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2. CODOMINANCE, INCOMPLETE DOMINANCE AND MULTIPLE ALLELES. Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123. Lecture outline . 2 nd Topic – extention of Mendelian Genetics Codominance Incomplete dominance Multiple alleles Lethal alleles Epistasis Polygenic inheritance

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2 codominance incomplete dominance and multiple alleles

2. CODOMINANCE, INCOMPLETE DOMINANCE AND MULTIPLE ALLELES

Compiled by

Siti Sarah Jumali

Level 3 Room 14

Ext 2123

lecture outline
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
codominance
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.
slide5

Pay attention on how to write codominance;

Writing it is with superscript

unlike law of segregation
Unlike Law of Segregation..
  • Using the calico cat as example, The genotype for fur can be represented as CB or CO.
  • CBis for black color and
  • CO denotes orange color
  • Therefore the codominance is written as
  • CBCBx COCO  100% CBCO
common situation of codominance
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

other example
Other example
  • Human blood type AB
    • Two types of protein A and B appear together on the surface of blood cells
question
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
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 Snapdragon

Rosendahl

example1
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
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??????
try this
Try this
  • 1. Predict the phenotypic ratios ofoffspring 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?
multiple 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 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 codominancebecause both IAand IBare fully expressed
several genes and the environment can influence a single multifactorial characteristic
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
slide25
Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences
slide30

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
slide31

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
Why is that?
  • Caused by tyrosinase; producing melanin
  • Two types of melanin: eumelanin (black pigment) and phaeomelanin (orange/yellow pigment)
slide34

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