Question

In hounds, one enzyme is needed to produce a substance required for smell. Gene S produces this enzyme. Another enzyme is needed to produce another substance required for smell (produced by gene R). These two genes are not linked. Individuals homozygous for either one or both of the nonfunctional recessive alleles, s or r, are incapable of smelling.

i) What are the expected proportions for phenotypes in a dihybrid cross for two unlinked genes controlling two traits (the simple Mendelian expectation)?

ii) For the scenario given here, if a large number of matings were made between two double heterozygotes, what phenotypic ratio would be expected in the progeny?

iii) The phenotypic ratio found in (ii) is a result of what well-known phenomenon?

Answer 1

a dihybrid cross is a cross between two heterozygotes.

RrSs * RrSs

	RS	Rs	rS	rs
RS	RRSS ( can smell)	RRSs( can smell)	RrSS( can smell)	RrSs( can smell)
Rs	RRSs( can smell)	RRss (can`t smell)	RrSs( can smell)	Rrss(can`t smell)
rS	RrSS( can smell)	RrSs( can smell)	rrSS(can`t smell)	rrSs(can`t smell)
rs	RrSs( can smell)	Rrss(can`t smell)	rrSs(can`t smell)	rrss(can`t smell)

i) expected proportion of individuals who can smell= number of individuals who can smell/total number=9/16

expected proportion of individuals who can`t smell= number of individuals who can`t smell/total number=7/16

ii) expected phenotypic ratio is

individuals can smell: individuals who can`t smell=9:7

iii) this is an example of complementation, functional copies of two genes are required to produce a normal phenotype, this is known as complementation.