1. Terms in genetics
Genetics is the study of heredity (passing of characters from parents to offspring) and variation.
| Term | Meaning |
|---|---|
| Gene | A unit of heredity: a segment of DNA on a chromosome that controls a character |
| Allele | One of the alternative forms of a gene at the same position on homologous chromosomes (V and v) |
| Dominant | Allele that expresses itself in the presence of another allele (V) |
| Recessive | Allele that expresses itself only when both alleles are alike (vv) |
| Homozygous | Having two identical alleles (VV or vv); breeds true |
| Heterozygous | Having two different alleles (Vv); a hybrid |
| Genotype | The genetic make-up for a character (VV, Vv, vv) |
| Phenotype | The observable character (violet or white flowers) |
| Mutation | A sudden heritable change in a gene or chromosome |
| Variation | Differences among individuals of the same species |
| Genotype | Phenotype |
|---|---|
| VV | Violet |
| Vv | Violet |
| vv | White |
Key terms
- Allele
An alternative form of a gene.
- Genotype
The genetic make-up for a character.
- Phenotype
The observable character.
2. Mendel and his laws
Gregor Johann Mendel studied inheritance in the garden pea (Pisum sativum). He chose it because it has clearly contrasting characters, is self-pollinating (pure lines) yet can be cross-pollinated by hand, and has a short life cycle with many seeds.
Law of dominance: when two contrasting alleles are present, only the dominant one is expressed in the hybrid.
Law of segregation (purity of gametes): the two alleles of a gene separate during gamete formation, so each gamete receives only one allele.
Law of independent assortment: alleles of different genes are distributed to gametes independently of one another (seen in a dihybrid cross).
Key terms
- Law of segregation
Alleles separate at gamete formation; each gamete is pure.
- Law of independent assortment
Alleles of different genes are distributed independently.
3. Monohybrid cross
A cross involving one pair of contrasting characters.
Example: pure violet (VV) × pure white (vv) → F1 all Vv, violet. F1 selfed (Vv × Vv) → gametes V, v from each parent → F2: VV, Vv, Vv, vv.
- Phenotypic ratio 3 violet : 1 white.
- Genotypic ratio 1 VV : 2 Vv : 1 vv.
Test cross: crossing an organism of dominant phenotype with the homozygous recessive to find its genotype. All dominant offspring → parent homozygous; 1 : 1 → parent heterozygous.
Key terms
- Monohybrid cross
A cross involving one pair of contrasting characters.
- Test cross
A cross with the homozygous recessive to reveal a genotype.
4. Dihybrid cross (phenotypic ratio only)
A cross involving two pairs of contrasting characters.
Example: tall violet (TTVV) × dwarf white (ttvv) → F1 TtVv, tall violet. The F1 forms four kinds of gametes (TV, Tv, tV, tv) in equal numbers. F2 (16 boxes):
- Tall violet 9 : tall white 3 : dwarf violet 3 : dwarf white 1.
- Tall white and dwarf violet are new combinations (recombinants), showing independent assortment.
(The genotypic ratio of the dihybrid F2 is not in the syllabus.)
Key terms
- Dihybrid cross
A cross involving two pairs of contrasting characters.
- Recombinant
An offspring with a new combination of characters.
5. Sex determination in human beings
Human body cells have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
- Female: 44 + XX; all eggs 22 + X (homogametic).
- Male: 44 + XY; sperms 22 + X or 22 + Y in equal numbers (heterogametic).
X sperm + egg → 44 + XX, girl; Y sperm + egg → 44 + XY, boy. So the father’s gamete decides the sex, and the chance of a boy or a girl is one half at every birth.
Key terms
- Autosomes
Chromosomes other than the sex chromosomes.
- Sex chromosomes
The X and Y chromosomes that determine sex.
6. X-linked inheritance: haemophilia and colour blindness
Genes carried on the X chromosome are X-linked. The alleles for haemophilia (blood fails to clot) and red–green colour blindness are recessive.
- Male (XY): one faulty X → affected (no matching allele on Y).
- Female (XX): one faulty X → carrier (normal); two faulty X → affected (rare).
Carrier mother × normal father: daughters half normal, half carriers; sons half normal, half affected. Affected father × normal mother: all daughters carriers; all sons normal (sons get his Y).
So X-linked disorders are commoner in males and pass from a carrier mother to her sons (criss-cross inheritance).
Key terms
- Carrier
A heterozygous person who carries a recessive allele without showing it.
- Haemophilia
An X-linked disorder in which blood does not clot normally.
- Colour blindness
An X-linked disorder in which red and green cannot be told apart.
Exam tips
Common mistakes
- Writing the genotypic ratio for a dihybrid F2 (not in the syllabus) or giving 3 : 1 for it.
- Using different letters for the alleles of one gene (V and w).
- Saying the mother decides the sex of the child.
- Calling a heterozygous woman "affected" instead of a carrier.
- Putting X-linked alleles on the Y chromosome.
How marks are usually awarded
- 1 mark: a term, a genotype, a ratio, an odd one out with its category.
- 2 marks: a law stated with evidence, or two reasons.
- 3 marks: a cross — parents and gametes, Punnett square, and correct ratios or conclusions.