What you'll learn
This revision guide covers all aspects of reproduction and genetics required for WJEC GCSE Biology. You'll understand the differences between sexual and asexual reproduction, how meiosis produces genetically different gametes, and how genetic information passes from parents to offspring. The guide also explains DNA structure, genetic variation, and inheritance patterns including monohybrid crosses.
Key terms and definitions
Gamete — a specialised sex cell (sperm or egg) containing half the normal number of chromosomes (haploid).
Meiosis — cell division that produces four non-identical haploid cells from one diploid cell, used to form gametes.
Allele — an alternative form of the same gene; different alleles control variations of a characteristic (e.g. blue or brown eye colour).
Genotype — the genetic makeup of an organism; the combination of alleles an organism has for a particular characteristic.
Phenotype — the observable characteristics of an organism resulting from the interaction of its genotype with the environment.
Heterozygous — having two different alleles for a particular gene (e.g. Bb).
Homozygous — having two identical alleles for a particular gene (e.g. BB or bb).
Dominant allele — an allele that is expressed in the phenotype even when only one copy is present; represented by a capital letter.
Core concepts
Sexual and asexual reproduction
Sexual reproduction involves the fusion of male and female gametes during fertilisation. Each gamete contains half the genetic information (haploid), so the offspring receives half its genetic material from each parent. This produces genetic variation in offspring.
Advantages of sexual reproduction:
- Genetic variation increases the probability that some offspring will survive environmental changes
- Enables selective breeding to develop new varieties
- Allows evolution through natural selection
Disadvantages of sexual reproduction:
- Requires finding a mate, which takes time and energy
- Slower than asexual reproduction
- Not possible for isolated individuals
Asexual reproduction involves only one parent and produces genetically identical offspring (clones). No gametes are involved; instead, cells divide by mitosis.
Examples of asexual reproduction:
- Binary fission in bacteria
- Budding in yeast
- Runners in strawberry plants
- Tubers in potatoes
Advantages of asexual reproduction:
- Only one parent needed
- Faster than sexual reproduction
- Many identical offspring produced quickly
Disadvantages of asexual reproduction:
- No genetic variation
- Whole population vulnerable to same diseases or environmental changes
- Cannot adapt to changing conditions
Meiosis and gamete formation
Meiosis produces four genetically different haploid cells from one diploid parent cell. It occurs in the reproductive organs (testes and ovaries in animals).
Key stages of meiosis:
- DNA replicates, forming copies of each chromosome
- Chromosome pairs line up at the cell equator
- First division — pairs separate; one chromosome from each pair moves to opposite poles
- Second division — chromosomes split; chromatids separate to opposite poles
- Four haploid cells form, each genetically different
Meiosis produces variation through:
- Independent assortment — random distribution of maternal and paternal chromosomes into gametes
- Crossing over — exchange of genetic material between chromosome pairs during the first division
Human body cells contain 46 chromosomes (23 pairs). Meiosis produces gametes with 23 chromosomes. At fertilisation, the full chromosome number (46) is restored.
DNA structure and genes
DNA (deoxyribonucleic acid) is a polymer made of two strands forming a double helix. The genetic material in the nucleus carries coded information.
DNA structure:
- Two polynucleotide strands twisted together
- Each strand contains repeating units called nucleotides
- Each nucleotide consists of a sugar, phosphate group, and one of four bases
- The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G)
- Bases pair specifically: A pairs with T; C pairs with G
- Base pairs held together by weak hydrogen bonds
A gene is a small section of DNA on a chromosome that codes for a particular protein. Different genes control different characteristics by coding for specific proteins.
The human genome is the entire genetic material of an individual. Understanding the genome helps scientists:
- Search for genes linked to diseases
- Understand and treat inherited disorders
- Trace human migration patterns
Genetic variation
Genetic variation arises from mutations and sexual reproduction.
Mutations are random changes in DNA sequence. Most have no effect, but some can:
- Alter protein structure, changing characteristics
- Increase or decrease survival chances
- Rarely, produce new phenotypes that give advantages
Factors increasing mutation rate:
- Ionising radiation (X-rays, gamma rays, UV light)
- Chemical mutagens (found in tobacco smoke)
Sexual reproduction creates variation through:
- Random fertilisation — any sperm can fertilise any egg
- Independent assortment during meiosis
- Crossing over during meiosis
Environmental variation results from conditions during development:
- Nutrition affects growth and body mass
- Sun exposure affects skin colour
- Exercise influences muscle development
Most characteristics result from interaction between genes and environment.
Monohybrid inheritance
Monohybrid inheritance examines how one characteristic controlled by a single gene passes from parents to offspring.
Dominant alleles are expressed when present (shown by capital letters, e.g. B). Recessive alleles are only expressed when two copies are present (shown by lowercase letters, e.g. b).
For example, in seed shape in pea plants:
- R = round seeds (dominant)
- r = wrinkled seeds (recessive)
Possible genotypes and phenotypes:
- RR (homozygous dominant) — round seeds
- Rr (heterozygous) — round seeds
- rr (homozygous recessive) — wrinkled seeds
Genetic diagrams and Punnett squares
Punnett squares predict offspring ratios from genetic crosses.
