Non-Mendelian genetics — A/L Biology

What must I remember about Non-Mendelian genetics?

• Incomplete dominance (Intermediate inheritance): Heterozygote displays an intermediate phenotype between the two homozygotes (e.g., Snapdragons: Red RR × White WW produces Pink RW, phenotypic ratio 1:2:1).
• Codominance: Both alleles are equally and simultaneously expressed in the heterozygote without blending (e.g., ABO blood group system: IA and IB are codominant, producing type AB blood).
• Multiple alleles: More than two allelic forms of a gene exist in the population, though each diploid organism inherits only two (e.g., ABO blood groups controlled by IA, IB, and i alleles).
• Sex-linked inheritance: Genes located on sex chromosomes (primarily X chromosome); X-linked recessive disorders (hemophilia, red-green color blindness) affect males more frequently due to hemizygosity.
• Polygenic inheritance: Quantitative continuous phenotypic variation determined by the additive effect of two or more independent genes (e.g., human skin color, height).

Test yourself (1 minute)

1. ABO blood group genetics table

   Blood group A = IAIA or IAi; Blood group B = IBIB or IBi; Blood group AB = IAIB (codominant); Blood group O = ii (universal donor).

2. Criss-cross inheritance of X-linked traits

   Carrier mothers pass the recessive trait to 50% of their sons; affected fathers cannot pass X-linked traits to sons (only to daughters as carriers).

3. Epistasis and gene interaction

   One gene masks or modifies the phenotypic expression of a second independent gene (deviating from 9:3:3:1 dihybrid ratio to 9:7, 12:3:1, or 9:3:4).

Practise Non-Mendelian genetics questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 6

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Next in the syllabus

Where does Non-Mendelian genetics fit in the syllabus?

Year 1 · Term 2