15.8: Maternal and paternal effects

Like any other process or trait, embryonic development can be studied and underlying mechanisms identified through the generation and analysis of mutations in the genes that influence the processes involved. From a genetic perspective, there are two general types of mutations - those that effect the formation of gametes, particularly the egg and those that effect development of the embryo directly or indirectly. Mutations (alleles) that influence oocyte formation and the maternally-constructed developmental environment are known as “maternal effect mutations”. Take for example a recessive allele “a” - it may be a typical zygotic effect allele or a “maternal effect” allele. How to distinguish them? Begin with a standard cross between homozygous individuals, the outcome will be the same whether the male or the female is homozygous for the "a" allele (↑). The traits Mendel use all behave in this way. The outcome of the cross will be dramatically different for a maternal effect allele if the female is homozygous for the wild type "A" or mutant "a" allele (↑). The genotype of the female parent (aa) rather than the genotype of the offspring (Aa) determines (influences) the phenotype, a decidedly non-Mendelian behavior. A similar situation arises if the maternal effect allele is dominant, assuming it specifically effects female reproductive success (fertility).

Gamete dimorphism (the differences in gamete size, morphology, and behavior)(↓) implies that some genes preferentially influence oocyte/egg or sperm behaviors and functions. In a number of organisms, particularly those that develop rapidly and outside the mother, most of the gene products and nutrients needed to support the early development of the new organism are supplied by the egg. Defects in the oocyte, due for example to recessive alleles in a homozygous mother, may lead to defects in the behavior of the fertilized egg and subsequent embryo that cannot be rescued by a sperm cell carrying a wild type (dominant) allele - they are dependent upon the maternal genotype and independent of the offspring's genotype. Paternal effects have also been identified.^[526]

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