Principles of Inheritance and Variation
Unit II, Genetics and Evolution. Covers Mendelian inheritance; Deviations from Mendelism; Chromosomal theory and linkage; Sex determination; Mutation and genetic disorders.
About this unit
Genetics studies how characters are passed from parents to offspring (inheritance) and why offspring differ from each other and from their parents (variation). This unit starts with Mendel's experiments on the garden pea and his laws, then moves to exceptions such as incomplete dominance, co-dominance, multiple alleles, pleiotropy and polygenic inheritance. It links Mendel's factors to chromosomes, explains linkage, sex determination, mutations and pedigree analysis, and describes common genetic disorders in humans.
Key ideas
- Monohybrid ratio 3:1 (phenotype) and 1:2:1 (genotype); dihybrid ratio 9:3:3:1.
- Law of dominance, law of segregation and law of independent assortment.
- Incomplete dominance gives 1:2:1 phenotypes; ABO blood groups show co-dominance and multiple alleles.
- Genes on the same chromosome are linked; recombination frequency reflects distance.
- Sex determination: XX–XY in humans, ZW–ZZ in birds, haplodiploidy in honey bees.
- Mendelian disorders arise from single-gene changes; chromosomal disorders from changes in chromosome number.
How to prepare
Practise Punnett squares until you can solve monohybrid, dihybrid, test cross and blood-group problems quickly. Learn pedigree symbols and how to recognise dominant, recessive and X-linked patterns. Memorise karyotypes of Down's, Turner's and Klinefelter's syndromes.
Topics in this chapter
Mendelian inheritance
Mendel’s laws of dominance, segregation and independent assortment; test and back crosses.
5 questionsDeviations from Mendelism
Incomplete dominance, co-dominance, multiple alleles and inheritance of blood groups, pleiotropy and polygenic inheritance.
5 questionsChromosomal theory and linkage
Chromosomal theory of inheritance; linkage and recombination.
5 questionsSex determination
Sex determination in humans, birds and honey bees.
5 questionsMutation and genetic disorders
Mutation, pedigree analysis, Mendelian disorders (colour blindness, haemophilia, sickle-cell anaemia, phenylketonuria, thalassaemia) and chromosomal disorders (Down’s, Turner’s and Klinefelter’s syndromes).
5 questions