In chickens, rose comb (R) is dominant to single comb (r). A homozygous rose-combed rooster is mated with a si

In chickens, rose comb (R) is dominant to single comb (r). A homozygous rose-combed rooster is mated with a single-combed hen. All of the chicks in the F1 generation were kept together as a group for several years. They were allowed to mate only within their own group. What is the expected phenotype of the F2 chicks?





1. 100% rose comb





2. 75% rose comb and 25% single comb





3. 100% single comb





4. 50% rose comb and 50% single comb

In chickens, rose comb (R) is dominant to single comb (r). A homozygous rose-combed rooster is mated with a si
I believe its 2. All of the F1 generation chicks are heterozygous dominant. If all of them mate, there's 1 RR 2 Rr and 1 rr.
Reply:The answer is:





2. 75% rose comb and 25% single comb





You're told that the rooster is homozygous (RR).


Since single comb is recessive, you know the hen must be homozygous recessive (rr).


From this you know that all of the F1 have to be Rr.





That means that the resulting F2 would be:





25% RR, showing a rose comb


50% Rr, also showing a rose comb, because R is dominant over r.


25% rr, showing a single comb
Reply:The original parents were each homozygous, so the first cross was RR x rr. That means ALL the F1 chicks were Rr, so all the F2 chicks are the offspring of Rr x Rr crosses. You can use a Punnett square to do this. The gametes each parent can produce would be R and r. So the possible offspring would be in a ratio of 1RR: 2Rr: 1rr. Since rose comb is dominant, 3/4 (75%) would be rose, and the rest would have single combs.





It's actually not a great question. After several years, you would have several generations, so why even add that part in!