Why does continuous variation occur

When they pair, they also exchange pieces of DNA with each other in a process called crossing-over or recombination. This generates a new opportunity for genetic variation, as it results in the shuffling of alleles on each chromosome to create new combinations of genes.

Discontinuous variation describes variation in which different alleles of one gene have a large effect on the phenotype of the organism. With this type of variation, individuals can be grouped into distinct classes. One example is in the case of blood types. Another example is tongue rolling, you either can or cannot roll your tongue. Continuous variation describes variation in which many alleles have small effects on the phenotype. There is no clear separation into distinctive categories, as measurements are on a continuous scale.

An example of continuous traits are height and IQ. With changing allele frequencies comes changing genotype and phenotype frequencies. Different genotypes can be associated subtly or dramatically with positive or negative effects on fitness in that environment. Join now If you're ready to pass your A-Level Biology exams, become a member now to get complete access to our entire library of revision materials. Join over 22, learners who have passed their exams thanks to us!

Sign up below to get instant access! Or try a sample Quick Navigation. Summary How does variation arise? Types of variation The effect of variation Further reading and references:. We can all identify examples of continuous variation in plant or animal populations that we have observed — many examples exist in human populations. One area of genetics in which continuous variation is important is in plant and animal breeding.

Many of the characters that are under selection in breeding programs, such as seed weight or milk production, have complex determination , and the phenotypes show continuous variation in populations. Animals or plants from one extreme end of the range are chosen and selectively bred. Before such selection is undertaken, the sizes of the genetic and environmental components of the variation must be known.

We shall return to these specialized techniques in Chapter 20, but, for the greater part of the book, we shall be dealing with the genes underlying discontinuous variation. Consider the difference between the pigmented and the albino phenotypes in humans. The dark pigment melanin has a complex structure that is the end product of a biochemical synthetic pathway.

Each step in the pathway is a conversion of one molecule into another, with the progressive formation of melanin in a step-by-step manner. Each step is catalyzed by a separate enzyme protein encoded by a specific gene.

Most cases of albinism result from changes in one of these enzymes — tyrosinase. The enzyme tyrosinase catalyzes the last step of the pathway, the conversion of tyrosine into melanin. To perform this task, tyrosinase binds to its substrate, a molecule of tyrosine, and facilitates the molecular changes necessary to produce the pigment melanin.

The active site is a pocket formed by several crucial amino acids in the polypeptide. If the DNA of the tyrosinase-encoding gene changes in such a way that one of these crucial amino acids is replaced by another amino acid or lost, then there are several possible consequences. First, the enzyme might still be able to perform its functions but in a less efficient manner. Such a change may have only a small effect at the phenotypic level, so small as to be difficult to observe, but it might lead to a reduction in the amount of melanin formed and, consequently, a lighter skin coloration.

Note that the protein is still present more or less intact, but its ability to convert tyrosine into melanin has been compromised. Second, the enzyme might be incapable of any function, in which case the mutational event in the DNA of the gene would have produced an albinism allele , referred to earlier as an a allele.

It results in normal pigmentation because transcription of one copy of the wild-type allele A can provide enough tyrosinase for synthesis of normal amounts of melanin. Alleles are termed haplosufficient if roughly normal function is obtained when there is only a single copy of the normal gene.

Alleles commonly appear to be haplosufficient, in part because small reductions in function are not vital to the organism. The molecular basis of albinism is represented in Figure Molecular basis of albinism.

Expression in cells containing 2, 1, and 0 copies of the normal tyrosinase allele on chromosome Melanocytes are specialized melanin-producing cells. The mutational site in the DNA can be of a number of types. The simplest and most common type is nucleotide-pair substitution , which can lead to amino acid substitution or to premature stop codons. Small deletions and duplication also are common.

Such mutations are called frameshift mutations. At the protein level, mutation changes the amino acid composition of the protein. The most important outcomes are change in shape and size. Such change in shape or size can result in no biological function which would be the basis of a null allele , or reduced function. More rarely, mutation can lead to new function of the protein product. New alleles formed by mutation can result in no function, less function, or new function at the protein level.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed. Turn recording back on. National Center for Biotechnology Information , U. There are no values in between, so this is discontinuous variation. Here are some examples:. Continuous and discontinuous variation Some of the features of the different organisms in a species show continuous variation, and some features show discontinuous variation.

Continuous variation Human height is an example of continuous variation. Examples of such characteristics are: height weight If you record the heights of a group of people and draw a graph of your results, it usually looks something like this: A bar chart to represent variation in height The more people you measure, and the smaller the categories you use, the closer the results will be to the curved line.

Discontinuous variation A characteristic of any species with only a limited number of possible values shows discontinuous variation.