Genetic Basis of Variation

Learning Outcomes

Use genetic diagrams to solve problems involving monohybrid inheritance. (Genetic diagrams involving autosomal linkage or epistasis are not required)
Explain co-dominance and multiple alleles with reference to the inheritance of the ABO group phenotypes – A, B, AB, O, gene alleles IA, IB, and IO.

🕜Estimated Time: 1 hour

Learning Outcomes

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Genetic Diagrams

Monohybrid inheritance involves only ONE pair of contrasting trait.
- - Height – Tall or dwarf
Genetic diagram is used to explain how the traits are being passed down from the parents to the offsprings.
Given that the tall plant is homozygous dominant (pure-bred) for height and the dwarf plant is homozygous recessive (pure-bred) for height.

Test Cross

A tall plant has two possible genotypes – TT , or Tt
- - Since the dominant trait (T) is expressed in either homozygous dominant (TT) or heterozygous (Tt) form.

The genotype of an organism showing the dominant trait can be determined by crossing it with an organism that is a homozygous recessive (tt).
- - This is known as test cross.

Codominance

In the previous cross, it is an example of complete dominance.
- - Where one allele (T) is completely dominant over the other (t).
  - Thus heterozygous plant (Tt) shows the dominant trait of tall.
However, there are other cases where neither allele is completely dominant over the other.
- - Both the dominant allele and the recessive allele express themselves equally in the heterozygous form.
  - E.g. In the example given on the right, the heterozygote is oval, a phenotype that is intermediate between long and round.

Multiple Alleles

So far, we have only assumed that there are only 2 possible alleles for a given trait.
- - T and t for height in pea plant
There may be more than 2 alleles for a given trait.
- - If a gene exists in more than 2 alleles, it is said to have multiple alleles.
For example, in human ABO blood group, there are 3 alleles – IA, IB and IO.
- - IA and IB are dominant over IO
  - IA and IB are codominant.

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