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Inheritance and Genetics: Question 1

Syllabus 17.1

Multiple choice Core 1 mark

A pair of budgerigars is being bred for feather colour. In this species, feather colour is controlled by a single gene, which exists as two alternative forms: G, which produces green feathers, and g, which produces blue feathers. G is dominant to g.

Which statement correctly describes the relationship between this gene and its alleles?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the definitions of chromosome, gene and allele

A chromosome is a structure made of DNA found in the nucleus. A gene is defined as a length of DNA that codes for a specific protein. An allele is an alternative form of a gene: different alleles of the same gene occupy the same position (locus) on a pair of chromosomes, and an organism inherits one allele of each gene from each parent.

Step 2: Apply these definitions to the budgerigar example

Here, the feather-colour gene is one particular length of DNA, coding for a protein involved in producing feather pigment. G and g are not two different genes and not the feathers themselves, they are the two alternative forms that this one gene can take, sitting at the same locus on a pair of chromosomes. A budgerigar inherits one of these alleles from its mother and one from its father.

Why the other options are wrong

  • B: this confuses a gene with a whole chromosome, and wrongly splits G and g into two separate genes on different chromosomes, when they are really two alleles of one gene.
  • C: this confuses the alleles (G and g, which are lengths of DNA) with the phenotypes they produce (green and blue feathers), and wrongly separates the gene from its alleles as if they were different things.
  • D: this wrongly treats G and g as alleles of two different genes on two different chromosome pairs, and wrongly implies only one gene is inherited, when in fact one allele of the same gene is inherited from each parent.

Final answer

The feather-colour gene is a length of DNA coding for a protein, and G and g are two alleles of this one gene, found at the same locus on a pair of chromosomes, option A.