Example: Cross between two heterozygous parents (Bb × Bb)
- B = brown eyes (dominant)
- b = blue eyes (recessive)
Parents' genotypes: Bb × Bb
Parents' gametes: B or b (from each parent)
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Offspring ratios:
- Genotype ratio: 1 BB : 2 Bb : 1 bb
- Phenotype ratio: 3 brown eyes : 1 blue eyes
Sex determination and sex chromosomes
Humans have 23 pairs of chromosomes. The 23rd pair are sex chromosomes:
- Females have XX
- Males have XY
During meiosis, sex chromosomes separate:
- Female gametes (eggs) always carry an X chromosome
- Male gametes (sperm) carry either X or Y
The sex of offspring is determined at fertilisation:
- XX produces female offspring
- XY produces male offspring
The ratio of males to females is approximately 1:1 (50:50).
Inherited disorders
Some genetic disorders are inherited through recessive alleles:
Cystic fibrosis is caused by a recessive allele (f). The dominant allele (F) produces normal mucus. Genotypes:
- FF — unaffected
- Ff — carrier (unaffected but carries one copy)
- ff — affected (thick mucus in lungs and digestive system)
Polydactyly (extra fingers or toes) is caused by a dominant allele (D):
- DD or Dd — affected
- dd — unaffected
Genetic screening can identify:
- Carriers of recessive alleles
- Embryos with genetic disorders
- Individuals at risk of developing genetic conditions
Ethical considerations include:
- Implications for insurance and employment
- Decisions about continuing pregnancy
- Accuracy and reliability of tests
Worked examples
Example 1: Meiosis and chromosome number
Question: A horse body cell contains 64 chromosomes. How many chromosomes will be present in a horse sperm cell? Explain your answer. [2 marks]
Answer: 32 chromosomes [1 mark]. Meiosis halves the chromosome number to produce haploid gametes [1 mark].
Examiner note: Always state the number clearly and explain that meiosis produces haploid cells with half the chromosome number.
Example 2: Monohybrid cross
Question: In cattle, the allele for horns (H) is dominant to the allele for no horns (h). A farmer crosses a homozygous horned bull with a cow with no horns. Use a Punnett square to determine the genotypes and phenotypes of the offspring. [4 marks]
Answer: Parents: HH × hh [1 mark]
Gametes: H (from bull), h (from cow) [1 mark]
| H | H | |
|---|---|---|
| h | Hh | Hh |
| h | Hh | Hh |
Offspring genotypes: all Hh [1 mark]
Offspring phenotypes: all horned [1 mark]
Examiner note: Always show parents' genotypes, gametes, Punnett square, and resulting genotypes/phenotypes for full marks.
Example 3: Sex determination
Question: Explain why approximately equal numbers of male and female babies are born. [3 marks]
Answer: Females produce eggs containing only X chromosomes [1 mark]. Males produce equal numbers of sperm containing X or Y chromosomes [1 mark]. Therefore, there is a 50% chance of X or Y sperm fertilising the egg, giving equal probability of male (XY) or female (XX) offspring [1 mark].
Examiner note: Link the types of gametes produced to the equal probability outcome.
Common mistakes and how to avoid them
Confusing mitosis and meiosis: Remember meiosis produces four haploid, genetically different gametes; mitosis produces two diploid, genetically identical cells for growth and repair.
Mixing up genotype and phenotype: Genotype refers to alleles present (letters); phenotype describes observable characteristics (words describing appearance).
Incorrect use of upper and lowercase letters: Always use uppercase for dominant alleles and lowercase for recessive. Keep the same letter for both alleles of one gene (e.g. B and b, not B and r).
Forgetting gametes in genetic diagrams: Always show what gametes each parent can produce before completing the Punnett square. This demonstrates understanding of how alleles separate.
Stating mutations are always harmful: Most mutations have no effect. Some are harmful, but a few can be beneficial and drive evolution.
Confusing carriers with affected individuals: Carriers are heterozygous (e.g. Ff) and show the dominant phenotype but carry one recessive allele. Only homozygous recessive individuals (ff) show the recessive condition.
Exam technique for "Reproduction and Genetics"
Command words matter: "Describe" requires you to state features or characteristics; "Explain" requires reasons or mechanisms. For genetic crosses, "determine" or "predict" means complete a Punnett square and state outcomes.
Genetic diagram structure: Always follow the sequence: parents' genotypes → parents' phenotypes → gametes → Punnett square → offspring genotypes → offspring phenotypes → ratios. Missing steps lose marks even if your answer is correct.
Show working: Even if the question doesn't explicitly ask for a Punnett square, draw one to avoid errors. Examiners can award method marks if your final answer is incorrect.
Use correct terminology: Use "alleles" not "genes" when discussing different versions; "chromosome" not "gene" when discussing meiosis; "haploid/diploid" when referring to chromosome numbers in cells.
Quick revision summary
Sexual reproduction involves gamete fusion, creating genetic variation; asexual reproduction produces identical clones. Meiosis halves chromosome number and creates variation through independent assortment and crossing over. DNA is a double helix of complementary base pairs; genes code for proteins. Alleles are gene variants; dominant alleles mask recessive ones. Genotype is genetic makeup; phenotype is observable characteristics. Punnett squares predict inheritance ratios in monohybrid crosses. Sex chromosomes (XX/XY) determine biological sex. Mutations and sexual reproduction create genetic variation essential for evolution.