Principles of Inheritance and Variation · Class 12 NEET Notes

• This is one of the highest scoring chapters in NEET Biology, and it is almost all reasoning.
• Learn the logic once and you can derive every ratio, instead of memorising a list of them.
• Blue marks a defining idea. Red marks a trap.
• Section 9 gathers what NEET keeps repeating. Read the chapter first, then use it.

• Mendel worked on the garden pea, Pisum sativum, for seven years.
• He chose seven pairs of contrasting characters, each with two clear-cut forms.
• The pea was a good choice: it is normally self-pollinating, so pure lines are easy to keep.
• It also has a short life cycle, many offspring, and flowers that can be hand-pollinated.
• The seven traits he chose happened to be on different chromosomes, or far apart on the same one. Had they been linked, he would not have found independent assortment.

• One character, two contrasting forms. Cross a pure tall with a pure dwarf.
• The F1 generation is all tall. The dwarf trait has vanished, not been destroyed.
• Self the F1 and the dwarf reappears in a quarter of the F2.

• Law of Dominance: of a contrasting pair, one form expresses itself and the other stays hidden.
• Law of Segregation: the two alleles separate during gamete formation, so each gamete carries only one.
• Phenotype ratio 3 : 1, but genotype ratio 1 : 2 : 1. Read which one the question wants.

• A tall plant may be TT or Tt. You cannot tell by looking.
• So cross it with the recessive parent, tt.
• The recessive parent contributes only t, so whatever the unknown carries will show.

• Two characters at once: seed shape and seed colour.
• Cross round yellow with wrinkled green, and the F1 is all round yellow.
• The F2 shows four phenotypes, including two combinations neither parent had.

• Law of Independent Assortment: each pair of traits assorts independently of the other.
• The quickest route is to treat each trait as its own 3 : 1 and multiply the two.
• That is why new parental combinations appear in the F2.

• Dominance is not always complete, and one gene does not always act alone.

• Incomplete dominance: the hybrid is intermediate. Red and white snapdragon give pink. F2 is 1 : 2 : 1 for both genotype and phenotype.
• Codominance: both alleles show fully and separately. The AB blood group.
• Multiple alleles: a gene has more than two alleles in the population, but any individual still carries only two. ABO blood grouping.
• Pleiotropy: one gene affects several traits. Phenylketonuria and sickle cell anaemia.
• Polygenic inheritance: several genes control one trait, giving a continuous range. Human skin colour and height.

• Genes on the same chromosome do not assort independently.
• They tend to be inherited together, which is linkage.
• Morgan showed this in the fruit fly, Drosophila melanogaster.

• Tightly linked genes are close together, so they rarely recombine.
• Loosely linked genes are further apart, so they recombine more often.
• Recombination frequency is proportional to distance along the chromosome.
• Sturtevant used this to build the first genetic map.

• In humans, females are XX and males are XY.

• XX-XY: humans and Drosophila. The male is heterogametic.
• XX-XO: grasshoppers. The male is simply XO, with no Y at all.
• ZZ-ZW: birds. Here the FEMALE is ZW, so the mother determines the sex.
• Haplodiploidy: in honey bees the male is haploid, developing from an unfertilised egg.

• A pedigree traces a trait through a family, using a standard set of symbols.

• Mendelian disorders come from a change in a single gene.
• Autosomal recessive: sickle cell anaemia, phenylketonuria, thalassemia.
• Autosomal dominant: Huntington's chorea, myotonic dystrophy.
• X-linked recessive: haemophilia, colour blindness.
• Chromosomal disorders come from a change in number or structure.
• Down syndrome is trisomy of chromosome 21. Klinefelter is XXY. Turner is XO.

Gathered in one place. Cover the gold band and test yourself.