Download multiple alleles lecture

Multiple
Alleles
allele = (n) a form of a gene which codes for one
possible outcome of a phenotype
For example, in Mendel's pea investigations, he
found that there was a gene that determined the
color of the pea pod. One form of it (one allele)
creates yellow pods, & the other form (allele)
creates green pods.
Two possible phenotypes of one trait (pod color)
are determined by two alleles (forms) of the one
"color" gene.
Review of incomplete dominance
GENOTYPES
BB = Homozygous Dominant
Bb = Heterozygous
bb = Homozygous recessive
RESULTING PHENOTYPE
BB = Black Fur
Bb = Grey Fur
bb = White Fur
Review of codominance
GENOTYPES
BB = Homozygous ?
BW = Heterozygous
WW = Homozygous ?
RESULTING PHENOTYPE
BB = Black Fur
BW = Black AND White Fur
WW = White Fur
• There may be multiple alleles within the
population, but individuals have only TWO
of those alleles.
Why?
Because individuals have only two biological
parents. We inherit half of our genes
(alleles) from mom, & the other half from
dad, so we end up with two alleles for
every trait in our phenotype.
Example: Human Blood Type
• An excellent example of multiple allele
inheritance is human blood type. Blood
type exists as four possible phenotypes: A,
B, AB, & O.
• There are 3 alleles for the gene that determines
blood type.
(Remember: You have just 2 of the 3 in your genotype --- 1 from mom & 1 from dad).
• The alleles are as follows:
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
• According to the symbols used in the table below,
the allele for "O" (i) is recessive to the alleles for "A"
& "B".
• With three alleles we have a higher number of
possible combinations in creating a genotype.
GENOTYPES
IAIA
IAi
IBIB
IBi
IAIB
ii
RESULTING PHENOTYPES
Type A
Type A
Type B
Type B
Type AB
Type O
Sample Problem
A woman with Type O blood and a man who
is Type AB are expecting a child. What
are the possible blood types of the kid?
Solve this using the symbols for blood type alleles & the good
old punnett square.
1. Figure out the genotypes of mom & dad using the given info.
2. "Woman with Type O" must be ii, because that is the one & only
genotype for Type O.
3. "Man who is AB" must be IAIB, again because it is the one & only
genotype for AB blood.
4. So our cross is: ii x IAIB. Make a punnett square
As you can see, our results are as follows:
50% of kids will be heterozygous with blood Type A
50% will be heterozygous with blood Type B
Multiple Alleles vs. Polygenic
• Polygenic = is the inheritance pattern of a
trait that is controlled by two or more
genes. Each gene may have two or more
alleles
• The resulting phenotype usually shows a
range of value for the trait. (ie. human
height or human hair color)