BIOL10005 Lecture Notes - Lecture 5: Dihybrid Cross, Mendelian Inheritance, Abo Blood Group System
Lecture 5 - Wednesday 3 August 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 5
MULTIPLE ALLELES & OTHER EXTENSIONS
TO MENDEL
CHAPTER 9 - INHERITANCE
INHERITANCE OF A SINGLE GENE
BLOOD GROUPS, CODOMINANCE AND MULTIPLE ALLELES
•Although an individual can only carry a max of 2 alleles, more than 2 alleles of a gene can exist
in a population.
•The ABO blood group is an example of a gene that has multiple allelic forms in a population. 3
alleles, IA, IB and IO influence the ABO blood type of individuals.
•In some cases, the phenotype of both alleles is evident in a heterozygote. The phenotype here is
the tendency of RBCs from one person to clump (agglutinate) when exposed to blood serum from
another individual.
•Blood groups are due to two different carbohydrates on the surface of RBCs whose presence or
absence is determined by the ABO gene on chromosome 9.
•We do not produce antibodies against our own antigens.
•Group A people have A antigen on the plasma membranes of their RBCs and anti-B antibodies.
•Group B people have B antigen on their cells and anti-A antibodies.
•Group O people have neither A or B antigen on their cells and have both anti-A and anti-B
antibodies in their serum. Group O serum will clump all other red cell types, A, B and AB.
•AB people have both A and B antigens on their RBCs but their serum has neither anti-A or anti-
B antibodies. Cells from an Ab individual are clumped by serum from either A or B but do not
clump other RBC types themselves.
•Antibodies in serum bind to their target antigen on the foreign BCs, causing clumping. Thus a
group A person’s serum will clump a group B person’s cells and vice versa.
INHERITANCE OF COMBINATIONS OF GENES
DIHYBRID CROSS
•Mendel did crosses between pure breeding lines differing in two unrelated characters; seed shape
and seed colour.
•The dihybrid cross occurred when the F1 plants were allowed to self pollinate. Firstly, when this
happened, 4 different types of F2 progeny seed were created, showing that a dihybrid cross can
produce new combinations of traits that weren’t present in the parents. This suggests that the
alleles of the different genes were being shuffled with respect to each other.
•Secondly, particular ratios of different phenotypes are consistently observed.
•The ratio 9:3:3:1 can be derived from knowledge of the segregation of alleles and phenotypic
dominance. Thus, the alleles of two different genes assort independently of one another when they
segregate into the gametes.
•Say Y = yellow allele, y = green allele, R = round seed allele and r = wrinkled seed allele.
•Parents: YYRR x eery.
•Fq: YyRr, yellow round.
•Cross 2 parents: YyRr x YyRr.
•F2: YYRR, YYRr, YyRR & YyRr = 9 yellow round
• YYrr & Yyrr = 3 yellow wrinkled
• yyRR & yyRr = 3 green round
• Yyrr = 1 green wrinkled
•The 9:3:3:1 ratio is a result of the two independent 3:1 segregations.
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Document Summary
Biol10005 - genetics & the evolution of life. 3 alleles, ia, ib and io influence the abo blood type of individuals: in some cases, the phenotype of both alleles is evident in a heterozygote. Group o serum will clump all other red cell types, a, b and ab: ab people have both a and b antigens on their rbcs but their serum has neither anti-a or anti- Cells from an ab individual are clumped by serum from either a or b but do not clump other rbc types themselves: antibodies in serum bind to their target antigen on the foreign bcs, causing clumping. Thus a group a person"s serum will clump a group b person"s cells and vice versa. Dihybrid cross: mendel did crosses between pure breeding lines differing in two unrelated characters; seed shape and seed colour, the dihybrid cross occurred when the f1 plants were allowed to self pollinate.
