ANSWERS TO REVIEW QUESTIONS
CHAPTER 6
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; existence in many species; twin studies in which identical
twins are more likely to both be homosexual than are fraternal twins.
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 Xbearing 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 sexinfluenced 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, due to random X inactivation, 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 could reveal why men die younger in these
countries. It could be due to deaths during military service. b. 97 c. Bangladesh
2. a. 1/2 b. 1/2 c. 1/2
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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. This is a less severe phenotype than that seen in
males.
ANSWERS TO WEB ACTIVITIES
1. Answers vary depending on disorder selected. An example is Rett syndrome, MIM 312750,
which causes autism, dementia, loss of hand use, jerky movements, seizures and poor growth.
2. Answers vary depending on disorders selected. Examples: 1) Cornelia de Lange
syndrome, which causes growth retardation, characteristic facial features, and intellectual
disability. 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.
ANSWERS TO CASE STUDIES AND RESEARCH RESULTS
1. a. G b. B, F c. A d. D e. E f. B, F g. D h. H i. D
2. 1/2
3.
Pedigree.
I
Edgar
Florence
II
Phil
Harold
Maude
Shirley
III
Marsha
Alvin
Simon
4, SRY for maleness
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