Download Multiple Alleles and Polygenic Inheritance

Multiple Alleles, Polygenic, and Sex-linked Notes
Multiple alleles – more than 2 alleles produces 3 or more genotypes.
Blood Types:
Genotypes
Phenotypes
IaIa or Iai
A
IbIb or Ibi
B
ii
O
IaIb
AB
Why is blood type important?
- Because transfusion of the wrong blood type can cause death
- Type O is the universal donor, type AB is the universal acceptor
- Can be used in parental disputes:
Example: if the mother is type A and the child is AB, can the
dad be type O? NO why not?
Epistasis= One allele hides the effects of another allele
Example: Labradors. The dominant allele (E) determines whether the skin will have
pigment. The dominant allele (B) determines how dark it is (color).
Possible Genotypes
BBee
Bbee
bbee
bbEE
bbEe
BBEE
BbEE
BBEe
BbEe
Phenotype
Yellow
Explanation
No Pigment
Chocolate
Recessive Color + Pigment
Black
Dominant Color + Pigment
Polygenic Inheritance- more than one pair of genes influencing a trait.
Examples: hair color, skin color, eye color
Sex-linked Traits
- 46 chromosomes- 23 pairs ( 22 pairs of autosomes and 1 pair of sex
chromosomes)
- Male  XY
- Female  XXXX
-
Genes located on the X or Y chromosomes
Most sex-linked genes are found on the X chromosome
The human Y chromosome is much smaller and appears to contain only few
genes.
Father determines the sex of the offspring
The chance is always 50-50 for either sex
A recessive gene has no matching gene on the Y
More Sex linked disorders are found in males
Sex linked disorders:
1. Colorblindness- the inability to distinguish certain colors
- Three human genes associated with colorvision are located on the X
chromosomes
- Red-green colorblindness is the most common form
- In the US, 1 in 10 males and 1 in 100 females have color- blindness.
- Why the difference? Males have just one X chromosome.
C
- Therefore, their genotypes will be X Y or XcY.
-
C C
Females have two X chromosomes. Therefore, their genotypes would be X X
C
(normal), X Xc (carrier), and XcXc is colorblind.
Let’s practice:
2. Hemophilia
- A sex-linked disorder in which a protein for normal blood clotting is missing.
- About 1 in 10,000 males has a form of hemophilia.
- Hemophiliacs can bleed to death from minor cuts; may suffer internal bleeding
from bumps or bruises.
3. Duchenne Muscular Dystrophy
- A sex-linked disorder that results in the progressive weakening and loss of
skeletal muscle.
- 1 in 3000 males (US) has the condition.
Testcross – A Way to Determine Certain Genotypes
- A testcross is a genetic cross of an organism showing the recessive phenotype
with an organism showing the dominant phenotype.
- This is done to determine if the organism with the dominant phenotype is
homozygous dominant or heterozygous dominant.