Answer to Question #158887 in Genetics for Julia Verity

Question

Does crossing two plants, each heterozygous for two traits, always produce a phenotypic ratio of 9:3:3:1 in the offspring? A) Yes, unless there are linked genmes, incomplete dominance, or, in animals, sex chromosomes involved. B)Yes, regardless of the situation. C) No, because the results of each cross are completely random D) None of these is correct

Answer 1

If we suggest that the two alleles of the first gene are "A" and "a" and the two alleles of the second gene are "B" and "b" after the dihybrid crossing we get the following ratio of genotypes:

9 A*B* : 3 A*bb : 3 ааB* : 1 aabb 

("*" stands for any of the alleles of the appropriate gene)

In the case of complete dominance, the ratio of phenotypes would be the same as the ratio of genotypes above, because only the dominant trait would manifest in "A*" or "B*" trait combination so there is no difference what is the second trait in the combination.

But in the case of incomplete dominance both dominant and recessive gene alleles contribute to the phenotype so there would be more phenotypes and another ratio.

In case genes are linked (are located in the same chromosome) they would inherit together in most cases. But sometimes chromosomes crossing over leads to a swap of genes between homologous chromosomes and new genes combination. The frequency of that event is not high so more gametes would have chromosomes that didn't undergo crossing over than chromosomes that did. It leads to the predominance of several phenotypes on another so the ratio of them would be different.

If the genes are located in the sex chromosome they would be linked to the sex and these chromosomes would not cross over in case they aren`t homologous (contribute to the formation of different sex, for example, X and Y chromosomes of mammals). It also would lead to the different phenotype ratio.

Answer:

Crossing two plants, each heterozygous for two traits, always produce a phenotypic ratio of 9:3:3:1 in the offspring unless there are linked genes, incomplete dominance, or, in animals, sex chromosomes involved.