Genotype is the genetic constitution of an organism. The term usually refers to the pair of alleles present at a particular locus.

Phenotype is the observable expression of the genotype. Phenotype results from the interaction between the alleles present and the environment.

What is the difference between a dominant and a recessive allele?

A dominant allele is expressed in the phenotype whenever it is present. A recessive allele is expressed only when two copies are present and no dominant allele is.

What are codominant alleles? Give an example.

Codominant alleles are both expressed in the phenotype of a heterozygote. The alleles for blood group antigens A and B are codominant, giving blood group AB.

What do homozygous and heterozygous mean for a diploid organism?

A homozygous organism carries two identical alleles at a locus. A heterozygous organism carries two different alleles at that locus.

CCEA A-Level Life and Health Sciences 0008

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Genetics, Stem Cell Research and CloningA2 option

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Genetics, Stem Cell Research and Cloning

Genotype, phenotype and monohybrid inheritance

3 things to know
A2 5

Overview

Genotype and phenotype, dominant, recessive and codominant alleles, carrier crosses and probabilities, test crosses, sickle cell trait, sex linkage and haemophilia, and ABO multiple alleles.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Define genotype.

Key term

Define phenotype.

Key term

Why can identical twins differ in body mass even though their genotypes are the same?

Questions from this subtopic

Each one with the model answer underneath.

1
Define genotype.
Model answer
Genotype is the genetic constitution of an organism. The term usually refers to the pair of alleles present at a particular locus.
2
Define phenotype.
Model answer
Phenotype is the observable expression of the genotype. Phenotype results from the interaction between the alleles present and the environment.
3
Why can identical twins differ in body mass even though their genotypes are the same?
Model answer
Body mass is influenced by the environment, including diet and exercise, as well as by genes. Twins with identical genotypes can therefore develop different phenotypes.
4
What is the difference between a dominant and a recessive allele?
Model answer
A dominant allele is expressed in the phenotype whenever it is present. A recessive allele is expressed only when two copies are present and no dominant allele is.
5
What are codominant alleles? Give an example.
Model answer
Codominant alleles are both expressed in the phenotype of a heterozygote. The alleles for blood group antigens A and B are codominant, giving blood group AB.
6
What do homozygous and heterozygous mean for a diploid organism?
Model answer
A homozygous organism carries two identical alleles at a locus. A heterozygous organism carries two different alleles at that locus.

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