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