3. Extensions to Mendelian Inheritance

How many Barr bodies are found in a normal human female nucleus? In a normal male nucleus?

An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the F₁ and F₂ generations from a cross between a female with reduced wings and a male with normal-sized wings when the male is the heterogametic sex.

Consider the case where a mutation occurs that disrupts translation in a single human mitochondrion found in the oocyte participating in fertilization. What is the likely impact of this mutation on the offspring arising from this oocyte?

What is the basis for homology among chromosomes?

In humans, SRY is located near a pseudoautosomal region (PAR) of the Y chromosome, a region of homology between the X and Y chromosomes that allows them to synapse during meiosis in males and is a region of crossover between the chromosomes. The diagram below shows SRY in relation to the pseudoautosomal region.

About 1 in every 25,000 newborn infants is born with sex reversal; the infant is either an apparent male but with two X chromosomes or an apparent female but with an X and a Y chromosome. Explain the origin of sex reversal in human males and females involving the SRY gene. (Hint: See Experimental Insight 3.1 for a clue about the mutational mechanism.) 

In C. elegans there are two sexes: hermaphrodite and male. Sex is determined by the ratio of X chromosomes to haploid sets of autosomes (X/A). An X/A ratio of 1.0 produces a hermaphrodite (XX), and an X/A ratio of 0.5 results in a male (XO). In the 1970s, Jonathan Hodgkin and Sydney Brenner carried out genetic screens to identify mutations in three genes that result in either XX males (tra-1, tra-2) or XO hermaphrodites (her-1). Double-mutant strains were constructed to assess for epistatic interactions between the genes (see table). Propose a genetic model of how the her and tra genes control sex determination.

Genotypeᵃ             XX Phenotype             XO Phenotype
Wild-type               Hermaphrodite            Male
tra-1ʳᵉᶜ                   Male                            Male
tra-2ʳᵉᶜ                   Male                            Male
her-1ʳᵉᶜ                  Hermaphrodite            Hermaphrodite
tra-1ᵈᵒᵐ/+               Hermaphrodite            Hermaphrodite
tra-1ʳᵉᶜ tra-2ʳᵉᶜ        Male                            Male
tra-1ʳᵉᶜ her-1ʳᵉᶜ       Male                            Male
tra-2ʳᵉᶜ her-1ʳᵉᶜ       Male                            Male
tra-2ʳᵉᶜ tra-1ᵈᵒᵐ/+   Hermaphrodite            Hermaphrodite
ᵃrec = recessive mutation; dom = dominant mutation.

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Propose a genetic hypothesis to explain these data.

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Design an experiment that will test your hypothesis, using the wild-type progeny identified above. Describe the results you expect if your hypothesis is true.