Exam 5: Sex Determination and Sex Chromosomes

The Lygaeus mode of sex determination is the _.

A cross is made between a female calico cat and a male cat having the gene for black fur on his X chromosome. What fraction of the offspring would one expect to be calico?

Give the sex -chromosome constitution (X and Y chromosomes) and possible genotypes of offspring resulting from a cross between a white -eyed female (Xw XwY) and a wild -type male (normal chromosome complement) in Drosophila melanogaster. Include all zygotic combinations whether viable or unviable.

Dosage compensation in mammals typically involves the random inactivation of one of the two X chromosomes relatively early in development. Such X chromosome inactivation often leads to phenotypic mosaicism. Assume that black fur in cats is due to the X -linked recessive gene b, whereas its dominant allele B produces yellow fur. A Bb heterozygote is a mosaic called "tortoise shell" or "calico." Using appropriate gene symbols, diagram a mating between a black male and a calico female. Give the phenotypes and genotypes of all the offspring.

In Drosophila melanogaster, sex is determined by the ratio of the number of X chromosomes to the number of haploid sets of autosomes.

Normally in humans, all the sons of a male showing a sex -linked phenotype will inherit the trait.

A color -blind woman with Turner syndrome (XO) has a father who is color -blind. Given that the gene for the color -blind condition is recessive and X -linked, provide a likely explanation for the origin of the color -blind and cytogenetic conditions in the woman.

Studies done in the 1960s suggested that individuals with the XYY condition were prone to criminal behavior. What conclusions currently seem appropriate concerning this chromosomal condition?

Individuals have been identified who have two different karyotypes, such as 45,X/46,XY, or 45,X/46,XX. Such individuals are called _.

A color -blind, chromatin -positive male child (one Barr body) has a maternal grandfather who was color -blind. The boy's mother and father are phenotypically normal. Construct and support (using appropriately labeled diagrams) a rationale whereby the chromosomal and genetic attributes of the chromatin -positive male are fully explained.