Transcript Document
LECTURE CONNECTIONS 5 | Extensions and Modifications of Basic Principles © 2009 W. H. Freeman and Company Sex-influenced and sex-limited characteristics • Sex-influenced characteristics are determined by autosomal genes and are inherited according to Mendel’s principles, but they are expressed differently in males and females. • The trait has higher penetrance in one of the sexes. Example: the presence of a beard on some goats is determined by an autosomal gene Bb that is dominant in males and recessive in females. Sex-influenced and sex-limited characteristics • Sex-limited characteristics is encoded by autosomal genes that are expressed in only one sex. • The trait has zero penetrance in the other sex. Example: In domestic chicken, some males display a pattern called cock feathering; other display a pattern called hen feathering. Cock feathering is an autosomal recessive trait that is sex limited to males. Genotype HH Hh hh Male Phenotype Female Phenotype Sex-influenced and sex-limited characteristics • Cytoplasmic inheritance: some characteristics are encoded by genes located in the cytoplasm. Example: Chloroplasts and mitochondria. Mitochondria in humans:15,000 nucleotides that encode 37 genes • For most organisms, all the cytoplasm is inherited from the egg. Sex-influenced and sex-limited characteristics • Genetic maternal effect: the phenotype of the offspring is determined by the genotype of the mother. • The genes are inherit from both parents but the offspring’s phenotype is determined not by its own phenotype but by the genotype of the mother. • The substances present in the cytoplasm of an egg are pivotal in early development. Example: shell coiling of the snail Limnaea peregra. Sex-influenced and sex-limited characteristics • Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent. • Epigenetics: phenomena due to alterations to DNA that do not include changes in the base sequence; often affects the way in which the DNA sequences are expressed (DNA methylation). 5.7 Anticipation Is the Stronger or Earlier Expression of Traits in Succeeding Generations • Anticipation: A genetic trait becomes more strongly expressed or is expressed at an earlier stage as it is passed from generation to generation. Example: The mutation causing myotonic dystrophy consists of an unstable region of DNA that can increase or decrease in size as the gene is passed from generation to generation. 5.8 The Expression of a Genotype May Be Influenced by Environmental Effects • Temperature-sensitive allele: an allele whose product is functional only at a certain temperature In rabbits, the Himalayan allele is temperature dependent.