IG2 - Inheritance (Online Lesson 5)

A gene is a blueprint for a trait (example: eye colour)

These genes may have several different versions (example: brown/blue)

A version of a gene is called an allele

Chinchillas are rodents native to South America (Andes mountains)

Genes determine the colour of a chinchilla's fur

These genes are instructions for producing proteins that cause different fur colours

In this example, chinchillas can have 2 different fur colours: grey or charcoal

Therefore, there are 2 alleles for the fur colour gene

When looking at the inheritance of gene alleles, we simplify each allele to a single letter

Tip: Always choose a letter that looks distinctly different when capital and lowercase

A/a and B/b are good

C/c and S/s are bad

In each cell of the chinchilla, there are 2 copies of the fur colour gene (1 from mother, 1 from father)

Let's say that the gene for fur colour is represented by the letter G or g

Grey fur is G

Charcoal fur is g

There are 3 possible combinations of this gene for any chinchilla to have in their cells:

GG

Gg

gg

If the 2 alleles are the same (GG or gg), we say that the chinchilla is homozygous

If the 2 alleles are different (Gg), we say that the chinchilla is heterozygous

The genetic makeup of an organism in terms of the alleles present is called its genotype

The observable features/traits an organism has is called its phenotype

Example: GG/Gg/gg = genotype Grey/charcoal fur = phenotype

The chinchilla has 3 possible genotypes (GG, Gg, gg) but only 2 possible phenotypes (grey and charcoal)

GG = grey

gg = charcoal

Genotype Gg corresponds with the phenotype Grey

This is because the G allele is dominant over the g allele (it will silence g)

A dominant allele is an allele that is expressed when it is present

A heterozygous chinchilla (Gg) is also called a carrier because it has the allele for charcoal fur but doesn't express it

The allele g is called recessive because it is only expressed when there is no dominant allele present