FACTORS THE ALTER SINGLE-GENE PHENOTYPIC RATIOS: Variations of Mendelian Genetics

All about different phenomenon’s that effect Mendel’s classic ratios - Connects straight from Module 5 : Mendelian Genetics – Quick review o Single Gene inheritance – either/or situation ▪ 2 forms (yellow/green; tall/short) ▪ few human phenotypes that are so straight forward ▪ genes interact with other genes and environmental factors that influence phenotype may not seem like perfect autosomal dominant or recessive patterns, but the underlying genotypic ratios are present - Lethal Alleles: o A lethal genotype (a combination of alleles that causes a lethal outcome) causes death before the individual is old enough to reproduce – this particular combination of alleles cannot be passed off to the next generation(s) o In animals, early lethal alleles (allelic combinations that) cause embryonic death so that particular (genotypic) class of progeny is not observed among the offspring. o Lethal recessive – homozygous lethal recessive alleles result in spontaneous abortion (causes embryo to die) o In typical Mendelian cross, it appears that the phenotypic ratios are 2/3 heterozygotes and 1/3 are homozygous - Autosomal dominant lethal alleles: example: