Complex Patterns of Inheritance

1
Complex Patterns of Inheritance
Beyond Mendels rules of simple
dominant/recessive inheritance
2
Incomplete Dominance

• Neither allele is dominant heterozygote is a
  blend of phenotypes of the two homozygotes.

Red White Pink
3
Genotype CC phenotype chestnut
4
Genotype WW phenotype white
5
Genotype CW phenotype palomino
6
Practice incomplete dominance with Four OClocks
(flowers)
P RR x WW
Red parent plant
R
R


W
RW
RW
White parent plant
RW
RW
W
Genotype of F1
100 RW
Phenotype of F1
All pink!
7
F1 RW x RW
Pink parent plant
R
W


R
RR
RW
Pink parent plant
WW
RW
W
Genotype ratio of F2
1RR 2RW 1WW
Phenotype ratio of F2
1 red 2 pink 1 white
8
Codominance

• Both alleles are dominant expressed separately

Red White Both Red and White (NOT pink!)
9
Genotype RW
10
Genotype RR phenotype red
11
Genotype WW phenotype white
12
Genotype RW phenotype roan
13
Show the cross between two roan cows (use R and
W).

• What is the probability that two roan cows will
  have at least one roan
  offspring?
• 50
• What is the probability that two roan cows will
  have a white offspring?
• 25

R
W


RR
RW
R
WW
W
RW
14
Multiple Alleles

• More than 2 alleles possible for a trait
• Example Blood type in humans
• Three alleles possible (A, B, and O), each person
  only has two of those alleles

• Four phenotypes (blood types) possible A, B, AB,
  and O

• A and B are codominant O is recessive

• Alleles notated as IA, IB, i

15
Notice the new allele notation!
16
Polygenic Traits

• Trait is controlled by interaction between 2 or
  more genes
• Examples skin color, eye color, height, hair
  color
• Results in a continuum of expressed phenotypes

17
Lethal Recessive Alleles

• Having two mutated copies of an essential gene
  may be deadly
• E.g., Cystic Fibrosis, Sickle-cell Anemia,
  Tay-Sachs Disease
• Heterozygote survivse because it has one
  nonmutated form of the gene
• Heterozygote is carrier for disorder
• Heterozyous condition is sometimes linked with
  beneficial traits
• e.g., sickle cell carrier and malaria resistance

18
Other Complex inheritance patterns still to come

• Sex-linked traits
• More about blood
• Environmental effects
• Chromosomal disorders