Physical characteristics, many disorders, and some aspects of behaviour are at least partly determined by genes passed from parents to children. Genes for each characteristic are always found at the same place on the same chromosome. At fertilization, the 23 chromosomes from the egg cell and 23 from the sperm cell come together to make the full set of 46 chromosomes containing two copies of each gene.
How genes are inherited
Half of a child’s genes are inherited from its mother and half from its father. In turn, the child’s parents inherited half of their genes from each of their own parents. Therefore, approximately one-quarter of a child’s genes has been inherited from each of its grandparents.
How gender is determined
There are two sex chromosomes, X and Y, that determine gender. Females have two X chromosomes and males have an X and a Y chromosome, in addition to the 22 other chromosomes. Therefore, all eggs have an X chromosome, while sperm may contain an X or a Y chromosome. The gender of a child depends on whether the sex chromosome in the sperm that fertilizes the egg is X or Y.
Dominant and recessive inheritance
Many characteristics are determined by a single pair of genes. Each gene in a pair may have a dominant or recessive effect. A gene with a dominant effect (a gene for a dominant trait) overrides a gene with a recessive effect (a gene for a recessive trait), with the result that a recessive trait occurs only if no genes for dominant traits are present to override the recessive trait. For example, blue eye colour is recessive and brown eye colour is dominant, as shown here.
Sex-linked traits and disorders are due to genes on the X chromosome. Males with an altered gene on the X chromosome are therefore affected as they have only one X chromosome. Females with one altered gene on one of their two X chromosomes are carriers if the genetic condition is recessive but affected if the condition is dominant. One example of an X-linked recessive trait is colour blindness. Its pattern of inheritance is illustrated in the diagram here.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.