Genetic Maternal Effects

A genetic phenomenon that is sometimes confused with cytoplasmic inheritance is genetic maternal effect, in which the phenotype of the offspring is determined by the genotype of the mother. In cytoplasmic inheritance, the genes for a characteristic are inherited from only one parent, usually the mother. In genetic maternal effects, the genes are inherited from both parents, but the offspring’s phenotype is determined not by its own genotype, but by the genotype of its mother.

Genetic maternal effects frequently arise when substances present in the cytoplasm of an egg (encoded by the mother’s nuclear genes) are pivotal in early development. An excellent example is the shell coiling of the snail Lymnaea peregra (Figure 4.21), described in the introduction to this chapter. In Lymnaea peregra, the direction of coiling is determined by a pair of alleles: the allele for dextral (right-handed) coiling (s+) is dominant over the allele for sinistral (left-handed) coiling (s). However, the direction of coiling is determined not by a snail’s own genotype, but by the genotype of its mother. The direction of coiling is affected by the way in which the egg divides soon after fertilization, which in turn is determined by a substance produced by the mother and passed to the offspring in the cytoplasm of the egg.

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Figure 4.21: In genetic maternal effects, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits a genetic maternal effect

If a male homozygous for dextral alleles (s+s+) is crossed with a female homozygous for sinistral alleles (ss), all the F1 are heterozygous (s+s) and have a sinistral shell because the genotype of the mother (ss) encodes sinistral coiling (see Figure 4.21). If these F1 snails self-fertilize, the genotypic ratio of the F2 is 1 s+s+: 2 s+s : 1 ss. Notice, however, that the phenotype of all the F2 snails is dextral, regardless of their genotypes. The F2 offspring are dextral because the genotype of their mother (s+s), which encodes a right-coiling shell, determines their phenotype. With genetic maternal effects, the phenotype of the progeny is not necessarily the same as the phenotype of the mother because the progeny’s phenotype is determined by the mother’s genotype, not her phenotype. Neither the male parent’s nor the offspring’s own genotype has any role in the offspring’s phenotype. However, a male does influence the phenotype of the F2 generation: by contributing to the genotypes of his daughters, he affects the phenotypes of their offspring. Genes that exhibit genetic maternal effects are therefore transmitted through males to future generations. In contrast, genes that exhibit cytoplasmic inheritance are transmitted through only one of the sexes (usually the female). image TRY PROBLEM 38

CONCEPTS

Characteristics exhibiting cytoplasmic inheritance are encoded by genes in the cytoplasm and are usually inherited from one parent, most commonly the mother. In genetic maternal effects, the genotype of the mother determines the phenotype of the offspring.