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Lecture 5

Human Biology
Course Code
Stephen Wright

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Lecture 5
6.1 Interactions between the alleles of Single Gene: Variations on
1:2:1 ratio = inheritance pattern is based on TWO alleles of a single gene
but the heterozygotes = intermediate in phenotype => INCOMPLETE DOMINANCE
-INCOMPLETE DOMINANCE: situation in which a heterozygote shows a phenotype
quantitatively (but not exactly) intermediate between two homozygous phenotypes.
(Exact intermediacy means NO dominance)
Each wild type allele generally produces a set dose of its protein product
Number of doses of wild type allele determines the [ ] of a chemical made by the protein
(i.e pigment)
-Co-dominance: the expression of both alleles of a heterozygote
I.e human blood -> co-dominance in antigen alleles
ABO blood groups are determined by three alleles of one gene -> interact in several
ways = produce 4 different blood types
Major alleles = i , I B,IA person can only have TWO of the three alleles or two copies of
one of them; combinations result in 6 different genotypes : 3 homozygotes and
3 different types of heterozygotes
IA/IA & i /I A -> A -> IA is dominant over i
i / I B & I B/IB -> B -> IB is dominant over i
IA/IB -> AB -> each allele produces its own form of the antigen -> are co-dominant
i/i -> O
Alleles determine the presence and form of a complex sugar molecule present on the
surface of red blood cells. Sugar = antigen (cell surface molecule that can be
recognized by immune system), I B & IA = two different types of the antigen (i =
no antigen because is a NULL allele)
Sickle cell anemia -> gene concerned encodes for hemoglobin which is responsible for
transporting oxygen in blood vessel and is the major constituent of RBCs.
Two main alleles HbA (dominant -> if have this = no sickle cell)and HbS -> three different
HbA /HbA -> normal. No sickle cell
HbS/HbS -> severe, often fatal anemia abnormal hemoglobin causes RBCs to have
abnormal shape
HbA/HbS -> no anemia. RBC sickle only under low [O2] ; haplosufficient
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