Codominance, multiple alleles and lethal alleles

Overview

Phenotypic ratio from crossing two pink codominant heterozygotes, multiple alleles and three alleles of the human ABO blood group gene.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is codominance?

   Model answer

   The condition in which both alleles of a heterozygote are fully expressed in the phenotype.

2. 2

   How are codominant alleles distinguished in a genetic diagram?

   Model answer

   Each is given its own capital letter, or a common gene letter with a distinguishing superscript, because neither allele is recessive to the other.

3. 3

   In a codominant flower colour gene, one allele gives red and the other white. What colour is the heterozygote?

   Model answer

   Pink, or in some species a flower with red and white patches, because both alleles are expressed.

4. 4

   State the phenotypic ratio from crossing two pink codominant heterozygotes.

   Model answer

   One red to two pink to one white, a 1 : 2 : 1 ratio.

5. 5

   Why does a codominant cross give the same phenotypic and genotypic ratio?

   Model answer

   Each genotype produces its own distinct phenotype, so no two genotypes are grouped together.

6. 6

   What is meant by multiple alleles?

   Model answer

   A gene that exists in more than two allelic forms in the population, although any one diploid individual still carries only two.

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