Inheritance Patterns

1. Inheritance Patterns • Not all genes migrate and follow strict Medelian patterns. Because of this a spectrum of dominance was created. • Complete Dominance • Mendelian pattern of the F1 (heterozygote) being indistinguishable from the dominant (homozygote) phenotype • Codominance • both effect the phenotype in distinguishable ways • blood types are an example (A & B are dominant, O is recessive)

2. Inheritance Patterns • Incomplete Dominance • the expression of the dominant phenotype is lessened by the existence of the recessive gene • Red & White true breeding snapdragons form pink F1 offspring • the degree to which the dominant gene is expressed is called penetrance • Tay Sachs is a disease that causes fat accumulation in the neural tissues of children leading to death in the homozygous individual. The disease is due to the loss of function of a needed enzyme. • Although the allele for the disease is dominant the heterozygote does not exhibit disease symptoms. 1/2 of the enzyme production is sufficient to eliminate symptoms. • The disease has very low penetrance on the heterozygous state.

3. Aberrations of Mendelian genetics • Frequency of recessive alleles • in many instances the recessive allele is much more common than the dominant allele • polydactally (6 or more digits) is a dominant trait but only expressed in ~1/400 births because the majority of the population is homozygous for the recessive gene • Multiple alleles • most than 2 form of alleles exist for most human traits (unlike Mendel's 2 allele peas) • bood type IA, IB, & i (O) • Pleiotropy • most genes exert more than one phenotypic effect (pick any genetic disease)

4. Aberrations of Mendelian genetics • Epistasis • a gene at 1 locus alters the effect of a gene at another • hair color is effected by a gene at a different locus that is responsible for deposition of the pigment in the follicles • if the animal is homozygous for the color gene the animal is white even though it may be homozygous for the pigment (black) • Polygenic Inheritance (quantitative inheritance) • describes traits that are effected by inheritance of genes on different chromosomes • the effect of the traits may be additive or pleiotrophic • skin color is additive with at least 3 genes inherited on 3 different locus and different genes • gives us a distribution of skin color instead of dark brown, tan, or white • Nature vs Nurture • the environment in which an organism lives can alter its phenotypic expression

5. Pedigree Chart • Recessive disorders • homozygous individuals are called carriers • cystic fibrosis • sickle-cell • mating of close relatives increases the penetrance of recessive disorders • Dominant disorders • achondroplasia • Huntington's • Multifactoral (Nature + Nurture) • diseases that have a genetic basis but there penetrance is tied to environmental issues • heart disease • diabetes (type II)

6. Testing • Fetal testing • amniocentesis - looking at cells from the amniotic fluid • in the 14th to 16th week • chorionic villi sampling (CVS) • investigation of the cells from the placenta • newborn screening • PKU • Tay Sachs • Hemophilia • Genetic counseling • offered to individuals related to or who have genetic diseases