2.2Unit 2 · Reproduction & Inheritance

Inheritance — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions

10 min read 4 parts Lesson 3 of 3 1 quiz

📚 Edexcel IGCSE Biology Modular - UNIT 2 Hub / Topical Revision

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Lesson summary

Describe the structure of DNA and RNA and how genes are located on chromosomes.Human body cells have 46 chromosomes (23 pairs); gametes have 23.Base pairing in DNAA with T, C with G.

ABO blood groups
The genome and genes
DNA structure
RNA structure
Stages of protein synthesis
Key terms in genetics

Section 1 of 4Inheritance — Unit 2

01
The genome and genes:
the genome is all the DNA in an organism.
- DNA is packaged into chromosomes in the nucleus; a gene is a length of DNA on a chromosome that codes for a particular protein, and the base sequence determines the amino acid sequence.
02
DNA structure:
DNA is a polymer of nucleotides.
- Two strands wind into a double helix.
- The sugar-phosphate backbones are on the outside; bases on the inside pair by hydrogen bonds: adenine with thymine and cytosine with guanine (complementary base pairing).
RNA structure:
RNA is single-stranded, contains the sugar ribose and the base uracil in place of thymine. Messenger RNA (mRNA) carries a copy of a gene to the ribosome; transfer RNA (tRNA) carries amino acids to the ribosome.
04
Stages of protein synthesis:
in transcription a gene is copied into mRNA using one DNA strand as a template.
- In translation the ribosome reads the mRNA codons; tRNA molecules with complementary anticodons deliver the correct amino acids, which are joined into a polypeptide.
Alleles:
genes exist in different versions called alleles. Body cells have two copies of each gene, one from each parent, so an individual can be homozygous or heterozygous.
06
Key terms in genetics:
a dominant allele (capital letter) masks a recessive allele (lower case).
- Genotype describes the alleles (e.g. Tt), phenotype the characteristic (e.g. tall).
- Punnett squares predict the probability of offspring genotypes; family pedigrees can show how a condition such as cystic fibrosis is inherited.
Codominance:
both alleles are expressed. In ABO blood groups, I^AI^B gives group AB. In some flowers, red and white alleles are codominant and heterozygotes show both colours or an intermediate pink appearance.
08
Polygenic inheritance:
many characteristics, such as human height, are controlled by several genes, each with alleles adding a small effect.
- This produces continuous variation, shown by a normal distribution curve, and the environment also has a strong influence.
Inheritance of sex:
Females are XX and males are XY. Eggs always carry an X chromosome; sperm carry either X or Y. So the sperm decides the sex, and each child has a 50% chance of being male or female.
10
Mitosis:
Nuclear division that produces two genetically identical diploid cells.
- Each chromosome is copied first, the copies line up and are pulled apart, then the cell divides.
- It is used for growth, repair, replacing cells and asexual reproduction.
11
Meiosis:
Nuclear division that produces four genetically different haploid cells (gametes).
- Chromosomes are copied, then the cell divides twice so each gamete gets one chromosome from each pair.
- This halves the chromosome number so it is restored at fertilisation.
Random fertilisation:
Each gamete carries a different mix of alleles, and any sperm can fuse with any egg. This random fusion gives every zygote a unique combination of alleles, increasing genetic variation.
13
Genetic and environmental variation:
Variation can be caused by genes, by the environment (e.g. diet, climate, accidents, language learned) or both.
- Discontinuous variation is usually genetic only, while continuous variation such as height results from many genes plus the environment.
14
Mutation:
A rare, random change in the base sequence of DNA.
- Most mutations have no effect on the protein or phenotype; a few change a protein's shape and function (e.g. sickle cell anaemia), and occasionally a new allele gives an advantage.
Types of gene mutation:
Insertion and deletion of a base change every triplet after the mutation, so many amino acids may change. A substitution swaps one base, so at most one triplet and one amino acid change.
Causes of mutation:
Mutations happen spontaneously, but the rate increases with ionising radiation (gamma rays, X-rays, ultraviolet) and chemical mutagens such as tar in tobacco smoke. Higher mutation rates increase the risk of cancer.
17
Natural selection:
Individuals vary because of genes.
- Those with features suited to the environment are more likely to survive and reproduce, passing on their alleles.
- Over many generations the advantageous alleles become more common in the population.
- This is Darwin's theory of evolution.
18
Antibiotic resistance:
A random mutation gives a bacterium a resistance allele.
- When an antibiotic is used, non-resistant bacteria die but resistant ones survive and reproduce.
- Over generations the resistance allele becomes more common, producing a resistant strain that needs a different antibiotic.

Section 2 of 4Process & sequence

In the nucleus, the DNA double helix unzips at the gene as hydrogen bonds break.
Free RNA nucleotides pair with the template strand to build mRNA (transcription).
mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome.
The ribosome reads mRNA in triplets called codons.
tRNA molecules carrying specific amino acids bring them to matching codons (anticodons pair).
Amino acids are joined by peptide bonds in the order set by the codons.
The polypeptide chain folds into a functional protein.

Gene to protein — write these steps in this order; each step is a marking point.

DNA unzips at gene
mRNA made by transcription
mRNA moves to ribosome
Codons read in triplets
tRNA brings amino acids
Polypeptide folds into protein

Section 3 of 4Compare

DNA compared with RNA
Feature	DNA	mRNA
Number of strands	Two (double helix)	One
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Where found	Nucleus (chromosomes)	Made in nucleus, moves to ribosomes
Length	Very long	Short copy of one gene

Single-gene vs polygenic inheritance
Feature	Single gene	Polygenic
Genes involved	One gene	Many genes
Variation	Discontinuous (distinct groups)	Continuous (range of values)
Examples	ABO blood group, cystic fibrosis	Human height, skin colour
Environmental effect	Usually little	Often large

Section 4 of 4Worked examples

Two heterozygous tall pea plants (Tt) are crossed. Predict the ratio of tall to short offspring.
- Gametes: T or t from each parent.
  - Punnett square gives TT, Tt, Tt, tt.
  - TT and Tt are tall (3), tt is short (1).
  - Ratio 3 tall : 1 short; probability of short = 0.25 (25%).
A mother is blood group A (IAIO) and the father is group B (IBIO). What blood groups could the children have?
- Gametes: I^A or I^O; I^B or I^O. Offspring: I^AI^B (AB), I^AI^O (A), I^BI^O (B), I^OI^O (O). Each group has a 1 in 4 chance.

Last updated: 11 October 2026

Section 1 of 4Inheritance — Unit 2

The genome and genes:

DNA structure:

RNA structure:

Stages of protein synthesis:

Alleles:

Key terms in genetics:

Codominance:

Polygenic inheritance:

Inheritance of sex:

Mitosis:

Meiosis:

Random fertilisation:

Genetic and environmental variation:

Mutation:

Types of gene mutation:

Causes of mutation:

Natural selection:

Antibiotic resistance:

Section 2 of 4Process & sequence

Section 3 of 4Compare

Section 4 of 4Worked examples

Two heterozygous tall pea plants (Tt) are crossed. Predict the ratio of tall to short offspring.

A mother is blood group A (IAIO) and the father is group B (IBIO). What blood groups could the children have?

Marked practiceQuizzes for this lesson