2. CODOMINANCE, INCOMPLETE DOMINANCE AND MULTIPLE ALLELES

Compiled bySiti Sarah JumaliLevel 3 Room 14

Ext 2123

Lecture outline

• 2nd Topic – extention of Mendelian Genetics• Codominance• Incomplete dominance• Multiple alleles• Lethal alleles• Epistasis• Polygenic inheritance• Linked genes• Crossover value and genes mapping• Sex linked genes

TWO ALLELES

CODOMINANCE

• Co- means together

codominant means equal in dominance• alleles are approximately equal effect in individuals;

alleles are equally detectable in individuals. • Phenotypes for both alleles are exhibited in the

heterozygote • The hybrid shows neither of the parents’ trait,

instead, a third, different phenotype• Examples of this is blood types; ABO, iAiB.

Pay attention on how to write codominance;Writing it is with superscript

example

Calico cat

Unlike Law of Segregation..

• Using the calico cat as example, The genotype for fur can be represented as CB or CO.

• CB is for black color and• CO denotes orange color • Therefore the codominance is written as• CBCB x COCO 100% CBCO

Common situation of Codominance

• Common phenotype used is roan fur in cattle• Cattle can either be all red RR;

all white WW or;

Roan RW

Roan cattle

Other example

• Human blood type AB– Two types of protein A and B appear together on

the surface of blood cells

Question:

• What is the probability of a child having type AB blood if one of the parents is heterozygous for A blood and the other is heterozygous for B blood?

• What other genotypes are possible for children of these parents?

INCOMPLETE DOMINANCE

• A form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele.

• The heterozygote has an intermediate phenotype between that of either homozygote

• This results in a combined phenotype.

Pink SnapdragonRosendahl

Example

• In cross-pollination experiments between red and white snapdragon plants, the resulting offspring are pink.

• The dominant allele that produces the red color is not completely expressed over the recessive allele that produces the white color. 

Incomplete dominance eg

• Crossing of organisms that has two different phenotypes produces offspring with another different phenotype which is a blend of the parental traits

• Examples??????

Snakes

Test cross for Incomplete dominance

Punnet Square for Incomplete dominance

Try this

• 1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

• 2. What should the genotypes & phenotypes for parent cattle be if a farmer only wanted a cattle with white fur

• 3. A cross between a black cat & a tan cat produces a tabby pattern (black and tan fur together)?– A) What pattern of inheritance is this?– B) What percent of kittens would have tan fur if a tabby

cat is crossed with a black cat?

THREE ALLELES

MULTIPLE ALLELES

• More than three alleles• Many genes have multiple alleles• Three or more different alleles• Exclude dominant and recessive effects• All alleles show its own effects in inheritance• Examples: Blood type, hair color

Multiple alleles cont’d

• Multiple alleles - gene has several allelic forms– Example: blood type is determined by multiple alleles

• IA = A antigen on red blood cells• IB = B antigen on red blood cells• i = Neither A nor B antigen on red blood cells

– Possible phenotypes and genotypes for blood type:

• This is an example of codominance because both IA and IB are fully expressed

Several genes and the environment can influence a single

multifactorial characteristic

• Polygenic inheritance occurs when a trait is governed by two or more genes– Multifactorial traits - controlled by polygenes

subject to environmental influences

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Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences

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Iris color

The Human Blood

Skin color

• An interesting example is coat color in rabbits– Four different alleles

• C (full coat color)• cch (chinchilla pattern of coat color)

– Partial defect in pigmentation

• ch (himalayan pattern of coat color)– Pigmentation in only certain parts of the body

• c (albino)– Lack of pigmentation

– The dominance hierarchy is as follows:• C > cch > ch > c

– Figure 4 illustrates the relationship between phenotype and genotype

Phenotype Genotype• Agouti (wild type) c+c+, c+cch, c+ch, c+c• Chinchilla (mutant) cchcch

• Himalayan(mutant) chch,chc• Light grey cchch, cchc• Albino (mutant) cc

FIGURE 4

Why is that?

• Caused by tyrosinase; producing melanin• Two types of melanin: eumelanin (black

pigment) and phaeomelanin (orange/yellow pigment)

• The himalayan pattern of coat color is an example of a temperature-sensitive conditional allele – The enzyme encoded by this gene is functional only at

low temperatures• Therefore, dark fur will only occur in cooler areas of the body• This is also the case in the Siamese pattern of coat color in cats