BIOL10005 Lecture Notes - Lecture 11: Mendelian Inheritance, Abo Blood Group System, Zygosity
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 11
MAPPING THREE GENES & BLOOD GROUPS
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.
LINKAGE ON AUTOSOMES
LINKAGE & RECOMBINATION
•Multiple genes reside on each chromosome.
•Homologous chromosomes carry the same set of genes, each homologue carrying one allele of
each gene. Homologs segregate at meiosis, as do the alleles that reside on them (accounting for
Mendel’s Principle of Segregation).
•Alleles of different genes located on the same chromosome should be transmitted together; they
should be linked and do not follow Mendel’s Principle of Independent Assortment.
•However, linkage between genes is never complete because of the crossing over between
homologous chromosomes.
•Crossing over leads to recombination between genes on a pair of homologs, and the level of
recombination varies greatly. If the genes are close together, alleles of the genes may recombine
only rarely, but if separated far enough apart, the high frequency of crossing over can mean that
alleles of the two genes appear to sort independently as if they were on different chromosomes.
•The locus of a gene on a chromosome is its position.
•D direct test for independent assortment is to test cross a double heterozygote with a double
recessive homozygote.
•Eg. Aa;Bb x aa;bb
•From Mendel’s Principle of Independent Assortment, the 4 possible combinations of gametes
arising from meiosis in the double heterozygote are AB, Ab, aB and ab, and they are expected to
form in equal proportions.
•Thus, the 4 possible genotypes of the test gross offspring (A/a;B/b, A/a;b/b, a/a;B/b and a/a;b/
b) are expected to appear in the ratio 1:1:1:1 if the two genes do sort independently.
•Deviations from the ratio indicate that independent assortment has not occurred.
Document Summary
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. Linkage & recombination: multiple genes reside on each chromosome, homologous chromosomes carry the same set of genes, each homologue carrying one allele of each gene. Homologs segregate at meiosis, as do the alleles that reside on them (accounting for.
