T

A snails

Compound A

-Enzyme I*

aa snails

"1

B snails

Compound B -Enzyme II*

bb snails

Compound C

Pigmented snails must produce enzymes I and II, which requires genotype A_ B_.

Compound C

Pigmented snails must produce enzymes I and II, which requires genotype A_ B_.

^ Albinism arises from the absence of enzyme I (aa B_), so compound B is never produced,...

^ .. .or from the absence of enzyme II (A_ bb), so compound C is never produced, or from the absence of both enzymes (aa bb).

I 5.9 Pigment is produced in a two-step pathway in snails.

In interpreting the genetic basis of modified ratios, we should keep several points in mind. First, the inheritance of the genes producing these characteristics is no different from the inheritance of genes coding for simple genetic characters. Mendel's principles of segregation and independent assortment still apply; each individual possesses two alleles at each locus, which separate in meiosis, and genes at the different loci assort independently. The only difference is in how the products of the genotypes interact to produce the phenotype. Thus, we cannot consider the expression of genes at each locus separately, but must take into consideration how the genes at different loci interact.

A second point is that in the examples that we have considered, the phenotypic proportions were always in sixteenths because, in all the crosses, pairs of alleles segregated at two independently assorting loci. The probability of inheriting one of the two alleles at a locus is 1/2. Because there are two loci, each with two alleles, the probability of inheriting any particular combination of genes is (/2)4 = /16. For a trihybrid cross, the progeny proportions should be in sixty-fourths, because (1/2)6 = 1/64. In general, the progeny proportions should be in fractions of (1/2)2n, where n equals the number of loci with two alleles segregating in the cross.

T b| 5"

2| Modified dihybrid — phenotypic ratios due to gene interaction

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