Genetic terms and monohybrid inheritanceAQA A-Level Biology: Revision notes

Section 1

Genetic terms

A gene is a length of DNA that codes for a polypeptide. Different forms of a gene are alleles, and there may be many alleles of one gene. The position of a gene on a chromosome is its locus.

The genotype is the genetic constitution of an organism, meaning the alleles it carries. The phenotype is the expression of that genotype and its interaction with the environment. The same genotype can give different phenotypes in different environments, for example a plant that is yellow when grown in the dark.

In a diploid organism an individual has two alleles at each locus, one from each parent. If they are the same the individual is homozygous; if different, heterozygous.

Key termsgeneallelelocusgenotypephenotypehomozygousheterozygous

Common mistake

A phenotype is not just what you can see. Include the environment as well as the genotype when you define it.

Section 2

Dominant, recessive and codominant alleles

A dominant allele is expressed in the phenotype whether the individual is homozygous or heterozygous. A recessive allele is expressed only in the homozygous recessive. Heterozygous individuals with a recessive allele are carriers if they do not show it.

With codominant alleles, both alleles are expressed in the phenotype of the heterozygote. Examples are the red and white alleles in shorthorn cattle (roan) and the alleles of the haemoglobin gene.

Write alleles as letters: a capital letter for dominant and the same lower case for recessive. For codominance, use a capital letter with superscripts, such as $C^R$ and $C^W$ .

Key termsdominantrecessivecodominantcarrier

Section 3

Inheritance of alleles

In a diploid organism, meiosis produces gametes that each carry only one allele of each gene. At fertilisation, the offspring inherits one allele from each parent. Alleles are inherited at random, so any gamete from one parent can fuse with any gamete from the other.

If both parents are heterozygous ( $Aa$ ), each parent produces equal numbers of gametes $A$ and $a$ .

Key termsmeiosisgametefertilisation

Section 4

Genetic diagrams for monohybrid crosses

A monohybrid cross studies the inheritance of one gene. A fully labelled genetic diagram has these steps:

Parental phenotypes.
Parental genotypes.
The gametes from each parent, each with one allele.
The offspring genotypes, for example in a Punnett square.
The offspring phenotypes and the ratio.

Worked example: tall $Tt$ × dwarf $tt$ . Gametes: $T$ , $t$ and $t$ . Offspring: $Tt$ and $tt$ in a 1:1 ratio. A cross of two heterozygotes gives 3 dominant : 1 recessive for complete dominance, or 1:2:1 for codominance.

Key termsmonohybrid crossPunnett square

Exam tip

Always show the gametes separately and use circles or labels. An unlabelled Punnett square loses marks.

Section 5

Probability in inheritance

Each fertilisation is an independent event, so earlier children do not change the odds for the next. For two carriers of a recessive allele, the chance of an affected child is 1/4 each time.

Use two rules. To find the chance of two independent events both happening, multiply: 1/4 × 1/4 = 1/16. To find the chance of either of two outcomes, add the probabilities. When the outcomes could occur in different orders (one affected and one not, in either order), count each order: $2 × 1/4 × 3/4 = 3/8$ .

Key termsprobabilityindependent event

Common mistake

A family with one affected child does not have a lower risk next time. The probability stays 1/4.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Genetic terms and monohybrid inheritance

In shorthorn cattle, coat colour is controlled by a single gene with two alleles, $C^R$ (red hairs) and $C^W$ (white hairs). The alleles are codominant. Animals with both alleles have a roan coat, with a mixture of red and white hairs.
A red bull is crossed with a white cow. Predict the phenotype of the calves and explain your answer.2 marks
Cystic fibrosis is caused by a recessive allele, $f$ , of a gene on chromosome 7. The dominant allele, $F$ , codes for a functioning chloride ion channel. Two healthy parents, who have both already had one child with cystic fibrosis, plan to have more children.
The couple go on to have two more children. Calculate the probability that both children have cystic fibrosis.2 marks
In pea plants, tall stems are produced by the dominant allele $T$ and dwarf stems by the recessive allele $t$ . A plant breeder crosses a tall plant of unknown genotype with a dwarf plant. The offspring are 48 tall plants and 52 dwarf plants.
Deduce the genotype of the tall parent and explain how you reached your answer.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).