Question

You discover a natural mouse mutant that lacks some ribs on its first two thoracic vertebrae. You note that this trait is evident in embryos. Despite this trait, the mouse can breed and you establish a breeding a colony of these mutants. What might be the specific genetic cause of this phenotype (trait) ? ( 2 marks )

Outline an experiment to test whether environment could cause the “missing ribs” trait: ( 4 marks)

Answer 1

Hox-cluster genes control animal body patterning in radiata and in bilateria.

Hoxa-4 is a mouse homeobox-containing gene that is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues during embryogenesis.

Hoxa-4 transcripts could suppress cervical rib formation.The skeletons of neonates from matings between Hoxa-4 transgenic males and Swiss females. In this genetic background, 22 of 31 (71%) wild-type mice had small rib anlagen at C7, while only 6 of 28 (21%) transgenic mice had them. These results suggest that overexpression of Hoxa-4 suppresses the formation of ribs on C7 (P < 0.001, X2 analysis). This is consistent with the interpretation that the intermediate phenotype seen in mice heterozygous for the Hoxa4 mutant allele may be due to loss of expression of the wild-type gene from one allele rather than to a dominant action of the mutant allele, particularly since Northern analysis shows the intensity of the wild-type transcript from heterozygotes is decreased relative to wild types