Download Unit 6: Inheritance

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
Packet p. 11
Practice on 12
• Hybrids have “in-between” appearance
– FRFR = red flowers
– FrFr = white flowers
– FRFr = pink flowers
• make 50% less color
FR FR
F R F r F rF r
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. 11
Practice on 12 & 13
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
Blood donation
clotting clotting
clotting
clotting
clotting
clotting
clotting
Pleiotropy
• One gene can have many effects.
• eg sickle cell
Packet p. 11
Many genes: one character
• Polygenic inheritance
– additive effects of many genes
– humans
•
•
•
•
•
skin color
height
eye color
intelligence
behaviors
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?