19. Spotted fur (S) in rabbits is dominant and solid fur (s) is recessive. A pure-breeding spotted male is crossed with a solid female. The genotypes of these individuals are?
     a.  SS and Ss        b.Ss and Ss        c.Ss and ss         d.SS and ss

Pure breeding is another term for homozygous just as carrier (especially in sex linked problems means heterozygous).

20. The offspring of this cross would have what appearance?
      a.  all would have spotted fur
      b.  half would have spotted fur and half would have solid fur
      c.  three-fourths would have spotted fur and one-fourth would have solid fur
      d.  none of the above

The F₁ offspring are all Ss. Since they all contain one S allele (the dominant allelic form), all the offspring will exhibit the dominant trait, spotted fur.  When you work a problem like this, it is a good idea to put a legend at the top of your page so you don't get the dominant trait and recessive trait confused (especially a possibility if both descriptions start with the same letter as they so in this problem!).  Thus, before I attempted to answer this question, I would have at the top of my work the following :
                        S = spotted fur        s = solid fur

21. Two of these F₁ offspring mate. The genotypic ratio of the offspring from this cross would be
      a. 3 spotted: 1 solid
      b. all spotted
      c. 1SS : 2Ss : 1ss
      d. 1ss : 1ss

Remember that the F₁ offspring are all Ss.  Thus, the work to arrive at the answer should look like this:

                    genotypes of the parents of this cross:    Ss  X  Ss

                    gametes that each parent can produce:        S and s

                    Punnett square:                                                            

Note that the gametes produced by each parent are written at the top ( S and s by one parent) and along the left side (also S and s by the other parent).  The possible types of offspring are found in the squares below and to the right of the gametes.  One can readily determine that these are the gametes produced by the fusion of an egg cell and sperm cell because the offspring are diploid.  They each have two genes for coat color.   Remember, these are the theoretical results.  It does not mean that if four offspring were produced   by this cross, three would have spotted fur and one would have solid fur ( the expected phenotypic ratio). 

BIOWEB  Al BIO 101 Web Section  ELI BIO 101 Web Section 
nverdae@nvcc.edu
Emma Erdahl, Associate Professor of Biology
Northern Virginia Community College
Revised 8/23/00