Complex Inheritance

Mendelian Genetics Mendel dealt with simple inheritance…dominant vs. recessive Dominant traits only require one dominant allele to be present in order for the trait to be expressed Hitchhiker’s thumb, tongue rolling, free ear lobes, hair on fingers Recessive traits require two recessive alleles to be present in order for the trait to be expressed. Most genetic disorders are caused by recessive alleles: Cystic Fibrosis, Tay-Sachs, PKU

Incomplete Dominance Phenotype of the heterozygote is an intermediate between the two homozygous parents Neither allele is dominant…the alleles blend together if offspring is heterozygous

Incomplete Dominance In snapdragons, neither red or white is completely dominant Red is represented by R and white by R’ (R prime) RR=red flowers R’R’= white flowers RR’=pink flowers In humans, curly hair is incompletely dominant HH = straight hair H’H’ = curly hair HH’ = wavy hair

Codominance BOTH alleles are dominant AND BOTH alleles will be seen if genotype is heterozygous Chickens with black feathers are homozygous for the B allele (BB) Chickens with white feathers are homozygous for the W allele (WW) A heterozygous chicken (BW) would have black and white feathers

Codominance in Humans Sickle Cell Anemia Hemoglobin crystallizes, changing the shape of RBC Sickle shaped cells slow blood flow and block small vessels Healthy People with NO Sickle Cell are AA Affected people with severe sickle cell are SS Carriers who may show symptoms are AS

Multiple Alleles The presence of 3 or more alleles for a genetic trait. ALL alleles are on the SAME gene! 3 or more genotypes are possible for one trait…so you get 3 or more phenotypes are possible for one trait Ex: Pigeons There are three alleles that code for feather color, but each individual can have only two. BA=ash red > B=grey > b = brown BABA, BAB, BB, Bb, bb

Multiple Alleles in Humans In humans, there are 3 alleles that code for blood type 3 alleles (IA, IB, and i) determine 4 blood types (A, AB, B, and O). IAIA or IAi result in type A blood IBIB or IBi result in type B blood IAIB results in type AB blood ii results in type O blood If a person is given the wrong blood during a transfusion, incompatible blood cells can clump together and cause death.

Sex-Linked Inheritance Traits are controlled by genes located on sex chromosomes Sex chromosomes determine sex XX = female XY = male Y chromosome doesn’t have same genes as X Disorders affect males more than females, because males only have ONE X chromosome

Sex-linked Inheritance Ex: Fruit flies XR=Red Eyes Xr=White Eyes Females XRXR or XRXr = Red Eyes XrXr = White Eyes Males XRY = Red Eyes XrY = White Eyes

Sex-Linked Inheritance in Humans Red-green color blindness Person cannot tell the difference between red and green. Recessive allele affects the red and green receptors in the eye Inherited on the X chromosome

Sex-Linked Inheritance in Humans Hemophilia Missing clotting factor so cannot stop bleeding Treatments include clotting factor injections and blood transfusions Inherited on the X chromosome

Polygenic Inheritance in Humans Involves 3-4 different genes Offspring look like an intermediate or combination of parents’ skin color Eye and Hair Color are also polygenic