complex inheritance. mendelian genetics mendel dealt with simple inheritance…dominant vs....
TRANSCRIPT
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