How is genetic information stored in DNA, and how do we predict the offspring of a genetic cross?
The structure of DNA, genes, chromosomes and alleles, the meaning of genotype and phenotype, monohybrid inheritance and Punnett squares, sex determination, and the inheritance of genetic disorders.
A focused answer to the WJEC GCSE Biology section 2.3 topic on DNA and inheritance, covering the structure of DNA, genes, chromosomes and alleles, genotype and phenotype, monohybrid crosses with Punnett squares, sex determination, and the inheritance of genetic disorders.
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What this dot point is asking
WJEC wants you to describe the structure of DNA and the meaning of genes, chromosomes and alleles, define genotype and phenotype, carry out monohybrid crosses with Punnett squares, explain sex determination, and show the inheritance of a genetic disorder.
The structure of DNA
Each nucleotide contains a sugar, a phosphate and one of four bases: A, T, C and G. The two strands are held together by base pairing, where:
- A pairs with T, and
- C pairs with G.
The order of the bases along the DNA is a code that determines the order of amino acids in a protein, and so which proteins a cell makes.
Genes, chromosomes and alleles
- A gene is a section of DNA that codes for a particular protein (and so a characteristic).
- A chromosome is a long molecule of DNA carrying many genes. Human body cells have 23 pairs of chromosomes (46 in total).
- An allele is a different version of the same gene. For example, the gene for eye colour has different alleles for brown and blue.
Two more terms describe how alleles affect an organism:
- The genotype is the combination of alleles an organism has (for example RR, Rr or rr).
- The phenotype is the actual characteristic shown (for example red flowers).
Dominant and recessive alleles
- A dominant allele shows in the phenotype even if only one copy is present. It is written with a capital letter (R).
- A recessive allele only shows if both copies are present. It is written with a lower-case letter (r).
- Homozygous means both alleles are the same (RR or rr); heterozygous means they are different (Rr).
Monohybrid crosses and Punnett squares
A monohybrid cross follows the inheritance of one gene. A Punnett square sets out the gametes and predicts the offspring.
For a cross between two heterozygous red-flowered plants (Rr x Rr), where R (red) is dominant to r (white):
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
The offspring genotypes are 1 RR : 2 Rr : 1 rr. Because R is dominant, RR and Rr are red and rr is white, giving a phenotype ratio of 3 red : 1 white.
Sex determination
In humans, the sex chromosomes determine whether you are male or female.
- Females are XX.
- Males are XY.
The mother (XX) always passes on an X. The father (XY) passes on either an X or a Y, with an equal chance. This gives a roughly 50:50 ratio of female (XX) to male (XY) offspring.
Inheriting a genetic disorder
Some disorders are caused by a faulty allele. If the disorder is recessive, a person must inherit two faulty alleles (one from each parent) to have it; a person with one faulty allele is a healthy carrier. A Punnett square can predict the chance of a child having or carrying the disorder.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksA plant has a gene for flower colour. The allele for red (R) is dominant to white (r). Use a Punnett square to find the ratio of offspring from a cross between two Rr plants.Show worked answer →
A 4-mark genetic cross. Award marks for the gametes, the square, the genotypes and the ratio.
Each Rr parent makes gametes R and r. The Punnett square gives: RR, Rr, Rr and rr.
The genotypes are 1 RR : 2 Rr : 1 rr. Since R is dominant, RR and Rr are red and rr is white. So the phenotype ratio is 3 red : 1 white.
Markers reward: correct gametes (R and r from each); the four offspring genotypes; the 3:1 red to white ratio. Forgetting that R is dominant (so Rr is red) is the usual error.
WJEC style3 marksUse a genetic diagram to explain why there is roughly an equal chance of a baby being male or female.Show worked answer →
A 3-mark question on sex determination.
Females are XX and males are XY. The mother (XX) can only pass on an X. The father (XY) passes on either an X or a Y, with an equal chance. So half the offspring are XX (female) and half are XY (male), giving roughly a 50:50 ratio.
Markers reward: female XX, male XY; the mother gives X, the father gives X or Y; the resulting 1:1 (50:50) ratio of female to male. Saying the chance depends on the mother is incorrect; it depends on which sperm fertilises the egg.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)