some cells as the result of X inactivation.

4. Answers vary depending on disorder selected. An example would be Rett syndrome, MIM 312750, which causes autism, dementia, loss of hand use, jerky movements, seizures and poor growth.

5. Answers vary depending on disorders selected. Examples: 1) Cornelia de Lange syndrome, which causes growth retardation, characteristic facial features, and mental retardation. This comes from the mother. 2) Wolf-Hirschhorn syndrome, with severe growth retardation, microcephaly, “Greek helmet face,” heart defects, and cleft lip or palate. It comes from the father.

6. a. G b. B, F c. A d. D e. E f. B, F g. D h. H i. D

7. 1/4

8. Pedigree.

I Edgar Florence

II Phi Harold Shirley l Maude

III Alvi Simo Marsha n n