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Fundamentals of genetic variation

Genetic variation is the difference in DNA. The variation in DNA can be considered between individuals within a population and from different populations.

Genetic variation is the basis of evolution.

Genetic variation is ultimately the result of mutations.

The variety in all living organisms is due to genetic variation. Almost all organisms have a different genome.

People have used this to their advantage to help breed better crops and animals.

These carrots have been selectively bred. They display significant genetic variation.
These carrots have been selectively bred. They display significant genetic variation.

Inheritance is the transmission of genes from parent to offspring through reproduction (vertical gene transfer).

In asexual reproduction , the offspring will inherit all of its genes from a single parent . Only mutations can lead to genetic differences between parent and offspring.

A mutation in a gene will produce a new version of that gene.

In sexual reproduction , the offspring will inherit half of its genes from each parent .

Each parent can have different versions of each gene. During sexual reproduction, a unique combination of these genes is produced. Even siblings will differ in the combination they receive!

This is why siblings from the same parents are not identical.

Siblings look similar, but are not identical to each other or to their parents
Siblings look similar, but are not identical to each other or to their parents

Alleles are different versions of a single gene.

Within a species, almost all individuals share the same genes. However, the DNA sequence of the genes can differ between individuals. These different versions of genes are called alleles.

Alleles are often associated with certain characteristics.

The pea plant has a gene that determines flower colour. One allele codes for purple flowers and another allele codes for white flowers.

Animals normally have two alleles for each gene: one on each chromosome. The alleles can be the same or different.

Important! Even though an individual has two alleles for a gene, there may be many more possible alleles of that gene.

There are three alleles for red blood cell type, but an individual can only express two!

These two colour variants of the moth B. betularia are determined by different alleles.
These two colour variants of the moth B. betularia are determined by different alleles.

Diploid organisms (such as most animals) contain two alleles for each gene. There are two possible situations for a given gene. The organism can be:

  • Homozygous: The individual carries two identical alleles of a gene on both homologous chromosomes.

A pea plant that has the purple allele for flower colour on both chromosomes is homozygous with respect to flower colour.

A homozygous organism is said to be a homozygote with respect to that particular gene.

  • Heterozygous: The individual carries two different alleles of the gene on homologous chromosomes.

A heterozygous organism is said to be a heterozygote with respect to that particular gene.

Jamie has one allele for dimples and another allele for no dimples. He is therefore a heterozygote with respect to the gene for dimples!

Each chromosome has two alleles of every gene.
Each chromosome has two alleles of every gene.

The genotype of an organism is the combination of alleles that the organism has.

A genotype is typically described with respect to one gene.

The genotype of a plant that has alleles R and r for the flower colour gene is Rr (heterozygous dominant).

The phenotype of an organism is the observable characteristics (traits) that organism has.

The phenotype of red flowers is associated with the genotype Rr for the flower colour gene.

The genotype is entirely inherited (ignoring mutations). The phenotype depends on both the genotype and environmental factors.

The table below illustrates the relationship between genotype (first row) and phenotype (second row) in flower colour:

RR Rr rr

The genotype influences the phenotype of an organism.

The genotype is the inherited information (genes), while the phenotype is the outward physical manifestation of the genes.

The genotype-phenotype relationship is highly complex.

If there are two different alleles coding for the same gene (in a heterozygote), it is unlikely that the two alleles will both be expressed equally.

It is rare for a heterozygous phenotype to fall exactly halfway between the two homozygous extremes.

Alleles are expressed to different extents determined by dominance relationships between them.

Most traits are polygenic, meaning that many genes interact to determine the characteristic.

Genes are not the only factor that determines the phenotype. In most cases, the environment has a significant influence on the phenotype expressed.

Hair colour is due to the pigment melanin. The relative levels of two different types of melanin are responsible for hair colour.

Several genes interact to regulate the concentration of the two types of melanin. This makes hair colour a polygenic trait.

The amount of each type of melanin do not remain the same over the life time of a person. Gene expression gradually changes in one's lifetime, altering the phenotype.

There is a gradual loss of hair pigmentation (a decline in the production of both types of melanin), causing hair to turn grey and then white as people get older.

Environmental factors also influence hair colour. Exposure to sunlight, artificial dyes and even diet have an effect on hair colour.

Humans have a wide range of hair colours. Environmental factors such as dyeing also influence hair colour.
Humans have a wide range of hair colours. Environmental factors such as dyeing also influence hair colour.

Genetic variation describes the differences in alleles and genes between individuals.

Genetic variation can be distinguished by the type of phenotype:

  • Discontinuous variation is characterised by the phenotype falling into discrete categories. Discontinuous traits are controlled by a small number of genes.
  • Blood type is a discontinuous trait. People are either A, B, AB or O.

  • Continuous variation involves the phenotype taking on a range of values. It is possible to fall anywhere on the scale. Continuous traits are controlled by many genes.
  • Body weight is a continuous trait. Weight is determined by hundreds of genes, as well as environmental factors like diet.

Discontinuous traits can be displayed as a bar graph. Continuous traits are best shown using a histogram.

Discontinuous variation is shown as a bar graph. The continuous variable on the right fits a normal distribution.
Discontinuous variation is shown as a bar graph. The continuous variable on the right fits a normal distribution.