Download 122 lec 03 probs dominance mult alleles

Figure 14.8 Segregation of alleles and fertilization as chance events
Today’s Objectives
• Use rules of probability to solve genetics
problems
• Define dominance, incomplete dominance,
and co-dominance
• Extend Mendelian principles to one locus
with > 2 alleles
Probability rules apply to inheritance at
more than one locus
• Example 1- dihybrid cross
– One locus analysis
• What is the probability of the dominant phenotype at
the color locus? The recessive phenotype?
• What is the probability of the dominant phenotype at
the shape locus?
– Combined analysis
• What is the probability of the dominant phenotype at
both loci?
• What is the probability of the recessive phenotype at
both loci?
• What is the probability of the dominant phenotype at
one locus and the recessive phenotype at the other
one?
Example 2: Cross between F1 individuals
that are heterozygous at two loci
AaBb X AaBb
– What is the probability of a recessive
homozygote at locus ‘A’ and a
heterozygote at locus ‘B’?
– Write out genotype: aaBb
– Determine probability of each
– Use rules of probability to combine
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Example 2 solution
• Probability of recessive homozygote
at one locus
P(aa) = 1/4
• Probability of heterozygote at other
locus
P(Bb) = 1/2
• Use multiplication rule to combine
probabilities from both loci
Example 2: five loci
• Given: One crosses two individuals
that are heterozygous at 5 loci
AaBbCcDdEe X AaBbCcDdEe
• What is the probability that an
offspring will be a recessive
homozygote at all 5 loci?
aabbccddee
P(aaBb) =1/4 * 1/2 = 1/8
Example 2 solution
Example 3: three loci
• Probability of recessive homozygote
at each locus
• Given: One crosses two individuals
that are heterozygous at 3 loci
• What is the probability of a recessive
homozygote at one locus and a
heterozygote at the two other loci?
= 1/4
• Use multiplication rule to combine
probabilities
= (1/4)5
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Example 3: solution
• Write out possible ways of getting it
AaBbcc
aaBbCc
AabbCc
• Calculate probabilities of each way
AaBbcc = 1/2*1/2*1/4
aaBbCc= 1/4*1/2*1/2
AabbCc= 1/2*1/4*1/2
• Calculate sum of probabilities
Question 4, study guide 1
• Which genotypes might an individual’s
parents be if that individual possesses the
dominant phenotype at a locus?
• Genotypes for dominant phenotype: Aa, AA
• Parents:
–
–
–
–
–
Aa X Aa
Aa X AA
AA X aa
AA X AA
Aa X aa
1/16 +1/16 +1/16
Dominance relationships
• Complete dominance- phenotype of
heterozygote identical to one of the
homozygotes
• Incomplete dominance- phenotype of
heterozygote between the homozygotes
• Codominance- Phenotype of
heterozygote shows both homozygotes
Dominance does not:
• Mean that one allele ‘dominates’ or
‘subdues’ the other one.
• Relate to the frequency of an allele
(e.g. most peas are green, although
the yellow allele is dominant to the
green one)
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Figure 14.9x Incomplete dominance in carnations
Co-dominance
• Can detect both alleles in the
heterozygote.
• Codominance is usually found by
conducting chemical tests or
analyzing molecules
Figure 14.9 Incomplete dominance in snapdragon color
Multiple alleles at one locus
• Each individual is diploid and can be a
homozygote or a heterozygote
A1A1, A1A2, A2A2
• A population may contain more than two
alleles
A1, A2, A3
• With three alleles, three homozygotes
and three heterozygotes are possible
A1A1, A1A2, A1A3, A2A2, A2A3 , A3A3
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Codominance at an enzyme locus
using electrophoresis
F
M
S
1
2
3
4
•Proteins separated by
electrophoresis.
•3 alleles in five individuals
•Individuals with two bands
are heterozygotes
•Individuals with one band
5 are homozygotes
Human blood type example
• One locus determines blood type
• Three alleles are common in human
populations
• Two alleles are dominant to the third
• The ‘dominant’ alleles are
codominant with each other
MS MM FS MM MS
Figure 14.10 Multiple alleles for the ABO blood groups
Blood type phenotypes
• Four phenotypes exist: A, B, AB, O
• A substance, B substance or no
substance coats blood cells
• Blood type determined by whether
antibodies react to substance
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Blood type genotypes
• Phenotypes determined by three
alleles- Ia, Ib, i
– Ia and Ib are codominant
– i is recessive
• Possible genotypes:
– Ia Ia homozygote, Iai heterozygote,
– Ib Ib homozygote, Ib i heterozygote,
– ii homozygote, IaIb heterozygote
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