Complex Inheritance

Simple Recessive Heredity Most genetic disorders are caused by recessive alleles; the person must be homozygous recessive in order to express the disorder Cystic Fibrosis Tay-Sachs PKU

Simple Dominant Heredity Only one dominate allele must be present for the person to express the phenotype Some examples of traits that follow this pattern are: Hitchhiker’s Thumb Tongue rolling Free earlobes Mid-digital hair

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

Incomplete Dominance in Humans Curly hair is incompletely dominant HH = straight hair H’H’ = curly hair HH’ = wavy hair

Codominance Phenotypes of both homozygous parents are expressed in the heterozygous offspring in equal amounts BOTH alleles are dominant AND BOTH alleles will be seen if genotype is heterozygous

Codominance 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 Common in African-American populations Hemoglobin crystallizes, changing the shape of RBC Sickle shaped cells slow blood flow and block small vessels Sickle cells have short life span causing low blood cell count and anemia

Multiple Alleles The presence of 3 or more alleles for a genetic trait 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

Multiple Alleles BABA or BAB = Ash red feathers BB or Bb = Grey feathers bb = Brown feathers

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

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 Inheritance pattern of a trait controlled by two or more genes…genes may be on the same chromosome or different chromosomes 2 or more genes control the trait Genes can be on different chromosomes

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

Environmental Influences Temperature Siamese Cats: body temp determines fur color Warm areas—white fur Cooler areas—black/brown fur Light Sun exposure increases melanin production, resulting in tan skin Chemicals Pesticide use can result in mutations and resistance

Environmental Influences Hormones Feathers of peahens versus peacocks Females are dull and males have bright, attractive plumage Sex changing fish Age Cells cannot divide indefinitely, eventually cannot repair themselves