Principles of Inheritance — Part 2: Beyond Mendel, pedigrees and linkage: visual lesson, ISC Class 12 Biology

Lesson

Deviations from Mendelism

Part 2 — cases that Mendel’s simple rules do not fully describe.

In this lesson you will

Explain incomplete dominance, codominance and multiple alleles
Explain polygenic inheritance and pleiotropy with examples
Analyse pedigrees
Explain the chromosomal theory of inheritance, linkage and crossing over

Diagram

Incomplete dominance: snapdragon

In the snapdragon (dog flower), a cross between true-breeding red (RR) and white (rr) gives pink (Rr) F1 plants. The F2 shows red : pink : white = 1 : 2 : 1, so the phenotypic ratio equals the genotypic ratio.

Concept

Codominance and multiple alleles: ABO blood groups

The ABO blood group is controlled by gene I with three alleles, Iᴬ, Iᴮ and i. Iᴬ and Iᴮ produce slightly different sugars on red cells; i produces none. Iᴬ and Iᴮ are codominant (both expressed in Iᴬ Iᴮ), and both are dominant over i. A person carries only two of the three alleles.

Table of ABO genotypes and blood groups
Genotype	Blood group
IᴬIᴬ, Iᴬi	A
IᴮIᴮ, Iᴮi	B
IᴬIᴮ	AB (codominance)
ii	O

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