Suppose that true-breeding (homozygous), black female dogs are mated with true-breeding (homozygous) brown male dogs. The F1 generation offspring are all black – 6 females and 5 males. The F1 offspring were mated with one another, and there were 15 black females, 7 black males, and 8 brown males in the F2 generation.

a. Which trait is dominant - brown or black? How do you know?

Question

Suppose that true-breeding (homozygous), black female dogs are mated with true-breeding (homozygous) brown male dogs.  The F1 generation offspring are all black – 6 females and 5 males.  The F1 offspring were mated with one another, and there were 15 black females, 7 black males, and 8 brown males in the F2 generation.

a. Which trait is dominant - brown or black? How do you know?

confusionist · Answer

Dominant Black, Recessive Brown. 

...........X^B..........X^B 
X^b...X^BX^b.....X^BX^b 

All female children are black with hidden brown allele 
..................................... 
Y.......X^BY........X^BY . . . 

All male children are black with no hidden allele

anonymous · Answer

But how did you figure that out?

blues · Answer

You know that both parents are homoygous going into the cross. One is homozygous dominant, one is homozygous recessive - though you don't know which is which. The point is, your Punnett square looks like this:

|dw:1386567777453:dw|

All the offspring are heterozygous. And because they are all black, you know that the black allele must be dominant to the brown allele.