Modified Mendelian Ratios Ch. 13 1. Multiple alleles (more than 2 alleles for gene in population) Example: Blood Groups Karl Landsteiner 1900 s Chromosome 9 I gene
ABO blood system = polymorphic I gene Blood type genotype A I A I A or I A i B? AB I A I B 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
A,B,O carbohydrate antigens
Bombay phenotype A man I A I A (Type A) and a woman of type I A I B have a child with type O blood. How??
I gene encodes glycotransferases Glycotransferases add sugar group to glycolipid H gene encodes the glycolipid antigen HH I A I A Hh I A I i
2. Incomplete dominance Example: snapdragons Allelic symbols do not specify dominance C R = red color C w = white color C R C R X C w C w
3. Codominance Each allele encodes separate gene product distinct in heterozygote L gene for human blood cell surface protein L M = M antigen LMLN x LMLN L N = 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 spine MM = 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
6. Expressivity =the extent to which a trait is exhibited Piebald spotting
Both sisters have same genotype 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
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)
7. Epistasis- gene product interactions A product of one gene influences, or masks, the expression of another gene(s) Modification of dihybrid cross ratio AaBb X AaBb 9:3:3:1
Epistasis in labrador retrievers B and E color genes (labs) B black E color ee is epistatic b brown e no color (yellow) 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 SINGLE PEA WALNUT
Epistasis in Cats W = white w = not white B = black b = brown Mate 2 heterozygous cats What is the expected ratio?
8. Polygenic traits Vary continuously Weight, height, IQ
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 factors Predisposition genes + environment Cleft palate and identical twins, colon cancer may run in families
4. Dominance series C series/ rabbits c+ = full color c ch = chinchilla (hypomorphic) c h = himalayan (hypomorphic) c = albino (apomorphic allele = nonfunctional) Chinchilla Himalayan Albino
Genotype phenotype? c ch c ch c ch c h c h c c+ c ch c+ = full color c ch = chinchilla (hypomorphic) c h = himalayan (hypomorphic) c = albino (apomorphic allele = nonfunctional)