How are characteristics passed from parents to offspring, and how do we predict the results of a genetic cross?
Use the terms gene, allele, dominant, recessive, genotype and phenotype to explain monohybrid inheritance, and use genetic diagrams to predict the offspring of a cross
A focused answer to the O-Level Combined Science outcome on inheritance. Genes and alleles, dominant and recessive, genotype and phenotype, and using a genetic diagram (Punnett square) to predict the ratios of offspring in a monohybrid cross.
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What this dot point is asking
SEAB wants you to use the key genetics terms correctly, to explain monohybrid inheritance (one characteristic controlled by one gene), and to draw a genetic diagram to predict the genotypes, phenotypes and ratios of offspring. Drawing a clear cross and reading off the ratio is the central exam skill here.
The answer
The key terms
- A gene is a section of DNA that codes for a characteristic.
- An allele is a version of a gene (for example, the tall allele or the short allele).
- A dominant allele is expressed even when only one copy is present (shown by a capital letter, e.g. T).
- A recessive allele is expressed only when two copies are present (shown by a lower-case letter, e.g. t).
- The genotype is the alleles an organism has (e.g. TT, Tt, tt).
- The phenotype is the observable characteristic (e.g. tall or short).
- Homozygous means two identical alleles (TT or tt); heterozygous means two different alleles (Tt).
How alleles combine
Each parent has two alleles for a characteristic but passes only one to each gamete (because of meiosis). At fertilisation, the offspring receives one allele from each parent, giving its genotype. If a dominant allele is present, it masks the recessive one, so a heterozygous organism (Tt) shows the dominant phenotype.
The genetic diagram (Punnett square)
To predict a cross:
- write the genotypes of the parents,
- work out the gametes each can make,
- combine the gametes in a grid (the Punnett square) to find the offspring genotypes,
- read off the phenotypes and the ratio.
A worked cross pattern
Crossing two heterozygous tall plants (Tt with Tt) gives offspring TT, Tt, Tt, tt, a ratio of 3 tall : 1 short. Crossing heterozygous with homozygous recessive (Tt with tt) gives Tt, tt, Tt, tt, a ratio of 1 tall : 1 short. The diagram gives the ratio every time.
Examples in context
Example 1. Inherited conditions. Some human conditions are caused by recessive alleles, so a child can inherit the condition only if both parents pass on the recessive allele. Genetic diagrams let families understand the chance of a child being affected, a direct medical use of monohybrid inheritance.
Example 2. Breeding plants for a trait. A grower who wants only tall plants can cross two homozygous tall plants (TT x TT) so all offspring are tall, or use genetic diagrams to predict the outcome of crossing different genotypes. This shows how inheritance theory guides selective breeding.
Try this
Q1. Define the terms dominant allele and recessive allele. [2 marks]
- Cue. A dominant allele is expressed when only one copy is present; a recessive allele is expressed only when two copies are present.
Q2. A plant has the genotype Tt for height, where T (tall) is dominant. State its phenotype and the gametes it can make. [2 marks]
- Cue. Phenotype: tall; gametes: T and t.
Q3. Two heterozygous plants (Tt) are crossed. State the expected ratio of tall to short offspring. [2 marks]
- Cue. 3 tall : 1 short.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksIn pea plants, the allele for tall (T) is dominant to the allele for short (t). A heterozygous tall plant (Tt) is crossed with a short plant (tt). Draw a genetic diagram to show this cross, and state the expected ratio of tall to short offspring.Show worked answer →
Parents: Tt (tall) crossed with tt (short).
Gametes: the Tt parent makes gametes T and t; the tt parent makes gametes t and t.
Genetic diagram (combining the gametes):
T with t gives Tt; t with t gives tt; T with t gives Tt; t with t gives tt.
Offspring genotypes: Tt, tt, Tt, tt, which is 2 Tt and 2 tt.
Phenotypes: Tt is tall, tt is short. So the expected ratio is 2 tall : 2 short, that is 1 tall : 1 short.
Markers reward the correct parental genotypes and gametes, a correct genetic diagram combining the gametes, the offspring genotypes Tt and tt, and the final ratio of 1 tall : 1 short.
Original3 marksExplain the difference between genotype and phenotype, and explain why an organism with a recessive allele may not show the recessive characteristic.Show worked answer →
The genotype is the alleles an organism has for a characteristic (for example Tt or tt). The phenotype is the observable characteristic that results (for example tall or short).
An organism with one recessive allele may not show the recessive characteristic if it also has a dominant allele (it is heterozygous, e.g. Tt). The dominant allele is expressed and masks the recessive one, so the recessive characteristic only shows when both alleles are recessive (tt).
Markers reward genotype as the alleles present and phenotype as the observable characteristic, and the explanation that a dominant allele masks a recessive allele so the recessive trait shows only when homozygous recessive.
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