Question

1. Labrador retrievers come in several colors: brown, black, or yellow. In a cross of a black female with a brown male, results can be either all black puppies, 1/2 black to 1/2 brown puppies, or 3/4 black to 1/4 yellow puppies. What to these results indicate?

      
A. Yellow is dominant to black.

       
B. Black is dominant to brown and to yellow.

       
C. Brown is dominant to black.

       
D. There is incomplete dominance.

       
E. There must be epistasis involved.

2. The spines of cacti may be sharp or dull. Gene S controls the sharpness of spines in a type of cactus. Cactuses with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cactuses have dull spines. At the same time, a second gene, N, determines whether cactuses have spines or not. Homozygous recessive nn cactuses lack any spines at all. A cross between a true-breeding sharp-spined cactus and a spineless cactus would produce

      
A. 25% sharp-spined, 50% dull-spined, 25% spineless progeny.

       
B. all sharp-spined progeny.

       
C. It is impossible to determine the phenotypes of the progeny.

       
D. all spineless progeny.

       
E. 50% sharp-spined, 50% dull-spined progeny.

Answer 1

1. Condition all black puppies. will be due to genotype BB for black and bb for brown.

This cross will form Bb which will be all black.

The condition for 1/2 black to 1/2 brown puppies due to genotype Bb fro black and bb fro yellow.

This cross will form Bb and bb which will be black and yellow.

The condition for 3/4 black to 1/4 yellow puppies due to genotypes Bb for both black and yellow.

This cross will from BB, Bb for black, and bb for yellow.

The answer is B.

This cannot be incomplete dominance because in incomplete dominance the heterozygotes always have a third color whijc isyello in this . But for heterozygotes black is domint, not yellow.