Genetics is the study of the instructions, contained in
cells, for how our bodies develop and function. These instructions are
called GENES and are carried by tiny objects called chromosomes in
the nuclei (centres) of cells. Packed into the chromosomes is a chemical called
deoxyribonucleic acid (DNA), which holds the instructions in coded form.
People’s genes come from their parents. The passing on of biological
characteristics from parents to children via their genes is called INHERITANCE.
Chromosomes are tiny structures in the nucleus at the centre of each
body cell. Each cell carries 46 chromosomes. Just before a cell divides, each
chromosome makes a copy of itself. In this way, a complete set of chromosomes
can pass into each of the two new cells that form during division.
When a chromosome copies itself just before cell division, for a
short time the chromosome and its copy are joined at a point called the
centromere. This gives each chromosome an X shape, with four arms extending
from the centromere. Each arm consists of a long threadlike molecule of DNA,
coiled up on itself. A gene is a section of the DNA molecule.
If the DNA molecule could be unravelled, it would reveal a
structure like a twisted ladder, called the double helix. The sides of this
ladder consist of a chainlike substance called sugar-phosphate. The rungs of
the ladder are formed from chemicals called nucleotide bases. There are four
different types of base. Their sequence within a gene (a long section of DNA)
is a code that holds the instructions carried by that gene.
A full chromosome set consists of 23 pairs of chromosomes.
Twenty-two of these chromosome pairs are the same in girls and boys. The last
pair, called sex chromosomes, differ between the sexes. Girls have two sex
chromosomes called X chromosomes. Boys have just one X chromosome and a smaller
partner, called the Y chromosome. This Y chromosome contains genes that give
boys their male characteristics.
A gene is a section of DNA in a chromosome. People
receive half their genes from their mothers and half from their fathers, via
the egg and sperm cells from which they grew. Every gene plays a part in
determining how a person looks and functions. Genes influence the body through
complex mechanisms in cells that translate the coded messages in DNA into the
activities of each cell. The differences between people result partly from tiny
variations in their genetic makeup – that is, in the sequence of bases
in their DNA.
Different genes on a single human chromosome may hold the
instructions for a wide range of body activities. Scientists are gradually
building up maps that show the locations of specific genes on each chromosome.
As they work out what each gene does, they add this information to the maps.
Altogether, there are probably between 30,000 and 40,000 genes on the human
chromosomes.
Everyone’s DNA sequence is amazingly similar. The numbers
below indicate in percentage terms exactly how similar. Human DNA is also very
like that of chimpanzees, our closest animal relatives.
Table 23. GENETIC SIMILARITY
| Identical twins | 100% |
| Two brothers or sisters | 99.95% |
| Any two unrelated humans | 99.9% |
| Any human and a chimpanzee | 99% |
Family members often resemble each other in traits
(characteristics) controlled by genes, such as eye colour. Some traits are
determined by the combined action of just two genes, one from each parent. Each
member of this gene pair exists as one of two or more forms, with differing
effects. One of these forms, called the dominant form, may mask the action of
others and only has to be inherited in a single dose to produce its specific
effect. Other gene forms, called recessive, have no effect unless they are
inherited in a double dose, one from each parent.
An example of a simply inherited trait is the ability to taste
phenylthiocarbamide (PTC), a bitter substance found in some fruits and
vegetables. Only about two-thirds of us can taste PTC. To be a PTC taster
depends on inheriting at least one copy of the dominant “taster”
gene for this characteristic. People are nontasters only if they inherit an
alternative “nontaster” gene from both parents.
