Chapter at a Glance
Heredity — the essentials in one place.
- Variation arising during reproduction can be inherited; it may improve survival. Sexual reproduction generates more variation than asexual.
- Heredity = reliable inheritance of traits from parents to offspring. Each trait has two gene copies — one from each parent.
- If the two copies differ, the expressed one is dominant, the hidden one recessive.
- Traits can be inherited independently, giving new combinations.
- Sex in humans is set by the sex chromosomes: the father's X (girl) or Y (boy).
Mendel's Crosses & Ratios
| Cross | Parents | F1 | F2 ratio |
|---|---|---|---|
| Monohybrid | TT × tt | all Tt (tall) | 3 : 1 (genotype 1:2:1) |
| Dihybrid | RRYY × rryy | all RrYy | 9 : 3 : 3 : 1 |
| Test cross | Tt × tt | — | 1 : 1 |
- Dominant (T): shows with one copy. Recessive (t): shows only as tt.
- Genotype = letters (Tt); Phenotype = looks (tall).
- Dihybrid F2 new combinations ⇒ independent inheritance.
Genes, Proteins & Chromosomes
- A gene = a section of DNA coding for one protein.
- Genes control traits by controlling proteins: gene → enzyme → hormone amount → trait (e.g. plant height).
- Body cells carry two copies of each gene on paired chromosomes (one from each parent).
- Germ cells carry one set; fertilisation restores the pair → species DNA stays stable.
- Genes on different chromosomes assort independently → independent inheritance.
Sex Determination & Blood Groups
Sex determination (humans)
- 23 pairs = 22 autosome pairs + 1 sex-chromosome pair.
- Female XX, Male XY. Mother → all X; Father → half X, half Y.
- X-sperm → girl; Y-sperm → boy. Father decides the sex; ratio ~1:1.
- Other species: temperature (some reptiles), sex change (snails).
ABO blood groups
| Group | Genotype |
|---|---|
| A | I^A I^A / I^A i |
| B | I^B I^B / I^B i |
| AB | I^A I^B (codominant) |
| O | i i (recessive) |
Master One-Liners (Morning-of-Exam Revision)
- Sexual reproduction → more variation; environment selects variants → basis of evolution.
- Heredity = transfer of traits; each trait has two gene copies.
- Free/attached earlobes = inherited trait example.
- Mendel used pea plants; first to count offspring.
- Monohybrid F2 = 3:1; genotype 1:2:1.
- Dihybrid F2 = 9:3:3:1; new combinations → independent inheritance.
- Dominant shows with one copy; recessive needs both.
- Genotype = letters; phenotype = looks. TT & Tt both tall.
- Test cross Tt × tt = 1:1.
- Gene → protein → trait (genes control traits via proteins).
- Body cells 2 copies; germ cells 1; fertilisation restores pair.
- Genes on separate chromosomes → independent assortment.
- Female XX, Male XY; father decides sex; ratio 1:1.
- Humans: 22 autosome pairs + 1 sex pair.
- Blood group O = ii; I^A, I^B dominant over i, and codominant with each other.