There is a gene that controls formation of muscles in the human tongue
that allow people with those muscles to roll their tongues; people who
lack those muscles cannot roll their tongues. The ability to roll one’s
tongue is dominant over non-rolling.
The ability to taste certain substances is also genetically controlled. Some
people can taste the substance called phenylthiocarbamate (PTC), while
others cannot. To people who are tasters, PTC test paper is very bitter; to
non-tasters it just tastes like paper. The ability to taste PTC is dominant
over being a non-taster.
Let R represent tongue-rolling, r represents non-roller.
T represents the ability to taste PTC, and t represents non-tasting.
1. Write the genotypes for the cross if one parent is heterozygous
for both tongue-rolling and PTC tasting while the other parent is
heterozygous for tongue-rolling, but cannot taste PTC.
RrTt X Rrtt.
2. Determine the phenotypic ratios for the offspring of this cross.