Genetics & Inheritance

IGCSE Edexcel Biology
3.1–3.33 DNA, cell division, and monohybrid inheritance
Key Concepts: DNA carries genetic information in genes. Mitosis produces genetically identical cells for growth and repair. Meiosis produces genetically different gametes. Monohybrid inheritance follows predictable patterns based on dominant and recessive alleles.

Section A — DNA and Genes

1. Define the following terms. [6]

a) Gene

b) Allele

c) Chromosome

2. State the structure of DNA, including the names of the four bases and how they pair. [4]
3. State how many chromosomes are found in a normal human: [2]

a) Body (somatic) cell

b) Gamete (sperm or egg)

Section B — Cell Division

4. Compare mitosis and meiosis. [6]
Feature Mitosis Meiosis
Number of divisions
Number of daughter cells
Genetically identical to parent?
Where it occurs (example)
Purpose / role
Chromosome number (human)
5. Explain why meiosis produces genetic variation. [2]

Section C — Monohybrid Inheritance

6. Define the following terms. [6]

a) Dominant allele

b) Recessive allele

c) Homozygous

d) Heterozygous

e) Genotype

f) Phenotype

7. In pea plants, tall (T) is dominant over short (t). A homozygous tall plant is crossed with a homozygous short plant. [5]

a) State the genotypes of the parents. [1]

b) Complete the Punnett square for this cross.

TT
t
t

c) State the genotypic ratio of the offspring. [1]

d) State the phenotype of all offspring. [1]

8. Two heterozygous tall pea plants (Tt) are crossed. [4]

a) Complete the Punnett square.

Tt
T
t

b) State the phenotypic ratio of the offspring. [1]

c) What is the probability of getting a short plant? [1]

9. Cystic fibrosis is a genetic disease caused by a recessive allele (f). A carrier mother (Ff) and a carrier father (Ff) have children. What is the probability that a child has cystic fibrosis? Show your working. [3]

Section D — Sex Determination and Variation

10. Explain how sex is determined in humans. Use a Punnett square in your answer. [4]
11. Distinguish between continuous and discontinuous variation. Give an example of each. [4]

Total marks: 46

Mark Scheme

1. a) A section of DNA that codes for a specific protein [2]; b) A version/form of a gene [2]; c) A long molecule of DNA found in the nucleus [2] [6]
2. Double helix structure [1]; made of nucleotides; bases are Adenine (A), Thymine (T), Guanine (G), Cytosine (C) [1]; A pairs with T, G pairs with C (complementary base pairing) [2] [4]
3. a) 46 (23 pairs); b) 23 [2]
4. Divisions: 1 / 2; Daughter cells: 2 / 4; Genetically identical: Yes / No; Location: all body cells (e.g. skin) / gonads (testes/ovaries); Purpose: growth, repair, asexual reproduction / production of gametes; Chromosome number: 46 (diploid) / 23 (haploid) [6]
5. Chromosomes are shuffled randomly during meiosis (independent assortment) [1]; crossing over of chromatids also exchanges alleles between homologous chromosomes, creating new combinations [1] [2]
6. a) Always expressed / masks recessive; b) Only expressed when homozygous / masked by dominant; c) Two identical alleles (TT or tt); d) Two different alleles (Tt); e) The allele combination an organism has; f) The physical characteristics expressed [6]
7. a) TT × tt; b) All Tt; c) All Tt (ratio 1:0 or all heterozygous); d) All tall [5]
8. a) TT, Tt, Tt, tt; b) 3 tall : 1 short; c) 1 in 4 / 25% [4]
9. Punnett square Ff × Ff gives FF, Ff, Ff, ff [1]; probability of ff = 1 in 4 [1]; = 25% chance of having cystic fibrosis [1] [3]
10. Females: XX; Males: XY [1]; each parent passes one sex chromosome to offspring [1]; Punnett square: XX × XY → XX, XY, XX, XY → 50% male, 50% female [2] [4]
11. Continuous: varies across a range (no distinct categories) [1]; example: height, mass [1]; Discontinuous: falls into distinct categories [1]; example: blood group, tongue rolling, sex [1] [4]