Genetics Chapter 6 Review Questions

State two reasons why males with X-linked dominant conditions are very rare.

-The male contain only one X-chromosome -So no gene of second X-chromosome is found that can mask it.

Why is it unlikely one would see a woman who is homozygous for an X-linked dominant condition?

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.

List the events required for a fetus to develop as a female?

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

Traits that appear more frequently in one sex than the other may be caused by genes that are inherited in an X-linked, sex limited, or sex influenced fashion. How might you distinguish among these possibilities in a given individual?

An X-linked trait appears usually in males and may affect structures or functions no 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.

How is sex expressed at the chromosomal, gonadal, phenotypic, and gender identity level?

Chromosomal- inheriting XX or XY Gonadal- developing ovaries or testes Phenotypic- developing male or female internal and external structures Gender ID- feelings

Why are male calico cats very rare?

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.

How does X inactivation even out the "doses" of X-linked genes between the sexes?

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.

Cite evidence that may point to a hereditary component of homosexuality.

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.

Describe the phenotypes of: a person with a deletion of the SRY gene, a normal XX individual, and an XY individual with a block of testosterone synthesis?

Female, Female, Female

How do genes in the pseudoautosomal region of the Y chromosome differ from genes in the male-specific region?

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

Prader-Willi and Angelman syndromes are more common in children conceived with certain assisted reproductive technologies (in vitro fertilization and intracytoplasmic sperm injection) than among the general population. WHat process may these procedures disrupt?

Genetic Imprinting

How might X inactivation cause patchy hairiness in women who have congenital generalized hypertrichosis, even though the disease-causing allele is dominant?

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

Cite evidence that genetic contributions from both parents are necessary for normal prenatal development.

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.

What is the basis of sex ratio at birth?

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