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)

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.

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)

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

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)

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