Genetics and variation
Joanna George-Johnson, Contributor

Why do most of us look so different? Why do babies look so different from their parents? Why do we differ so much in height? Why do some people write with their right hands, while others write with their left? The answers to these and similar phenomena lie in the principles of genetics and variation.

Before we begin, I should warn you that what you learn at CSEC biology is just the basics of genetics. Therefore, do not expect to be professional geneticists by the end of this course. You won't be able to tell who is a 'jacket' from who is not!

There are numerous terms involved in the study of genetics and variation that you need to understand. First, genetics is the study and behaviour of genes. Now, your genes are basic units of heredity found in the cells of all living organisms, which determine the physical characteristics the organism inherits.

A gene for eye colour

For example, there is a gene for eye colour. This gene determines that the individual will have an eye colour. It is important to note that genes are segments of our heredity molecule, DNA. Alleles are alternative forms of the gene. This means the alleles you possess determine what eye colour you will have.

Now, genes are found on chromosomes in cells. You should remember from studying cell structure that chromosomes are found in the nucleus of cells. A chromosome is a long strand of DNA and is, therefore, formed from a single DNA molecule that contains many genes.

The diagram below shows a cell and how the DNA forms into a chromosome.

Coiled DNA double Helix

Figure 1

The diagram above shows a gene as a section of the DNA molecule.

Figure 2

You should now be able to appreciate how DNA, genes, and chromosomes are linked. Let's now study a gene more closely. During the study of mitosis, you would have seen that chromosomes work in pairs. This means that each pair consists of a maternal (from the mother) chromosome and a paternal (from the father) chromosome. Since genes are found on chromosomes, they also work in pairs. The point at which a gene is found on a chromosome is known as a locus. A gene found on one chromosome in a pair usually has the same locus as another gene in the other chromosome of the pair.

Seed shapes

For example, the gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Using this example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), wrinkled: (rr). Note that a dominant allele is represented by a CAPITAL LETTER at all times, while a recessive allele is represented by a common letter.

The diagram below shows the positioning of a gene on a pair of chromosomes.

Figure 3

What is expressed in an organism is determined by its genotype. This is the genetic make-up, comprising the combination of genes in an organism; The phenotype of the physical characteristics expressed in an organism.

An organism can either be homozygous or heterozygous for a particular trait. Homozygous means the allele is the same on both of the chromosomes (maternal and paternal chromosomes contain the same allele).

Therefore, if we are dealing with 'handedness', we can let H be the code for right handedness (which would be the dominant allele) and h the code for left handedness (recessive allele).

If both chromosomes contain H, the genotype would be HH and so the dominant trait would be expressed, therefore the phenotype is a right-handed organism.

Dominant characteristic

If the genotype was hh the phenotype would be left handedness. It is also possible to have the genotype Hh and this is classified as being heterozygous for the trait. In this example, the organism's phenotype would be right handedness. It is very important to note that as long as a dominant allele is present, the dominant characteristic will be expressed.

** Please note that for your exams, using the letters 'C' 'c' and 'S' 's' O, P,U,V,W,X etc., to represent alleles is not recommended as their representations can be confusing.

The alleles do not always combine that simply to produce a dominant or recessive phenotype. This is explained by incomplete dominance.

In cases of incomplete dominance, the inheritance of a dominant and a recessive allele results in a blending of traits to produce intermediate characteristics. For example, four-o'clock paint plants may have red, white, or pink flowers. Plants with red flowers have two copies of the dominant allele A for red flower colour (AA). Plants with white flowers have two copies of the recessive allele r for white flower colour (aa). Pink flowers result in plants with one copy of each allele (Aa), with each allele contributing to a blending of colours.

The diagram below represents a situation of incomplete dominance.

Figure 4

Look out for the second part of the genetics and variation series soon!

Questions

1) Differentiate between

(a) allele and genes

(b) Chromosome and DNA

2) Explain the following terms: dominant, recessive, incomplete dominance, genotype, phenotype.

3) Identify one situation of incomplete dominance in each of the following:

(a) Plants

(b) Animals

4) By representing with a letter, show the possible genotype(s) of a person who:

(a) Is a tongue roller

(b) Is not a tongue roller

Joanna George-Johnson teaches at Ardenne High School.