What is a gene?
Genes influence and direct the growth and function of everything in the human body. They control the pattern for growth, survival, and reproduction, and account for variations in height, hair and eye colour, body shape, and gender.
The “blueprint” of the body
Your genetic inheritance also decides how likely you are to have certain diseases. All cells come from the single egg fertilized by the male's sperm so the same genetic material is duplicated in every cell in your body, except for egg and sperm cells. But not all the genes in an individual cell are active; it's where the cell is and what it does that determines which genes are active. For example, different sets of genes are active in bone cells than blood cells.
Genes in chromosomes
Genes are carried in pairs along a chromosome and each gene is either dominant or recessive. A recognizable effect is the result of the dominant gene or genes in each pair; the effect of recessive genes will only be seen when there are two recessive genes. Every cell in your body, except the egg and sperm cells, normally contains 23 pairs of the thread-like structures called chromosomes. Egg and sperm cells contain only 22 chromosomes plus an X or Y chromosome. Each chromosome contains thousands of genes, which are arranged in single file along its length. Chromosomes are made up of two chains of DNA, which are arranged together to form a ladder-like structure, the sides of which are sugar-phosphate molecules. This spirals around upon itself and is known as a double helix. DNA has four bases - adenine, cytosine, guanine, and thymine - which are arranged in different combinations according to the functions of the genes on the different parts of the chromosome. Each combination of bases provides coded instructions that control and regulate the body's various activities.
Half of a baby's genes come from his mother, via the egg, and half come from his father, via the sperm.
Each egg and sperm contains a different “mix” of the parents' genes, so each child inherits a different and unique selection of his parents' genetic information.
Most of our individual mix of genes blends together, but some are dominant and others recessive. For example, the gene for brown eyes is dominant and the gene for blue eyes is recessive. So a child with one brown-eyed parent and one blue-eyed parent who receives a gene for each will have brown eyes because the gene for brown eyes prevails over the gene for blue eyes.
Sometimes when a cell divides and duplicates its genetic material the copying process is not perfect, and there's a fault. This leads to a small change, or mutation, in the structure of the genetic material.
Carrying a mutant gene normally has a neutral or harmless effect - most, if not all, of us have a mutant gene as part of our genetic make-up. But sometimes it can have a disadvantageous effect or, more rarely, a beneficial one.
The effects of a mutant gene depend largely upon whether it's carried within the fused egg and sperm, or whether it's a fault in the later copying process of the body cells.
A mutation in the egg or sperm will reproduce itself in all of the body's cells, and can cause genetic disease such as cystic fibrosis. A mutated body cell, at worst, will multiply to form a group of abnormal cells in a specific area. These may have only a minor effect in that part of the body, or they could cause deformity or disease. This type of mutation is usually triggered by an outside influence, such as radiation or exposure to cancer-forming agents.
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