Genetic Inheritance

Key Terms for Genetic Inheritance

Gametes are sex cells. In animals, gametes are either sperm (in males) or ova (in females). In plants, gametes are found in pollen grains (male gametes) and in ovules (female gametes).

Chromosomes are structures made of DNA that are found in the nucleus of nearly all cells in the human body. In humans, each cell contains 23 pairs of chromosomes. So, normal human cells contain two copies of each chromosome. One chromosome in the pair is inherited from the mother and one is inherited from the father. A mutation of a gene inside a chromosome may lead to an inherited disorder.

Genes are small sections of DNA on a chromosome that determine the sequence of amino acids that form a protein. As chromosomes come in pairs, there are two copies of every gene.

Alleles are a variation of a particular gene (e.g. brown eyes vs. blue eyes). Alleles can be dominant or recessive. For each gene, an individual has two alleles because one is inherited from the mother and the other from the father. The same is true in plants.

A dominant allele is always expressed, even if only one copy is present. We use capital letters to represent dominant alleles.

A recessive allele is only expressed when two copies are present, meaning no dominant allele is present. We represent recessive alleles using lowercase letters.

Being homozygous means that the individual inherited two identical copies of an allele. For example, FF or ff.

Being heterozygous means that the individual inherited two different copies of an allele. For example, Ff.

A genotype is the combination of alleles that determine a characteristic.

A phenotype is the observable characteristics of an organism, which are expressed by the alleles. For example, hair colour or blood type.

Single Gene Inheritance

Some characteristics in species are controlled by a single gene. Examples of this can be found in fur colour in dogs, or red-green colour blindness in humans. These genes may have different variations, which are called alleles.

However, most characteristics are controlled by multiple genes, rather than just a single gene.

You’ve used 0 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up