If the heterozygote form simultaneously expresses both alleles fully, then the relationship between the two alleles is called codominance. An example of codominance appears in human blood type. Blood type is determined by two alleles, A and B, that code for the presence of antigen A and antigen B on the surface of red blood cells. Allele A and B are codominant. If only the allele A is present, then only antigen A exists on the blood cell. If only allele B is present, then only antigen B exists on the blood cell. If both alleles A and B are present, neither dominates the other and both antigens appear on the red blood cell. A third allele, i, is recessive: if only it appears, then the blood is of type O. The following is a summary of the genotypes that result in the four different blood types:
AA and Ai
type A blood
BB and Bi
type B blood
AB and BA
type AB blood
ii
type O blood
Linkage and Crossing-Over
Fortunately for Mendel, the genes encoding his selected traits did not reside close together on the same chromosome. If they had, his dihybrid cross results would have been much more confusing, and he might not have discovered the law of independent assortment. The law of independent assortment holds true as long as two different genes are on separate chromosomes. When the genes are on separate chromosomes, the two alleles of one gene (A and a) will segregate into gametes independently of the two alleles of the other gene (B and b). Equal numbers of four different gametes will result: AB, aB, Ab, ab. But if the two genes are on the same chromosome, then they will be linked and will segregate together during meoisis, producing only two kinds of gametes.
For instance, if the genes for seed shape and seed color were on the same chromosome and a homozygous double dominant (yellow and round, RRYY) plant was crossed with a homozygous double recessive (green and wrinkled, rryy), the F1 hybrid offspring, as usual, would be double heterozygous dominant (yellow and round, RrYy). However, since in this example the R and Y are linked together on the chromosome inherited from the dominant parent, with r and y linked together on the other chromosome, only two different gametes can be formed: RY and ry. Therefore, instead of 16 different genotypes in the F2 offspring, only three are possible: RRYY, RrYy, rryy. And instead of four different phenotypes, only the original two will exist. Notice that the inheritance pattern now resembles that seen in a monohybrid cross, with a 3:1 phenotypic ratio, rather than the 9:3:3:1 ratio expected from the dihybrid cross. If physically linked on a single chromosome, the round and yellow alleles would segregate together, and the wrinkled and green alleles would segregate together: no round green seeds or wrinkled yellow seeds would ever appear.
The above explanation, however, neglects the influence of the crossing over of genetic material that occurs during meiosis. The farther away two genes are from one another, the more likely an exchange point for crossing over will form between them. At these exchange points, the alleles of one gene switch to the opposite homologous chromosome, while the other gene alleles remain with their original chromosomes. When alleles switch places like this, the resulting gametes are called recombinant. In the example above, the original parental gametes would be RY and ry, while the recombinant gametes would be Ry and rY. Thus four different kinds of gametes will be formed, instead of only two formed when the genes were linked.
