Chapter 4 – Modification of Mendelian Ratios
• Wild Type Allele
– Allele that occurs most frequently in population
– Designated as “+” or “wt”
• Mutant Allele
– Loss of Function mutation
• Null mutation if complete loss of function
– Gain of Function mutation
• Usually dominant
– Neutral mutation
• No effect
• When one allele is not completely dominant to another then it is said to show incomplete or
• The phenotype of the heterozygote is intermediate to those of the homozygotes.
• Classic example:
– Flower colour in snapdragons:
• Red flowered x White flowered
– Progeny are all pink flowered
Palomino Horses: Incomplete Dominance
• Palominos do not breed true when bred together:
– Progeny are 1/4 cremellos (extremely light colored), 1/2 palominos and 1/4 light
– 1:2:1 ratio is characteristic of incomplete dominacre. cr cr
• C/C produces a horse that is light chestnut (or sorrel), C/C palamino, and C /C the cremello.
– Other coat color genes are necessary but are at different loci.
– It is thought that there up to 7 different loci that contribute to the coat color of horses.
• In other words the palamino is a slight dilution of the sorrel, and the cremello is a further
dilution to give an even paler colour.
• The heterozygote displays both phenotypes of the two homozygotes.
– Incomplete dominance displayed intermediate phenotype between the two
• The ABO blood group i and i alleles are codominant alleles.
– Individuals can be AB blood type.
• Human M-NMbloMd group. M N N N
– L /L (homozygote), L L (heterozygote), L L (homozygote).
– Antigens present on surface of RBCs. The Nature of the ABO Blood Groups
• Red blood cells (RBCs) contain complex polysaccharides linked to the lipids in their membranes.
– These polysaccharides are on the outside of the RBCs.
– These are antigens, since in organisms where this molecule is foreign, antibodies will be
produced against it.
• People with type A blood have A antigens on their RBCs, while type B individuals have type B
antigens. Type AB individuals have both A and B antigens present on their RBCs.
• ABO locus encodes glycosyltransferases, enzymes that add sugar groups to existing
• RBCs have a precursor glycolipid called the H antigen present.
– The A allele produces a glycosyltransferase enzyme that adds a -N-
acetylgalactosamine to the H antigen.
• Produces the A antigen.
– The B allele produces a different glycosyltransferase that adds galactose to the H
• Produce the B antigen.
– Type O individuals lack these glycosyltransferase enzymes, so the H antigen is
Antibodies to the ABO Antigens
• People with blood type O
– Have anti-A and anti-B antibodies
• People with blood type A
– Have anti-B antibodies
• People with blood type B
– Have anti-A antibodies
• People with blood type AB
– Have not antibodies to A or B
• What about the H antigen?
– It is shared amongst all these individuals so antibodies are not raised against this
• The H antigen is produced by the H allele (FUT1 gene) at a distinct locus from the ABO locus.
• People who are homozygous h/h do not make the H antigen.
– Regardless of ABO locus, these individuals are similar to blood type O individuals.
• They lack the A and B antigens.
– Produce anti-O antibodies (antibodies against the H antigen).
– These people have the Bombay phenotype.
• Gene products essential to an organism’s survival need to be present.
– One copy may be sufficient, although in some cases it may lead to developmental
• Eg. Stunted growth • Two types of lethal alleles:
– Recessive lethals -essentially as described above
• Eg yellow coat colour allele in mice
– Dominant lethal - only need one copy for lethality
• Eg Huntington disease
• Gene interaction where several genes may influence a particular trait.
• Epistasis is where one gene or gene pair modifies or masks the expression of another gene.
– Three types:
• Recessive epistasis eg. Bombay phenotype over ABO locus
• Dominant epistasis eg. Fruit color in squash
• Duplicate Recessive eg. White flowered sweat peas
– Also called, Complementary Gene Action
• In dominant epistasis, A_B_ and A_bb individuals have the same phenotype.
– Observe 12:3:1 ratio rather than 9:3:3:1 ratio.
– One gene when dominant is epistatic to the other gene.
• Summer squash example:
– Three common fruit colours: white, yellow and green.
• White x Yellow gi