Download Complex Patterns of Inheritance

Complex Patterns
of Inheritance
Beyond Mendel’s rules of simple dominant/recessive
inheritance
Incomplete Dominance
Neither allele is dominant; heterozygote is a blend
of phenotypes of the two homozygotes.
Red + White = Pink
Genotype = CC; phenotype = chestnut
Genotype = WW; phenotype = white
Genotype = CW; phenotype = palomino
Practice incomplete dominance with Four O’Clocks (flowers)
P = RR x WW
W
R
Red parent plant
R
RW
RW
RW
RW
White
parent
plant
W
Genotype of F1 =
100% RW
Phenotype of F1 = All pink!
F1 = RW x RW
R
Pink parent plant
R
W
RR
RW
RW
WW
Pink
parent
plant
W
Genotype ratio of F2 = 1RR : 2RW: 1WW
Phenotype ratio of F2 = 1 red : 2 pink: 1 white
Codominance
Both alleles are dominant; expressed separately
Red + White =
Both Red and White (NOT pink!)
Genotype: RW
Genotype = RR;
phenotype = red
Genotype = WW; phenotype = white
Genotype = RW; phenotype = roan
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?
R
W
R
RR
RW
W
RW
WW
50%
• What is the probability
that two roan cows will
have a white offspring?
25%
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
Type A: IAIA or IAi
Type AB: IAIB
Type B: IBIB or IBi
Type O: ii
Notice the new allele notation!
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
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
Other Complex inheritance
patterns still to come:
•
•
•
•
Sex-linked traits
More about blood
Environmental effects
Chromosomal disorders