Post on 18-Jan-2016
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Dominance and Multiple Allele Notes
Mendel’s Principles – A Review
• Inheritance of traits is determined by genes.
• Genes are passed from parents to offspring.
• Alleles can be dominant or recessive.• In sexually reproducing organisms –
each adult has two copies of each gene – one from each parent.
Exceptions to Mendel’s Work
• Some alleles are neither dominant or recessive.
• Many traits are controlled by multiple alleles or multiple genes.
Incomplete dominance – When the alleles are blended and the offspring have a mix of their parent traits.
ex. Snap Dragons
R = red
W = white
Offspring can be pink!
Codominance – in this case both alleles are expressed.
BB= black corn
YY= yellow corn
BY = black and yellow corn
Heterozygous genotype
Incomplete Dominance or Codominance?
Incomplete Dominance or Codominance?
Incomplete or Codominance?
Polygenic Traits• A polygenic trait is determined by multiple
genes. (poly=many, genic=genes)
Example: eye color and height
Skin color is controlled by more than four genes
Homologous chromosomes - Chromosomes occur in
pairs.(homologous means “same”)
- The different alleles of a
gene occupy the same positions on each chromosome
So far each gene we have discussed has been made of two possible alleles.
Ex. B = blue b= yellow
R = red r = white
However, it is possible to have several different allele possibilities for one gene.
Multiple alleles is when there are more than two allele possibilities for a gene.
Coat color in rabbits is determined by a single gene with 4 possible alleles.
In traits with multiple alleles, each individual can carry any two of the
several possible alleles.
Ex. BLOOD TYPE
The gene for blood type has 3 possible alleles.
IA, IB, and i
In this case both A and B are dominant to O (recessive).
A and B are codominant (both expressed)
So... there are four human blood typesGenotype Phenotype
IAIA, IAi Blood type A
IBIB, IBi Blood type B
IAIB Blood type AB
ii Blood Type O
Alleles for blood type are:-IA (dominant)-IB (dominant)-i (recessive)
Practice Problems:1. A mother is AB and a father is O. Draw the punnett
square.IA IB
i i
What is the probability their offspring will have A type
blood? ____ Can they have an offspring with O blood? ____
2. A mother is IAi and a father is IB IB. Draw the punnett square.
What is the probability their offspring will have B
type blood? Can they have an offspring with O blood?
3. If a child has AB blood which of the following could NOT be the parents?
a) ii, IAIB
b) IAi, IBIB
c) IBIB, IAIA
d) IAi, IBi