What is DNA, and how are characteristics inherited through dominant and recessive alleles?
DNA, genes and chromosomes, dominant and recessive alleles, and using genetic diagrams to predict the inheritance of a characteristic.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on inheritance, covering DNA, genes and chromosomes, dominant and recessive alleles, genotype and phenotype, and using a genetic (Punnett) diagram to predict inheritance.
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What this dot point is asking
WJEC Double Award Unit 4 wants you to describe DNA, genes and chromosomes, explain dominant and recessive alleles, and use genetic diagrams to predict inheritance.
DNA, genes and chromosomes
Humans have 23 pairs of chromosomes in each body cell. Each pair carries genes for the same characteristics, one from each parent.
Alleles, genotype and phenotype
If both alleles are the same (RR or rr), the organism is homozygous; if they are different (Rr), it is heterozygous.
Using a genetic diagram
A genetic diagram (Punnett square) predicts the offspring of a cross. You write the alleles of each parent's gametes along the top and side, then fill in the combinations. For a cross between two heterozygous red plants (Rr x Rr):
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
The offspring are RR, Rr, Rr, rr. Because R (red) is dominant, the first three are red and only rr is white, giving a 3 red : 1 white ratio.
Reading ratios from a diagram
The diagram gives both a genotype ratio (1 RR : 2 Rr : 1 rr) and a phenotype ratio (3 red : 1 white). Read the question carefully: if it asks for the characteristic (colour), give the phenotype ratio; if it asks for the alleles, give the genotype ratio. Ratios are predictions of probability, so real offspring may not match exactly, especially with small numbers.
Inherited disorders
Some conditions are caused by faulty alleles and are inherited. Many are caused by a recessive allele, so a person must inherit two copies (one from each parent) to have the disorder; someone with only one copy is a healthy carrier. Cystic fibrosis is an example of a recessive inherited disorder. Genetic diagrams can be used to work out the chance that two carriers will have an affected child (a 1 in 4 chance from two carriers). Being able to use a Punnett square to find the probability of inheriting a disorder is a common exam task.
Sex determination
Sex is also decided by inheritance. Of the 23 pairs of chromosomes, one pair are the sex chromosomes: females are XX and males are XY. The egg always carries an X, while the sperm carries either an X or a Y, so it is the sperm that determines the sex of the child. A genetic diagram crossing XX with XY gives XX, XX, XY, XY, a 1 : 1 ratio, which is why there is roughly an equal chance of a boy or a girl. This is a neat application of genetic diagrams to a familiar example.
Try this
Q1. What is an allele? [1 mark]
- Cue. A different version of a gene.
Q2. A characteristic only shows when two copies of the allele are present. Is the allele dominant or recessive? [1 mark]
- Cue. Recessive.
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 is heterozygous for flower colour (Rr), where red (R) is dominant to white (r). Draw a genetic diagram for a cross between two such plants and give the ratio of offspring colours.Show worked answer →
A Unit 4 genetics question worth 4 marks. Reward: a correct Punnett square crossing Rr x Rr, giving offspring RR, Rr, Rr, rr (2 for a correct diagram); three red to one white (3 red : 1 white) phenotype ratio (1); because only rr is white and R is dominant (1). Markers credit the correct gametes and combinations, and the 3:1 ratio. A common error is to give the genotype ratio (1:2:1) when asked for the colour (phenotype) ratio.
WJEC style3 marksExplain the difference between the terms genotype and phenotype, and between dominant and recessive alleles.Show worked answer →
A Unit 4 explain question. Reward: the genotype is the alleles an organism has (for example Rr) (1); the phenotype is the characteristic that is shown (for example red flowers) (1); a dominant allele is expressed even if only one copy is present, while a recessive allele is only expressed when two copies are present (1). Markers credit genotype as the alleles, phenotype as the characteristic, and the dominant/recessive distinction. A common error is to swap genotype and phenotype.
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