Genetic Maternal Effect

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 effect, 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 effect frequently arises when substances present in the cytoplasm of an egg (encoded by the mother's genes) are pivotal in early development. An excellent example is shell coiling of the snail Limnaea peregra. In most snails of this species, the shell coils to the right, which is termed dextral coiling. However, some snails possess a left-coiling shell, exhibiting sinistral coiling. The direction of coiling is determined by a pair of alleles; the allele for dex-tral (s+) is dominant over the allele for sinistral (s). However, the direction of coiling is determined not by that snail's own genotype, but by the genotype of its mother. The direction of coiling is affected by the way in which the cytoplasm 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.

If a male homozygous for dextral alleles (s+s+) is crossed with a female homozygous for sinistral alleles (ss), all of the F1 are heterozygous (s+s) and have a sinistral shell, because the genotype of the mother (ss) codes for sinistral (I FIGURE 5.17). If these F1 snails are self-fertilized, the genotypic ratio of the F2 is 1 s+s+ :2 s+s:1 ss. The phenotype of all F2 snails will be dextral regardless of their genotypes, because the genotype of their mother (s+s) encodes a right-coiling shell and determines their phenotype.

P generation

Dextral S

Dextral S

P generation

Dextral, a right-handed coil, results from an autosomal allele (s+) that is dominant.

Dextral, a right-handed coil, results from an autosomal allele (s+) that is dominant.

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