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