Phenotypic variation: continuous and discontinuous

The measurable physical and biochemical characteristics of an organism, whether observable or not, make up its phenotype.The phenotype results from the interaction of the genotype and the environment. The genotype determines the potential of an organism, whereas the environmental factors to which it is exposed determine to what extent this potential is fulfilled. For example, in humans the potential height of a person is genetically determined, but a person cannot reach this height without an adequate diet. Phenotypic variation (commonly referred to simply as variation) is of two main types: continuous and discontinuous.

In continuous variation,differences are slight and grade into each other. Characteristics such as human height and weight show continuous variation, and are usually determined by a large number of genes (they are polygenic)and/or considerable environmental influence.

In discontinuous variation,the differences are discrete (separate) and clear cut: they do not merge into each other. Discontinuous variations are generally caused by different alleles of one, two, or only a few genes.

Continuous variations are usually quantitative (they can be measured) whereas discontinuous variations are qualitative (they tend to be defined subjectively in descriptive terms). Thus height in humans is a continuous variation given a value in metres, whereas height in sweet peas is a discontinuous variation described as 'tall' or 'dwarf.

Mutations: more variation

Genetic variation arises partly from sexual reproduction by a combination of independent assortment, crossing over, and random fertilisation. However, these processes merely shuffle the existing pack of genes so that new combinations are made. The ultimate source of inherited variations is mutations.

A mutationis a change in the amount or the chemical structure of DNA. If the information contained within the mutated DNA is expressed (that is, transcribed into mRNA and translated into a specific polypeptide chain) it can cause a change in the characteristics of an individual cell or an organism. Mutations in the gametes of multicellular organisms can be inherited by offspring. Mutations of the body cells of multicellular organisms (somatic mutations) are confined to the body cells derived from the mutated cell; they are not inherited.

Mutations can happen spontaneously as a result of errors in DNA replication or errors during cell division, or they can be induced by various environmental factors (such as certain chemicals, X-rays, and viral infection). Factors that induce mutations are called mutagens.

Chromosome mutations and gene mutations

Alterations in the number or structure of chromosomes are called chromosome mutations.Chromosome mutations can happen during mitosis and meiosis when chromosomes are being condensed and pulled apart. Homologous chromosomes may fail to separate, resulting in non-disjunction. Chromosome mutations also occur during interphase when DNA replicates, and during crossing over when sections of chromosomes are exchanged.

Gene mutationsare changes in the nucleotide base sequence in a cistron (the portion of DNA that makes up a single gene). A change of a single nucleotide base pair is called a point mutation.There are a number of types of point mutation, including:

▪ substitution- the replacement of one nucleotide with another containing a different base

▪ deletion - the loss of a nucleotide

▪ insertionor addition- addition of an extra nucleotide.

Sickle-cell anaemia is an example of an inherited condition that results from a substitution. Gene mutations may also result from duplication(repetition of a portion of a nucleotide sequence within a cistron) and inversion(reversal of the portion of the nucleotide sequence in the cistron).

Most mutations, if expressed, are harmful. Note, however, that in diploid organisms such as ourselves, mutations usually result in recessive alleles. These are expressed only in the homozygous condition unless the mutation is on the X chromosome. Many mutations result in a change in the shape of a protein so that the protein cannot function properly (for example, the mutation that causes sickle-cell anaemia). Mutations that affect large sections of a gene, and chromosome mutations are often lethal. However, some mutations have no effect: a mutation may occur in a non-coding part of DNA; it may produce a different codon for the same amino acid; or the altered amino acid sequence may not affect the protein's shape or function. Occasionally, a mutation is beneficial, changing the phenotype so that an organism has a better chance of surviving and reproducing. Although beneficial mutations are very rare events, they are bound to happen sooner or later if there is a large number of individuals in a population. These mutations are of immense importance because they are the ultimate source of all variation: the raw material for the evolution of new species by natural selection.

■ Glossary of essential terms for you to know

■ Your Essential Assignments

I.Quick check

1. What is a mutagen? Give one example.

2. Distinguish between the genotype and the phenotype of an organism.

3. If a diploid organism has two different alleles for the same gene, is it homozygous or heterozygous?

4. Is weight in humans an example of continuous variation or discontinuous variation?

II. Using a monolingual English dictionary define the following genetic terms:

allele; homozygous; heterozygous; dominant; recessive; polygenic.

III. Find Russian equivalents to the following word combinations:

IV. Fill in the gaps with the words and expressions from the text:

1. Mutations can either … spontaneously or … by agents called mutagens.

2. Mutations are usually thought of as … and they often are.

3. Occasionally, a mutation is …, changing the phenotype so that an organism has a better chance of … and ….

4. Genetic differences reflect… of an organism, that is, its genetic ….

5. Mutations that affect large sections of a gene and chromosome mutations are often ….

6. Although beneficial mutations are …, they are found to happen sooner or later if there is a large … in a population.

7. A mutation is … in the amount or the chemical structure of DNA.

8. The genotype determines the … of an organism, whereas the environmental factors to which it is exposed determine to ….

9. In continuous variations, differences are ….

10. In discontinuous variations, the differences are ….

V. Find English equivalents to the following word combinations:

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