Maternal Effects

• All embryos develop inside maternal tissue
• Some maternal genes affect embryo development
• Maternal effects can be genetic or environmental
• Can result from expression of genes in maternal tissue
• Can result from inheritance of maternal cytoplasm (and organelles)
• Can result from maternal environment (not genetic)

Snail coiling

• See Fig. 15.14 Inheritance of the direction of shell coiling in the snail
  Limnaea peregra
• Phenotype depends only on genotype of mother
• Example: We have a female F1 snail with genotype Dd, where right coiling is dominant to left. Which way does her shell coil?
• • We can't tell because we know nothing about her mother's genotype
• If she is mated to a sibling (also F1, Dd), what will the coiling phenotypes of her offspring be? I.e. what if her offsping genotype is:
• • DD ?
  • Dd ?
  • dd ?

Genomic Imprinting

• For some loci, only one allele is expressed, even though the individual has both copies.
• May be maternal or paternal, depending on the gene, but for any gene it is always the same.
• Imprinting occurs during gametogenesis
• • Persists in all somatic cells
  • Erased in germ line, and then re-imprinted as necessary
• Example: Igf2 (insulin-like growth factor)
• • Affects size and growth rate
  • Only paternal allele is expressed

Why imprinting?

• Lots of imprinted genes affect embryonic growth (e.g. Igf2):
• May be a solution to the genetic "conflict of interest" between mothers and offspring sired by different fathers
• • Mothers gain fitness by spreading resources evenly among offspring
  • Fathers gain fitness by having their offspring grow fastest (especially if multiple paternity is common).
• Paternal genes tend to be expressed for genes that increase growth rate (Igf2); maternal genes tend to be expressed for loci that decrease growth (Grb10, an Igf signaling inhibitor)

Mechanism of imprinting

• 5 methyl cytosine can block transcription factors from binding to DNA sequence
• • the methyl group sticks out into the major groove of DNA helix
  • may also keep chromatin in condensed form, and thus not allow gene transcription
• methylation occurs CpG sequences

Mitochondria and Chloroplasts

We didn't have time in lecture to talk very much about maternal inheritance of organelles, but you should read your text and understand: