Alleles and Inheritance

Quick revise

An Allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

Alleles are dominant or recessive

Homozygous = two same alleles (purebred)

Heterozygous = two different alleles


Dominant + recessive > dominant

Dominant + dominant > dominant

Recessive + dominant > dominant

Recessive + recessive > recessive


A heterozygous brown-eyed father and a blue-eyed mother


Result: 50:50 chance of being either brown eyed or blue eyed.


An individual is homozygous for a certain gene if they have two identical alleles. They are heterozygous for a certain gene if they two different alleles.

The genotype is the genetic makeup of an individual. For example, it is the particular combination of alleles

The phenotype is the characteristics expressed by an individual. For example, it is the actual eye colour

Monohybrid Cross


In a monohybrid cross two plants or animals, which differ at only one gene and which are bred together.

By looking at alleles of the genes that the parents have we can tell how the offspring will turn out. This can be very useful when carrying out selective breeding of plants or animals.

In humans with inherited diseases it is extremely useful to know about the alleles since we can inform them about the likely effect if they want to have children.

Thsi video below explains about Monohybrid crosses and Punnet Squares