The Genetics Problem Toolkit

Most hard NEET questions on this chapter are problems. Keep this toolkit ready.

Ratios and counting

  • Monohybrid F2: phenotype 3 : 1, genotype 1 : 2 : 1. Incomplete dominance / co-dominance F2: 1 : 2 : 1 for both phenotype and genotype.
  • Dihybrid F2: phenotype 9 : 3 : 3 : 1, and the F2 has 9 genotypes and 4 phenotypes.
  • Test cross: monohybrid 1 : 1, dihybrid 1 : 1 : 1 : 1 (only if the genes are unlinked).
  • Number of gamete types from a hybrid = 2^n, where n = number of heterozygous gene pairs (Aa -> 2, AaBb -> 4, AaBbCc -> 8).
  • In a self-cross with complete dominance, number of phenotypes = 2^n and number of genotypes = 3^n.

Probability

  • Use multiplication for independent events happening together ('and'): e.g. the chance an AaBb x AaBb cross gives an aabb child = 1/4 x 1/4 = 1/16.
  • Use addition for mutually exclusive outcomes ('either/or').

Blood-group and sex-linked logic

  • A group O (ii) child needs an i from each parent; a parent who is I^A I^B can never give an O child. An AB parent can never have an O child, and two O parents can have only O children.
  • X-linked recessive (haemophilia, colour blindness): a carrier mother (X^H X^h) x normal father (X^H Y) gives 1/2 of sons affected and 0 daughters affected (daughters may be carriers). An affected daughter needs an affected father AND a carrier/affected mother. Note the criss-cross pattern - an affected male passes the allele to all his daughters (carriers), who pass it to half their sons.

Traps: gamete types = 2^n (n = heterozygous pairs); AB x O never gives O or AB… it gives A and B children; X-linked recessive affects more males, and there is no father-to-son transmission of an X-linked gene.

Reading Pedigrees and Fine Distinctions

Pedigree clues

  • Autosomal recessive - the trait skips generations; two unaffected parents can have an affected child (both parents are carriers); males and females affected equally.
  • Autosomal dominant - the trait appears in every generation; every affected child has at least one affected parent; it does not skip.
  • X-linked recessive - many more males are affected than females; an affected male never passes it to his sons (he gives them his Y), but passes it to all his daughters as carriers.

Fine distinctions worth memorising

  • Incomplete dominance vs co-dominance: in incomplete dominance the heterozygote is intermediate (pink); in co-dominance both alleles are fully expressed (AB blood group) - no blending.
  • Linkage vs independent assortment: linked genes (same chromosome) give more parental types and a distorted test-cross ratio; unlinked genes give 1 : 1 : 1 : 1.
  • Sickle-cell vs thalassaemia: both are autosomal recessive haemoglobin disorders, but sickle-cell is a qualitative defect (a structurally abnormal globin, HbS), whereas thalassaemia is a quantitative defect (too little normal globin).
  • Pleiotropy vs polygeny: pleiotropy = one gene -> many traits; polygeny = many genes -> one trait (additive, continuous).
  • Chromosomal disorders by number: Down's = 47 (trisomy 21), Klinefelter's = 47 (XXY), Turner's = 45 (X0). Down's and Klinefelter's both have 47 chromosomes but differ in which chromosome is extra.
  • Aneuploidy vs polyploidy: aneuploidy = gain/loss of one chromosome (non-disjunction); polyploidy = extra whole sets.

Assertion-Reason watch-outs: the Law of Segregation has no exceptions, but the Law of Independent Assortment fails for linked genes. Incomplete dominance and co-dominance do not violate segregation - the alleles still segregate; only the dominance relationship differs.