The ratio given in the question is
the ratio of dihybrid test cross.
A test cross is a type of backcross
in which the individual is specifically crossed to a recessive
parent. In phenotypic ratio in dihybrid, test cross is 1:1:1:1.
Test cross is used to identify the heterozygosity of an
individual
Four
genotypes         Â
=Â Â Â Â Â Â Â Â Â TTDD, TtDD,
ttDD, TdDd
Total individuals are
=Â Â Â Â Â Â Â Â Â 160
No of individuals with TTDD
genotype     Â
=Â Â Â Â Â Â Â Â Â ?
All four genotypes are in 1:1:1:1 ratio so = 1/4*160
= 40
The No of individuals with TTDD genotype is
40.
Other genotypes having individuals are
TtDDÂ Â
=1/4*160Â Â Â Â Â Â Â Â
=Â Â Â Â Â Â Â Â Â 40
ttDD
=1/4*160Â Â Â Â Â Â Â Â Â
=Â Â Â Â Â Â Â Â Â 40
TdDd=
1/4*160Â Â Â Â Â Â Â Â
=Â Â Â Â Â Â Â Â Â 40
