The Role of Genes and Inheritance
Genes are located on rodlike structures called chromosomes that are found in the nucleus of every cell in the body. Each gene occupies a specific position on a chromosome. Because genes provide instructions for making proteins, and proteins determine the structure and function of each cell in the body, it follows that genes are responsible for all the characteristics you inherit.
The full genetic instructions for each person, known as the human genome, are carried by 23 pairs of chromosomes and consist of around 20,000-25,000 genes.
How Gene Inheritance Works
How Gender is Determined
Of the 23 pairs of chromosomes that are inherited, one pair determines gender. This pair is composed either of two X (female) chromosomes, in which case the baby will be a girl, or of one X and one Y (male) chromosome, in which case the baby will be a boy.
An egg always contains one X chromosome, while sperm can carry an X or a Y chromosome. Whether your baby is a boy or a girl will therefore always be determined by the father. If a sperm carrying an X chromosome fertilizes the egg, the resulting embryo will be a girl. If a sperm with a Y chromosome fertilizes the egg, the resulting embryo will be a boy. In the male, both the X and Y chromosomes are active. In females, however, one of the two X chromosomes is deactivated early in the development of the embryo in order to prevent duplicate instructions. This could be the X chromosome from either the mother or the father.
Gene Variations
Each gene within a cell exists in two versions, one inherited from each parent. Often these genes are identical. However, some paired genes occur in slightly different versions, called alleles. There may be two to several hundred alleles of a gene, although each person can only have two. This variation in alleles accounts for the differences between individuals, such as the color of eyes or the shape of ears. One allele may be dominant and "overpower" the other recessive one.
Why Genetic Disorders Occur
Genes usually exist in a healthy form, but sometimes a gene is faulty. Genetic disorders arise either when an abnormal gene is inherited or when a gene changes or mutates. Genetic disorders may follow a dominant or recessive pattern of inheritance. They can also be passed on via the X chromosome. Such sex-linked disorders are usually recessive, which means that a woman can carry the faulty gene without being affected because she has another healthy X chromosome to compensate. If a boy receives an affected X chromosome, he will be affected; a girl will be a healthy carrier like her mother. An affected male could pass on the affected gene only to his daughters.
