New News Biology #46

DNA (Part 2): Alleles/Dominant/Heterozygous/Phenotypes & More

Recap: A gene is a segment of DNA that codes for a particular type of protein.

The genes that we inherit will determine our characteristics. Sometimes it might be a single gene (for example, fur color in mice or red/green colour blindness in humans). However, in most other cases, our characteristics are determined by several different genes which interact with each other (for example, there are loads of genes which code for height).

If you pay attention, the definition of gene is that it codes for a particular type of protein. This is because there maybe a lot of different forms of a single protein, and there’s a different code for each of those forms. These different codes are known as ‘alleles’. Alleles = different versions of a gene.

As we have two copies of every gene (see pt.1 of DNA), there are 2 alleles of every single gene. They can either be the same, in which we call them ‘homozygous’. Or they can be different alleles, in which it is known as ‘heterozygous’.

This is where things get interesting. Say we have a mouse, who has two genes, one from each parent, for its fur colour, and there are only two kinds of alleles for fur colour, purple & green.

If the mouse is heterozygous, it has one of each. So what colour should it be? Here’s a hint, it isn’t the mix of the two. In this case, there will be a dominant allele and the other one will be recessive. Let’s assume that purple is dominant (making green recessive). In this case, whenever purple is present, no matter if the mouse is heterozygous or ‘homozygous dominant (both alleles are purple)’, its fur colour will always be purple.

However, this doesn’t mean that a green mouse is impossible. There is a case in which the mouse is green in colour, where both alleles are green, which is known as ‘homozygous recessive’.

Differences between ‘Genotype’ & ‘Phenotype’:

Genotype means the collection of alleles we have, in the mouse case, there are three. Each of the possible cases are a different genotype.

Phenotype is the characteristics you will get from genotype. In the heterozygous and homozygous dominant genotypes of the mouse, they share the same phenotype, because the outcome is both of them are purple. Meanwhile, because the homozygous recessive genotype is green, it is both different in genotype and phenotype from the other two.