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1. Multiple alleles (more than 2 alleles for gene in
population)
Example: Blood Groups Karl Landsteiner 1900’s
Chromosome 9 I gene
Modified Mendelian Ratios Ch. 13
ABO blood system = polymorphic I gene
Blood type genotypeA IAIA or IAi
B ?
AB IAIB
O ii
What is the mechanism of inheritance of A, B, AB, O?
Autosomal or sex chromosome?
Which two alleles are codominant?
Which allele is recessive?
Example
A child has type O blood. The mother of the child has Type B blood. What could the blood types of the f IBi
ii
I gene encodes glycotransferasesGlycotransferases add sugar group to glycolipid
H gene encodes the glycolipid antigen
HH IAI A Hh IAIi
2. Incomplete dominance
Example: snapdragons
Allelic symbols do not specify dominance
CR = red color
Cw = white color
CR CR X Cw Cw
3. Codominance
Each allele encodes separate gene
product distinct in heterozygote
L gene for human blood cell surface protein
LM = M antigen LMLN x LMLN
LN = N antigen 1M
A man with the M bloodtype has a child with a woman of the MN bloodtype
Expected ratio of offspring?
4. Lethal alleles
MM = normal spineMM’ = manx cat (no
tail)M’M’ = lethal
Cross two manx, what is ratio of phenotypes in offspring?How do breeders obtain manx cats?
Lethal alleles in humans
Tay Sach disease tt is fatal
Huntington disease HH is fetal lethal Hh causes death ~ age 50.
5. Penetrance % individuals that exhibit
phenotype corresponding to genotype
Polydactyly, dominant
Pp pp
5,5 6, 5 6, 6
Human piebald trait autosomal dominant defect in melanocyte development mutation in the KITgene, which encodes cell surface receptor tyrosine kinase
http://dermatlas.med.jhmi.edu/derm/indexDisplay.cfm?ImageID=386927278
Both sisters have same genotype
Incomplete penetrance AND expressivity NF-1 = Neurofibromatosis1
(1/4000, 17q11.2)
(350 kb gene, 60 exons)
Autosomal dominant trait NN and Nn with 50- 80% penetrance
Variable expressivity
Mild form tumors on nerve CT coverings, skin, eyes, organs, face
speech, blood pressure, spine curvature, headaches
Gene expression also affected by: Sex (baldness) Temperature (melanin in Siamese cats) Chemicals (PKU) Diet (height, cancer)
A product of one gene influences, or masks, the expression of another gene(s)
Modification of dihybrid cross ratioAaBb X AaBb 9:3:3:1
7. Epistasis- gene product interactions
Epistasis in labrador retrievers B and E color genes (labs)
B black b brown
E color e no color (yellow)
ee is epistatic
Cross two double heterozygotes
Phenotypes of parents?
Phenotypes of offspring? ratio?
The genotype rrpp gives roosters a single comb.R-P- gives a walnut comb. rrP- gives a
pea comb, R-pp gives a rose comb.
Determine the ratio of phenotypes in the F2 generation of the following cross
P ?
F1 ?
F2 ?
ROSE PEA
SINGLE WALNUT
Epistasis in Cats
W = white w = not white B = black b = brown
Mate 2 heterozygous cats
What is the expected ratio?
Example: CAD is a multifactorial trait affected by
Obesity, diabetes, high blood pressure, high levels of LDL-cholesterol, and gum disease
Although CAD runs in families, does not show Mendelian inheritance patterns
CAD occurs more often in men Risk higher among US African Americans than
Caucasians or Asians >200 genes identified
LDLR and familial hypercholesterolemia (dominant)
Influenced by many genes and environmental factorsPredisposition genes + environment
Cleft palate and identical twins, colon cancer may run in families
4. Dominance series – C series/ rabbits
c+ = full color cch = chinchilla (hypomorphic)
ch = himalayan (hypomorphic)
c = albino (apomorphic allele = nonfunctional)
Chinchilla
Himalayan Albino