Words to know
| Word |
Simple meaning |
Example |
| Gene |
A unit of heredity that controls a trait |
Gene for plant height |
| Allele |
Different forms of the same gene |
T (tall) and t (dwarf) |
| Dominant |
The allele that shows up when paired with another allele |
T |
| Recessive |
The allele that is hidden when paired with a dominant allele |
t |
| Homozygous |
Both alleles are the same |
TT or tt |
| Heterozygous |
The two alleles are different |
Tt |
| Genotype |
The genes an organism has |
TT, Tt, tt |
| Phenotype |
How it looks |
Tall or dwarf |
Why did Mendel choose peas?
- Pea plants are easy to grow and have a short life cycle.
- They have many clear, opposite traits (tall/dwarf, green/yellow).
- They self-pollinate naturally but can also be cross-pollinated by hand.
- Mendel studied 7 pairs of contrasting traits from 1856 to 1863.
Monohybrid cross: one trait at a time
Mendel crossed a pure tall (TT) plant with a pure dwarf (tt) plant.
Parents TT (tall) × tt (dwarf) → F1: all Tt (all tall) → self-pollinate Tt × Tt → F2: TT, Tt, Tt, tt → 3 tall : 1 dwarf (genotype 1 TT : 2 Tt : 1 tt).
Punnett square for F1 × F1 (Tt × Tt):
| ♀ \ ♂ |
T |
t |
| T |
TT (tall) |
Tt (tall) |
| t |
Tt (tall) |
tt (dwarf) |
Test cross: find the hidden genotype
A tall plant could be TT or Tt. To find out, cross it with a dwarf (tt) plant:
- If all the offspring are tall → the plant was TT.
- If half are tall and half dwarf (1 : 1) → the plant was Tt.
Mendel's 3 laws
| Law |
In simple words |
| 1. Law of Dominance |
In a Tt plant, only the dominant trait (T, tall) shows |
| 2. Law of Segregation |
The two alleles separate when gametes form, so each gamete gets only one allele. Also called the "law of purity of gametes" |
| 3. Law of Independent Assortment |
Genes for different traits are passed on independently of each other |
Dihybrid cross: two traits together
Mendel crossed round yellow (RRYY) seeds with wrinkled green (rryy) seeds. The F1 were all RrYy (round yellow). In the F2:
| Round yellow |
Round green |
Wrinkled yellow |
Wrinkled green |
| 9 |
3 |
3 |
1 |
- 4 types of gametes from each parent: RY, Ry, rY, ry. That gives a 4 × 4 = 16 box Punnett square.
- This cross proved the Law of Independent Assortment.
When Mendel's rules bend: exceptions
1. Incomplete dominance: blending
- In the snapdragon (dog flower, Antirrhinum): red (RR) × white (rr) → pink (Rr) in F1.
- F2 = 1 red : 2 pink : 1 white. Here the phenotype ratio is the same as the genotype ratio (1:2:1).
2. Co-dominance: both show
- ABO blood groups in humans are controlled by the gene I, which has 3 alleles: Iᴬ, Iᴮ and i.
- Iᴬ and Iᴮ are co-dominant (both show together as AB). i is recessive.
- This also shows multiple alleles (more than 2 alleles for one gene in a population).
| Genotype |
Blood group |
| IᴬIᴬ or Iᴬi |
A |
| IᴮIᴮ or Iᴮi |
B |
| IᴬIᴮ |
AB (co-dominance!) |
| ii |
O |
3. Pleiotropy and polygenic inheritance
|
Pleiotropy |
Polygenic inheritance |
| Meaning |
One gene → many effects |
Many genes → one trait |
| Example |
Phenylketonuria |
Human skin colour, height |
Chromosomal theory of inheritance
- Sutton and Boveri said genes are on chromosomes, and chromosomes move in pairs like Mendel's factors.
- Thomas Hunt Morgan proved it using fruit flies (Drosophila).
- Linkage: genes on the same chromosome tend to be inherited together.
- Recombination: new combinations appear when linked genes separate (by crossing over).
- Tight linkage gives few recombinants; loose linkage gives more.
- Alfred Sturtevant used recombination frequency to make the first gene maps.
Sex determination: boy or girl?
| System |
Example |
Female |
Male |
| XX–XO |
Grasshopper |
XX |
XO (one X only) |
| XX–XY |
Humans, Drosophila |
XX |
XY |
| ZW–ZZ |
Birds |
ZW |
ZZ |
| Haplodiploidy |
Honey bee |
Diploid (32 chromosomes), from a fertilised egg |
Haploid (16 chromosomes), from an unfertilised egg |
Mutation
- A mutation is a change in DNA.
- Point mutation: one base changes. Example: sickle-cell anaemia.
- Frameshift mutation: a base is added or deleted, which shifts the whole reading frame.
- Mutagens: things that cause mutation, such as UV rays and some chemicals.
Pedigree analysis
A pedigree is a family tree that shows how a trait is passed down. It helps track genetic diseases.
| Symbol |
Meaning |
| Square |
Male |
| Circle |
Female |
| Shaded (filled) square or circle |
Affected person |
| Horizontal line joining a square and a circle |
Mating (marriage) |
| Double line |
Consanguineous mating (between relatives) |
Genetic disorders
A. Mendelian disorders (a single gene change)
| Disorder |
Type |
Key fact |
| Haemophilia |
X-linked recessive |
Blood doesn't clot. Mostly affects males; carrier mothers pass it to sons. Famous in Queen Victoria's family |
| Colour blindness |
X-linked recessive |
Can't tell red from green. About 8% of males and 0.4% of females |
| Sickle-cell anaemia |
Autosomal recessive |
Glutamic acid → valine at the 6th position of the β-globin chain (GAG → GUG). RBCs become sickle-shaped |
| Phenylketonuria |
Autosomal recessive |
Can't convert phenylalanine → tyrosine, which leads to mental retardation |
| Thalassemia |
Autosomal recessive |
Less globin is made (a quantity problem). Sickle-cell makes the wrong globin (a quality problem) |
B. Chromosomal disorders (whole chromosome changes)
- Aneuploidy: one chromosome extra or missing, caused by failed separation during cell division.
- Polyploidy: whole extra sets of chromosomes. Common in plants.
| Disorder |
Chromosomes |
Features |
| Down's syndrome |
Trisomy 21 (47 chromosomes) |
Short, small round head, furrowed tongue, partly open mouth, palm crease, slow mental development |
| Klinefelter's syndrome |
47, XXY |
Male with some female features (breast growth); sterile |
| Turner's syndrome |
45, XO |
Female with underdeveloped ovaries; sterile |
30-second revision
- Monohybrid 3:1 (phenotype), 1:2:1 (genotype); dihybrid 9:3:3:1; test cross 1:1.
- Incomplete dominance: snapdragon → pink (1:2:1).
- ABO blood groups = co-dominance + multiple alleles (3-6-4).
- Morgan → Drosophila → linkage; Sturtevant → gene maps.
- Birds: female ZW. Honey bee male: haploid (16).
- Haemophilia and colour blindness: X-linked recessive. Sickle-cell: Glu6Val.
- Down's = 21 trisomy; Klinefelter = XXY; Turner = XO.