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

– FRFR = red flowers– FrFr = white flowers– FRFr = pink flowers

• make 50% less color

FRFR FRFr FrFr

In humans, hypercholesterolemia is an example of incomplete dominance.

CHCH= normal

CHCh= elevated cholesterol (2x’s the normal level

ChCh= extremely high cholesterol (5x’s the normal level, VERY dangerous).

Packet p. 11Practice on 12

Codominance (& multiple alleles)

• Equal dominance (expressed equally)– human ABO blood groups– 3 versions

• IA, IB, 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. 11Practice on 12 & 13

Blood donation

clotting clotting

clotting clotting

clotting clotting 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?