Genetics information sheet

In document NEW SPEC UNIT 1 (TOPIC 2) (Page 106-108)

All cells contain hereditary information that is encoded by a chemical called DNA (deoxyribonucleic acid). DNA is an extremely long molecule, with up to a metre in every cell (Figure 1). The DNA molecule is coiled and packaged, together with a type of protein called histones, to form a

chromosome. Just before the cell divides, the chromosomes become even more tightly packaged and they can be seen under a microscope.

Figure 1 DNA is coiled up.

Each chromosome has a separate molecule of DNA, so a cell with eight chromosomes has eight different molecules of DNA. The DNA molecule in each of the chromosomes in Figure 1 has

replicated to form two chromatids and these are packaged ready for cell division. A gene is a segment on a DNA molecule. Different genes may be very different lengths. Each gene codes for a certain protein molecule, which is then made in the cell cytoplasm. The proteins produced by the genes can generally be sorted into two different types: ones that run the chemical reactions in the organism and ones that will be the structural components of the organism. How an organism looks and functions are a result of the cumulative effect of all of these proteins.

Organisms produced by reproduction from two parents have an even number of chromosomes (unless an error has occurred), because one half of the chromosomes come from the father and the other half from the mother. The male and female sex cells, that is the gametes, contain the father's or mother's contribution. These two cells combine to make a single cell, which grows into the offspring. Humans have 23 pairs of chromosomes, giving a total of 46 chromosomes. One chromosome in each of the 23 homologous pairs is from the person’s father, the other from the person’s mother. Other organisms have different numbers of pairs.

Since chromosomes come in pairs, genes do too. One gene is located on one member of a

chromosome pair; the other gene is in the same location on the matching chromosome. The precise location where the gene is found on the chromosome is referred to as its locus.

A gene can consist of a variety of different forms known as alleles. Normally two alleles are present. The two alleles on the pair of chromosomes may be identical or different. Both members of the pair contribute to the same feature, such as having hair on the middle segment of your fingers.

For example, in the Reebops activity the gene for tail shape has a T form and a t form. T and t are alleles for the tail shape gene. If both chromosomes have a T form, or if both have a t form, the gene is said to be homozygous (two of the same form). If one chromosome has a T form and the other has a t

form, the gene is said to be heterozygous (two different forms).

If you look at the Key to Reebop features, you will notice that two Ts (TT) or a T and a t (Tt) code for the same thing: a curly tail. If the Reebop has a small t on each chromosome, he or she will have a straight tail. Because both the heterozygous (Tt) form and one of the homozygous (TT) forms code for the same variation of tail shape, curly tail is said to be the dominant variation and straight tail the

Purpose

 To examine how characteristics are inherited.

 To illustrate one of the ways in which meiosis is responsible for the tremendous variation that exists in every sexual species.

 To confirm some key genetic terms.

SAFETY

Students should not eat edible parts as hygiene cannot be guaranteed in the laboratory.

Notes on the procedure

‘Reebops’ are imaginary animals, made out of marshmallows, pins and cocktail sticks. They have an affectionate place in the homes of most students who make them and they are fun to ‘create’. Far from being an exercise suited to younger students, this activity has proved to be very popular with post-16 students.

Foam packaging pieces can be used instead of marshmallows to allow reuse and to avoid the issue of eating in laboratories.

Before starting, ensure students have grasped the idea of one chromosome of each homologous pair coming from each parent. There is an information sheet (page 3), which could be read in advance of the lesson. Details of meiosis are not required for Topic 2.

The task involves three stages.

1 Before the lesson, you will need to make up Mum and Dad Reebop (Figure 1), so that you can show the group what they look like and use their features as the starting points for discussions on the inheritance of features from parents. Both parents are heterozygous for all their characteristics.

Figure 1 A Reebop parent.

One feature – nose colour – shows incomplete dominance, which links to Question 6 on Activity 2.18.

pipe cleaner

push pin small marshmallow

small nail or pin thumb tack cocktail stick large marshmallow

In document NEW SPEC UNIT 1 (TOPIC 2) (Page 106-108)