HMB265H1 Lecture Notes - Lecture 5: Glycosyltransferase, Mendelian Inheritance, Antigen
20 May 2018
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Lecture 5: Extensions of Mendelian Genetics Part 1
Dominance is not always complete
• Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents
• Incomplete Dominance:
o F1 hybrids that differ from bith parents express an intermediate
phenotype. Neither allele is dominant nor recessive to the other.
o Phenotypic ratios are same as genotypic ratios
• Codominance
o F1 hybrids express the phenotype of both parents equally
o Phenotypic ratios are same as genotypic ratios
o Equality between both parents
Incomplete Dominance
• The heterozygote phenotype is distinct from either homozygous phenotype – an intermediate phenotype
o Familial Hypercholesteraemia
▪ Excess amount of cholesterol being made
A Gene can have more than two alleles
• Genes may have multiple alleles that segregate in populations
• Although there may be many alleles in a population, each individual carries only 2 of the alterantives
o ABO blood group gene: I
▪ 3 alleles: IA, IB, and i
▪ 6 possible ABO genotypes: IAIA, IBIB, IAIB, IAi, IBi, or ii
• Dominance relations are unique to a pair of alleles
o Dominance or recessiveness is always relative to a second allele
o ABO blood group
▪ IA is completely dominant to i but codominant to IB
▪ 6 genotypes generate 4 phenotypes: Type A, B, AB, or O
Codominant Alleles
• Both allels are expressed in the heterozygotes
o The ABO gene encodes a cell surface protein: glycosyltransferase ( specifies the form and presence of an
antigen on the surface of a RBC)
▪ A allele produces A antigen
▪ B allele produces B antigen
▪ O allele does not produce antigens
▪ A and B antigens may be present on the same cell
▪ Alleles A and B are codominant
• If you get blood from someone with a different blood type, the immune system will attack
the different antigens
o Blood will aggregate and lead to death
Establishing Dominance Relations
• Perform reciprocal crosses between pure-breeding lines of all phenotypes
One gene may contribute to several visible characteristics
• Pleiotropy—single gene determines more than one distinct and seemingly unrelated
characteristics
• Some alleles may cause lethality
o Type of pleiotropy where alleles produce a visible phenotype and affect viability
o Alleles that affect viability often produce deviations from a 1:2:1 genotypic and
3:1 phenotypic ratio predictive by Mendel’s Laws
▪ Alleles that affect survival are lethal and usually need to be homozygous
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Document Summary
Lecture 5: extensions of mendelian genetics part 1. Dominance is not always complete: crosses between true-breeding strains can produce hybrids with phenotypes different from both parents. Incomplete dominance: f1 hybrids that differ from bith parents express an intermediate phenotype. Neither allele is dominant nor recessive to the other: phenotypic ratios are same as genotypic ratios, codominance, f1 hybrids express the phenotype of both parents equally, phenotypic ratios are same as genotypic ratios, equality between both parents. Incomplete dominance: the heterozygote phenotype is distinct from either homozygous phenotype an intermediate phenotype, familial hypercholesteraemia, excess amount of cholesterol being made. Ia is completely dominant to i but codominant to ib: 6 genotypes generate 4 phenotypes: type a, b, ab, or o. If you get blood from someone with a different blood type, the immune system will attack the different antigens: blood will aggregate and lead to death. Establishing dominance relations: perform reciprocal crosses between pure-breeding lines of all phenotypes.
