By Tamarin R.H.
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Among the genes having the greatest number of alleles are those involved in self-incompatibility in plants. In some cases, hundreds of alleles exist for a 20. Mendel self-fertilized dihybrid plants (RrYy) with round and yellow seeds and got a 9:3:3:1 ratio in the F2 generation. As a test of Mendel’s hypothesis of independent assortment, predict the kinds and numbers of progeny produced in testcrosses of these F2 offspring. 21. Four o’clock plants have a gene for color and a gene for height with the following phenotypes: RR: red flower Rr: pink flower rr: white flower TT: tall plant Tt: medium height plant tt: dwarf plant Give the proportions of genotypes and phenotypes produced if a dihybrid plant is self-fertilized.
How can we explain this? We must be dealing with the offspring of dihybrids with each gene segregating two alleles, because the ratio is in sixteenths. Furthermore, we can see that the F2 9:7 ratio is a variation of the 9:3:3:1 ratio. The 3, 3, and 1 categories here are producing the same phenotype and thus make up 7/16 of the F2 offspring. 22 outlines the cross. We can see from this figure that the purple color appears only when dominant alleles of both genes are present. When one or both genes have only recessive alleles, the kernels will be white.
As a rule of thumb, however, the hypothesis is valid, and it has served to direct attention to the functional relationship between genes and enzymes in biochemical pathways. Although a change in a single enzyme usually disrupts a single biochemical pathway, it frequently has more than one effect on phenotype. Multiple effects are referred to as pleiotropy. A well-known example is sickle-cell anemia, caused by a mutation in the gene for the  chain of the hemoglobin molecule. In a homozygote, this mutation causes a sickling of red blood cells (fig.