Principles of Inheritance and Variation
Class 12 · Biology
4 ideas in Class 12 Biology, explained in plain words, with games and quizzes to practise.
Monohybrid Cross and Laws
Part of: One Gene
True-breeding tall crossed with dwarf gives all tall F1, then 3 tall to 1 dwarf in F2 with genotypes 1 TT to 2 Tt to 1 tt. Dominant alleles mask recessive ones, pairs segregate so each gamete carries one allele, and crossing with the recessive parent reveals unknown genotypes.
Why it matters. Ratio reading and test-cross logic start every genetics paper.
Formulas
- F2 phenotype ratio is 3 dominant to 1 recessive
- F2 genotype ratio is 1 to 2 to 1
- Test cross of Tt gives a 1 to 1 ratio
Key words
- Allele
- : One of two forms of a gene.
- Homozygous
- : Carrying two identical alleles.
- Heterozygous
- : Carrying two different alleles.
- Genotype
- : The allele pair an organism holds.
- Phenotype
- : The visible trait expressed.
- Test cross
- : Crossing with the recessive parent.
Try it in two minutes
- Fill one Punnett square for TT by tt.
- Self the F1 and count 3 to 1.
- Test an unknown tall against tt.
Try at home. Explain why TT and Tt look alike but breed differently.
Dominance Variations and Two Genes
Part of: One Gene
Snapdragon red by white gives pink F1 with a 1 to 2 to 1 F2, while ABO groups show codominance with six genotypes and four phenotypes. Two genes assort into 9 to 3 to 3 to 1 unless linked, when parental combos dominate. Skin colour adds three genes with additive effects.
Why it matters. Ratio shifts signal which inheritance pattern a question tests.
Formulas
- Snapdragon F2 ratio is 1 red to 2 pink to 1 white
- Dihybrid phenotype ratio is 9 to 3 to 3 to 1
- Gamete types equal 2 raised to heterozygous loci
Key words
- Incomplete dominance
- : Heterozygote shows an intermediate trait.
- Codominance
- : Both alleles express together.
- Multiple alleles
- : Over two alleles in a population.
- Linkage
- : Joint travel of same-chromosome genes.
- Recombination
- : New combos from gene exchange.
- Polygenic trait
- : A trait summed over several genes.
- Pleiotropy
- : One gene affecting many traits.
Try it in two minutes
- Sort 3:1, 1:2:1 and 9:3:3:1 by pattern.
- Place six ABO genotypes into four groups.
- Split linked from assorted gene pairs.
Try at home. Explain why dihybrid boxes total sixteen.
Chromosomes and Male-Female Determination
Part of: Chromosomes Decide
Sutton and Boveri matched gene pairs to chromosome pairs, and Morgan proved it in fruit flies. Males may carry one X, an XY pair, or share ZZ while females are ZW; bees decide by chromosome sets with 32 in females and 16 in drones. Mutations from base changes to aneuploidy create variation.
Why it matters. Which gamete decides, and in which organism, is a classic trap.
Formulas
- Human cells hold 46 chromosomes in 23 pairs
- Drone bees hold 16, females hold 32
- Each pregnancy carries a half chance either way
Key words
- Chromosomal theory
- : Genes ride on chromosomes.
- Male heterogamety
- : Males making two gamete types.
- Female heterogamety
- : Females making two gamete types.
- Haplodiploidy
- : Males haploid, females diploid.
- Point mutation
- : A single base-pair change.
- Aneuploidy
- : Gain or loss of chromosomes.
Try it in two minutes
- Pair each organism with its system.
- Name the deciding gamete in each.
- Sort mutation from base to set.
Try at home. Explain why a drone has a grandfather but no father.
Mendelian and Chromosomal Disorders
Part of: When Genes Fail
Pedigrees trace single-gene disorders: X-linked recessive colour blindness and haemophilia strike males through carrier mothers, while sickle-cell, phenylketonuria and thalassemia are autosomal recessive. Extra or missing chromosomes cause Down (47, trisomy 21), Klinefelter (47, XXY) and Turner (45, X0).
Why it matters. Carrier-risk arithmetic and karyotype reading recur every year.
Formulas
- Normal human cells hold 46 chromosomes
- Down and Klinefelter hold 47, Turner holds 45
- Carrier by carrier gives one-fourth affected
Key words
- Pedigree
- : A family tree tracing traits.
- Carrier
- : A healthy heterozygote hiding a recessive allele.
- Haemophilia
- : X-linked clotting failure.
- Thalassemia
- : Too few globin chains made.
- Trisomy
- : An extra chromosome copy.
- Monosomy
- : A missing chromosome copy.
Try it in two minutes
- Mark X-linked paths mother to son.
- Halve carrier-by-carrier risks.
- Count 45, 46 and 47 karyotypes.
Try at home. Explain why haemophilic females are extremely rare.
What your child will practise
- Chromosomes and Male-Female Determination: Ocean Puzzle (Clue Puzzle)
- Chromosomes and Male-Female Determination: Ocean Quick Fire (Quick Fire)
- Dominance Variations and Two Genes: Jungle Puzzle (Clue Puzzle)
- Mendelian and Chromosomal Disorders: Candy Quick Fire (Quick Fire)
- Monohybrid Cross and Laws: Space Quest (Flip and Match)
- Quiz: Chromosomes and Male-Female Determination (8 questions)
- Quiz: Dominance Variations and Two Genes (8 questions)
- Quiz: Mendelian and Chromosomal Disorders (8 questions)
- Quiz: Monohybrid Cross and Laws (8 questions)