Day 85 MCAT Practice Question

Passage 6: Reproduction

Researchers have recently identified a novel mutation in the SPATA42 gene, which

is expressed exclusively in the testes and plays a critical role in spermatogenesis.

The mutation, a single base pair deletion, results in a frameshift and a premature

stop codon, leading to a truncated and non functional SPATA42 protein.

Males homozygous for this mutation are infertile due to severely impaired

spermatogenesis, with testicular biopsies revealing a complete absence of mature

spermatozoa.



Further investigation into the function of SPATA42 has shed light on its role in

the regulation of meiotic progression during spermatogenesis. In wild-type mice,

SPATA42 is highly expressed in primary spermatocytes during the pachytene stage

of prophase I. Immunofluorescence studies have shown that the protein localizes

to the synaptonemal complex, a proteinaceous structure that forms between

homologous chromosomes and facilitates synapsis and recombination.

Surprisingly, female mice homozygous for the SPATA42 mutation remain fertile,

despite the gene’s expression in the ovaries. Ovarian tissue from these females

appears histologically normal, with no apparent defects in oogenesis or follicular

development. This sex-specific effect suggests that SPATA42 may have a redundant

function in the female reproductive system, with other proteins compensating for

its loss.

If a male heterozygous for the SPATA42 mutation mates with a female who is

also heterozygous, what proportion of their male offspring is expected to be

infertile?

A) 1/4

B) 1/2

C) 3/4

D) All male offspring will be infertile