Variation Biology

What are the Genetic Sources of Variation?

While members of the same species share the same genes, genetic differences occur when different individuals harbour different alleles. In real-world populations, the frequencies of certain alleles might also change from generation to generation. In sexually reproducing organisms containing diploid cells, genetic variation arises from processes such as mutations, crossing over, independent assortment, and random fertilisation.

What is a Mutation?

Mutations describe changes in the DNA base sequence that results in the generation of one or more new alleles. These new alleles may be beneficial, harmful, or neutral to the individual’s phenotype. Depending on the organism, mutations can occur fairly frequently. However, they are not always expressed. Even when they are, they may have little to no effect on the phenotype.

Fig. 1 - Mutation in a base sequence

Mutations occur throughout our lives and can happen in any kind of cell. Most mutations are harmless, but some can be harmful, and some are even associated with the symptoms of aging. However, not all mutations are passed on between generations. Mutations must happen in the germline – that is, the cells that form our gametes– to be passed on to offspring. Offspring don’t inherit autosomal mutations.

What is crossing over?

Diploid organisms undergo the process of meiosis to generate haploid gametes. During meiosis, crossing over occurs during which non-sister chromatids exchange alleles. In meiosis I, homologous chromosomes pair up very closely to one another. Due to the proximity, non-sister chromatids can cross over and get entangled together at crossing points called chiasmata. This entanglement creates enough stress on the DNA molecule to cause it to break and rejoin with the chromatid from the other chromosome, effectively swapping out a chunk of DNA. This swapping can result in a new combination of alleles on the two chromosomes. It usually happens at least once in each bivalent pair and is more likely to occur further away from the centromere.

Fig. 2 - Crossing over

What is independent assortment?

During metaphase, chromosome pairs are pulled towards the equator of the mitotic spindle. Each pair can be randomly arranged with either chromosome on top. This orientation is completely independent of the actions of any other pair. The homologous chromosomes are then separated and pulled to different poles, leading to different combinations of alleles in each gamete. To determine how many chromosome combinations can result due to independent assortment, we use the formula 2n, where n is the number of chromosomes in a haploid cell.

What is the random fertilisation of gametes?

The process of meiosis creates genetic variation between gametes, which means that gametes will each carry various alleles. Fertilisation occurs when male and female gametes fuse to create a new individual. In humans, the gametes are sperm and egg cells. Any sperm cell can fuse with any egg cell and share its genetic information, thus creating genetic variation.

What are the environmental sources of variation?

The environment itself has relatively little impact on genetic variation. However, it can significantly influence organisms and change the way certain genes are expressed. While the genes can serve as a blueprint for an organism’s phenotypic limits, the environment determines where exactly within those bounds the organism lies.

Can environmental influences on phenotype be passed on to offspring?

Variation in phenotype due to environmental factors does not usually cause genetic variation, and because of this, it cannot be passed on to offspring. For instance, something like the presence or absence of nitrogen in an environment does not cause an individual to mutate a new allele that allows it to survive better in those conditions.

What is the normal distribution curve?

Often, quantitative characteristics of organisms (such as height and mass) form a continuum rather than discrete phenotypes. These traits are often not controlled by a single gene. Instead, they are controlled by polygenes (many genes.) Environmental factors are important in determining where on the continuum an organism lies. While genetics might predispose individuals towards taller heights, factors like diet might restrict them to more average heights. If we were to measure the heights of a large number of individuals, we would obtain a bell-shaped curve known as the normal distribution curve. An important characteristic of this curve is that its mean, median, and mode are all equal, and all points along the curve are equally distributed around the mean.

Fig. 3 - Normal distribution curve

Discrete phenotypes that are controlled by only one gene or set of genes, such as blood type or certain types of colouration, cannot be appropriately represented by a bell curve. In cases such as this, it is more appropriate to use bar charts to visualise trends in data.