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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 1 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 2 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) 3 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 4 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 5 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 6