Genetics

What you'll learn

This revision guide covers everything you need to know about genetics for Edexcel GCSE Biology. You'll understand DNA structure, how genes code for proteins, inheritance patterns including monohybrid crosses, sex determination, and the difference between continuous and discontinuous variation. These topics appear regularly in Paper 1 and Paper 2, particularly in extended response and calculation questions.

Key terms and definitions

Gene — a section of DNA that codes for a specific sequence of amino acids to make a protein

Allele — a different version of the same gene (e.g. genes for eye colour may have brown or blue alleles)

Genotype — the combination of alleles an organism has (e.g. Bb or bb)

Phenotype — the observable characteristics of an organism resulting from its genotype and environmental interactions

Dominant allele — an allele that is expressed in the phenotype even when only one copy is present (represented by a capital letter)

Recessive allele — an allele that is only expressed in the phenotype when two copies are present (represented by a lower-case letter)

Homozygous — having two identical alleles for a particular gene (e.g. BB or bb)

Heterozygous — having two different alleles for a particular gene (e.g. Bb)

Core concepts

DNA structure and function

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. Its structure is described as a double helix — two strands twisted around each other like a spiral staircase.

Key structural features:

• DNA consists of two complementary strands held together by bases
• Each strand is made of repeating units called nucleotides
• A nucleotide contains a sugar molecule, a phosphate group, and one of four bases
• The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G)
• Complementary base pairing follows strict rules: A pairs with T, C pairs with G
• These pairs are held together by weak hydrogen bonds

The genetic code:

• A gene is a section of DNA that codes for a protein
• The sequence of bases in a gene determines the sequence of amino acids in a protein
• Three bases (a triplet or codon) code for one amino acid
• The order of amino acids determines how the protein folds and functions
• Different proteins perform different functions (enzymes, antibodies, structural proteins like collagen)

DNA is located in the nucleus of eukaryotic cells, organized into structures called chromosomes. Humans have 46 chromosomes (23 pairs) in each body cell.

Protein synthesis

Genes control characteristics by controlling the production of proteins. This occurs in two main stages:

Transcription (in the nucleus):

• The DNA double helix unzips
• One strand acts as a template
• Complementary bases attach to form messenger RNA (mRNA)
• In RNA, uracil (U) replaces thymine (T)
• mRNA is a single-stranded copy of the gene
• mRNA leaves the nucleus through pores in the nuclear membrane

Translation (at the ribosome):

• mRNA attaches to a ribosome in the cytoplasm
• The ribosome reads the mRNA in triplets (codons)
• Transfer RNA (tRNA) molecules bring specific amino acids
• Each tRNA has an anticodon complementary to the mRNA codon
• Amino acids join together in the correct sequence
• The chain of amino acids folds to form a protein

Genetic variation and mutation

Variation refers to differences between individuals. There are two types:

Continuous variation:

• Characteristics that show a range of values with no distinct categories
• Examples: height, mass, skin colour, leaf length
• Usually controlled by multiple genes (polygenic)
• Significantly influenced by environmental factors
• Data forms a bell curve (normal distribution) when graphed

Discontinuous variation:

• Characteristics with distinct categories with no intermediates
• Examples: ABO blood group, ability to roll tongue, earlobe attachment
• Usually controlled by a single gene
• Little or no environmental influence
• Data forms a bar chart when graphed

Mutations are random changes to DNA base sequences. They occur spontaneously but the rate can be increased by:

• Ionising radiation (X-rays, gamma rays, UV light)
• Chemical mutagens (certain substances in tobacco smoke)

Most mutations have no effect because:

• They occur in non-coding regions of DNA
• The change doesn't alter the amino acid (degenerate genetic code)
• The change produces a similar amino acid

Some mutations can be harmful (causing genetic disorders), while very rarely they may be beneficial and provide an advantage.

Monohybrid inheritance

Monohybrid inheritance examines how a single gene with two alleles is inherited from parents to offspring.

Key principles:

• Each parent has two alleles for each gene
• Gametes (sex cells) contain only one allele for each gene
• During fertilisation, offspring inherit one allele from each parent
• This results in offspring having two alleles (one maternal, one paternal)

Genetic diagrams:

When constructing Punnett squares, always follow this method:

1. State the phenotypes of the parents
2. State the genotypes of the parents (use letters: capital for dominant, lower-case for recessive)
3. State the gametes each parent can produce (one allele from each)
4. Draw a Punnett square showing all possible combinations
5. Write out genotypes and phenotypes of offspring
6. Calculate ratios or probabilities

Homozygous × Homozygous cross:

• BB × bb produces all Bb offspring (100% heterozygous)
• All offspring show the dominant phenotype

Heterozygous × Heterozygous cross:

• Bb × Bb produces BB : Bb : bb in a 1:2:1 genotype ratio
• Phenotype ratio is 3:1 (dominant:recessive)
• 75% show dominant phenotype, 25% show recessive phenotype

Important terminology:

• F1 generation — first generation of offspring
• F2 generation — second generation (offspring of F1 × F1)
• The carrier — an individual who is heterozygous for a recessive condition (has the allele but doesn't show symptoms)

Sex determination and sex-linked inheritance

Sex chromosomes:

• Humans have 23 pairs of chromosomes; pair 23 determines biological sex
• Females have XX (two X chromosomes)
• Males have XY (one X, one Y chromosome)
• During gamete formation, females produce eggs with X only
• Males produce sperm with either X or Y
• There is a 50:50 ratio in offspring (50% male, 50% female)

Sex-linked characteristics:

• Genes located on sex chromosomes are called sex-linked
• Most sex-linked genes are on the X chromosome
• Males are more likely to express X-linked recessive conditions
• Males only need one recessive allele (they have only one X)
• Females need two recessive alleles (one on each X)

Examples of X-linked recessive conditions:

• Red-green colour blindness
• Haemophilia (blood doesn't clot properly)

Notation for sex-linked inheritance:

• X^D = X chromosome with dominant allele
• X^d = X chromosome with recessive allele
• Y = Y chromosome (usually carries no allele for X-linked genes)

A carrier female would be X^DX^d, while an affected male would be X^dY.

