04Class 12 Biology
Principles of Inheritance and Variation short notes
Short notes on Mendel's laws, non-Mendelian inheritance, sex determination and genetic disorders. Practise crosses, since many questions hinge on working one out.
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Mendel's work
- He studied seven traits of pea plants in true-breeding lines.
- Law of dominance: one allele can mask the other in the heterozygote.
- Law of segregation: alleles separate when gametes form.
- Law of independent assortment: genes on different chromosomes assort independently.
- Monohybrid F2 ratios are 3:1 for phenotype and 1:2:1 for genotype. The dihybrid F2 phenotype ratio is 9:3:3:1.
Other patterns
- Incomplete dominance: F1 is intermediate, as in snapdragon, and F2 is 1:2:1 for both phenotype and genotype.
- Codominance: both alleles show fully, as in the AB blood group.
- Multiple alleles: the ABO system has three alleles (Iᴬ, Iᴮ and i).
- Linkage: genes close together on a chromosome tend to be inherited together, which Morgan showed in fruit flies.
- Polygenic traits, such as human skin colour, depend on several genes.
- Pleiotropy: one gene affects several traits, as in phenylketonuria.
Sex determination
- Humans and fruit flies are XY, with XY males.
- Birds are ZW, with ZW females.
- Grasshoppers are XO, with XO males.
- Honeybees are haplodiploid: males develop from unfertilised eggs and are haploid.
Disorders
- Mendelian: sickle-cell anaemia and phenylketonuria (autosomal recessive), and haemophilia and colour blindness (X-linked recessive).
- Down's syndrome: trisomy 21.
- Klinefelter's syndrome: 47, XXY.
- Turner's syndrome: 45, XO.
Where students slip up
- Mixing up incomplete dominance and codominance. In one the phenotype blends, and in the other both show.
- Assuming all sex determination is XY. Birds and grasshoppers differ.
- Forgetting X-linked recessive traits are more common in males, who have just one X.
Try it on your weakest chapter.
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