Unit 6: Inheritance Part 2: Complex Patterns of Inheritance

Extending Mendelian genetics Mendel worked with a simple system –peas are genetically simple –most traits are controlled by single gene –each gene has only 2 versions 1 completely dominant (A) (complete dominance) 1 recessive (a) –Examples: albinism, earlobes, tongue rolling But it’s usually not that simple!

Incomplete dominance Hybrids have “in-between” appearance –F R F R = red flowers –F r F r = white flowers –F R F r = pink flowers make 50% less color FRFRFRFR FRFrFRFr FrFrFrFr In humans, hypercholesterolemia is an example of incomplete dominance. C H C H = normal C H C h = elevated cholesterol (2x’s the normal level C h C h = extremely high cholesterol (5x’s the normal level, VERY dangerous). Packet p. 11 Practice on 12

Codominance (& multiple alleles) Equal dominance (expressed equally) –human ABO blood groups –3 versions I A, I B, i A & B alleles are codominant both A & B alleles are dominant over i allele –the genes code for different carbohydrate "flags" on the surface of red blood cells Packet p. 11 Practice on 12 & 13

Blood donation clotting

Pleiotropy One gene can have many effects. eg sickle cell

Many genes: one character Polygenic inheritance –additive effects of many genes –humans skin color height eye color intelligence behaviors Packet p. 11

Polygenic inheritance Multiple genes affect one trait. Human skin color is controlled by at least 3 genes, each with at least two alleles. This Punnett square shows the potential offspring skin tones in the F2 generation, crossing two individuals who are triple heterozygotes.

Polygenic inheritance Eye color is controlled by 4 known genes and probably multiple others At least 3 pigment genes –Brown (B) dominant to blue (b) and to G & g (below) –Green (G) dominant to blue (g) –Melanin At least 1 structural gene

Linked genes So far we have talked about independent assortment, which is… But, some genes ARE inherited together On autosomal chromosomes, we call these linked genes On sex chromosomes, we call these sex-linked genes

Sex-linked genes X-linked –Recessive: hemophilia, red-green colorblindness –Recessive traits are more common in males. Why? –There are very few disorders that are X-linked dominant. Why do you think?

Pedigrees Pedigrees are family trees that show the presence/absence of specific traits or diseases. Squares represent males Circles represent females Filled shapes represent the presence of a trait or disease Some pedigrees show carriers (half-filled shapes) If this pedigree tracks the presence of an autosomal homozygous recessive trait, what are the genotypes of the very first parents at the top?