Inherited disorders

Some genetic conditions are inherited when individuals receive certain combinations of alleles:

Polydactyly (extra fingers or toes):

• Caused by a dominant allele (D)
• Can be inherited if one or both parents have the condition
• Genotypes: DD or Dd show the condition, dd is unaffected
• A heterozygous parent (Dd) has a 50% chance of passing the allele to offspring

Cystic fibrosis:

• Caused by a recessive allele (f)
• Must inherit two copies (ff) to have the condition
• Parents who are both carriers (Ff) have a 25% chance of having an affected child
• Affects cell membranes, causing thick mucus in lungs and digestive system
• Affects approximately 1 in 2,500 babies born in the UK

Ethical considerations:

• Genetic screening can identify carriers or affected individuals
• Allows informed reproductive choices
• Raises questions about discrimination and privacy
• Treatment availability varies globally

Worked examples

Example 1: Monohybrid cross

Question: In pea plants, tallness (T) is dominant to shortness (t). Two heterozygous tall plants are crossed. Calculate the ratio of phenotypes in the offspring. [4 marks]

Solution:

Parental phenotypes: Tall × Tall

Parental genotypes: Tt × Tt

Gametes: T and t (from each parent)

Punnett square:

	T	t
T	TT	Tt
t	Tt	tt

Offspring genotypes: TT : Tt : tt = 1:2:1 ✓

Offspring phenotypes: 3 tall : 1 short ✓

Ratio = 3:1 ✓

OR 75% tall, 25% short ✓

Marking points: Correct gametes (1), correct Punnett square (1), genotypes identified (1), phenotype ratio (1)

Example 2: Sex determination

Question: Explain why approximately 50% of babies born are male. Use a genetic diagram to support your answer. [4 marks]

Solution:

Females have XX sex chromosomes, males have XY ✓

Female gametes (eggs): all contain X ✓

Male gametes (sperm): 50% contain X, 50% contain Y ✓

	X (egg)	X (egg)
X (sperm)	XX (female)	XX (female)
Y (sperm)	XY (male)	XY (male)

There is an equal (50:50) probability of X or Y sperm fertilising the egg ✓

Marking points: State parental chromosomes (1), gametes identified (1), genetic diagram showing combinations (1), conclusion about 50:50 probability (1)

Example 3: Sex-linked inheritance

Question: Colour blindness is a sex-linked recessive condition. A woman who is a carrier marries a man with normal colour vision. What is the probability their son will be colour blind? [3 marks]

Solution:

Mother's genotype: X^DX^d (carrier) ✓

Father's genotype: X^DY (normal vision) ✓

	XD	Xd
XD	XDXD	XDXd
Y	XDY	XdY

Sons can be X^DY (normal) or X^dY (colour blind)

Probability = 50% or 1 in 2 or 0.5 ✓

Marking points: Correct parental genotypes (1), genetic diagram or working (1), correct probability (1)

Common mistakes and how to avoid them

• Confusing genotype and phenotype. Remember: genotype is the alleles present (letters), phenotype is what you observe (description). Always read the question carefully to determine which is required.

• Using the same letter for different alleles that aren't related. In a monohybrid cross, use the same letter in upper and lower case (e.g. B and b, not B and r). The capital represents dominant, lower-case represents recessive.

• Forgetting to show gametes in genetic diagrams. Examiners expect to see the gametes written out separately before completing the Punnett square. This shows you understand meiosis halves the chromosome number.

• Mixing up sex-linked notation. For sex-linked conditions, you must write the allele as a superscript on X (e.g. X^d), not separately. The Y chromosome typically has no corresponding allele.

• Stating percentages without showing working. Always draw the Punnett square or show working. Marks are awarded for method even if your final answer is incorrect.

• Confusing continuous and discontinuous variation. Continuous = range/spectrum (use histogram), discontinuous = distinct categories (use bar chart). Remember environmental factors heavily influence continuous variation.

Exam technique for "Genetics"

• Command words matter. "Describe" requires you to state what happens; "explain" requires you to say why it happens, often using "because" or "therefore". For genetic diagrams, "use a Punnett square" means you must draw one — a written description won't earn marks.

• Show all working in genetic crosses. Even if the question doesn't explicitly ask for a diagram, drawing one protects you. Write: parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes, ratio.

• One mark ≈ one relevant point. A 3-mark question typically requires three distinct points. If asked to "explain using a genetic diagram" [4 marks], expect 2-3 marks for the diagram itself and 1-2 for written explanation.

• Use appropriate terminology. Replace everyday words with scientific terms: say "gametes" not "sex cells" in formal answers, use "heterozygous" rather than "mixed", and write "recessive allele" not "weaker gene".

Quick revision summary

DNA is a double helix containing genes that code for proteins. Base pairing is specific: A-T and C-G. Inheritance follows predictable patterns shown through Punnett squares. Dominant alleles mask recessive ones. Heterozygous individuals have different alleles; homozygous individuals have identical alleles. Sex is determined by X and Y chromosomes; males are XY, females XX. Variation can be continuous (range of values, polygenic, environmental influence) or discontinuous (distinct categories, single gene). Mutations are random DNA changes, occasionally beneficial but often neutral or harmful.