What is meant by homozygous and heterozygous?

Homozygous: two identical alleles at a locus. Heterozygous: two different alleles.

What is a dominant allele?

An allele expressed in the phenotype when present in one or two copies.

When is a recessive allele expressed?

Only when the individual is homozygous recessive.

What are codominant alleles?

Alleles that are both expressed in the phenotype of a heterozygote.

A heterozygote with a recessive allele that does not show the recessive phenotype.

What ratio do two heterozygotes give with codominance?

1 : 2 : 1 (two homozygous phenotypes and the intermediate).

What are the steps in a fully labelled genetic diagram?

Parental phenotypes, genotypes, gametes, offspring genotypes, offspring phenotypes and ratio.

Genetic terms and monohybrid inheritanceAQA A-Level Biology: Flashcards

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Define genotype.

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Define genotype.: The genetic constitution of an organism, meaning the alleles it carries.
Define phenotype.: The expression of the genotype and its interaction with the environment.
What is an allele?: One of the different forms of a gene.
What is meant by homozygous and heterozygous?: Homozygous: two identical alleles at a locus. Heterozygous: two different alleles.
What is a dominant allele?: An allele expressed in the phenotype when present in one or two copies.
When is a recessive allele expressed?: Only when the individual is homozygous recessive.
What are codominant alleles?: Alleles that are both expressed in the phenotype of a heterozygote.
What is a carrier?: A heterozygote with a recessive allele that does not show the recessive phenotype.
What ratio do two heterozygotes give with complete dominance?: 3 dominant : 1 recessive.
What ratio do two heterozygotes give with codominance?: 1 : 2 : 1 (two homozygous phenotypes and the intermediate).
What are the steps in a fully labelled genetic diagram?: Parental phenotypes, genotypes, gametes, offspring genotypes, offspring phenotypes and ratio.
Two carriers have a child: probability of an affected child?: 1/4.
How do you find the probability of two independent events both happening?: Multiply their probabilities.

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).