Structure and Function: Inheritance

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.

Child’s genetic make-up

A child’s genes are a mix of genes from his or her parents and grandparents. About one-quarter of each child’s genes are inherited from each grandparent.

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.

Male or female?

Boys have one X and one Y chromosome while girls have two X chromosomes, in addition to the other 22 pairs of chromosomes.

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.

Recessive and recessive

Each child inherits two genes for the recessive trait blue eyes, one from each parent. Since there are no genes with a dominant effect to override the effect of the recessive trait, all of the children have blue eyes.

Recessive and mixed

Each child inherits a gene for the recessive trait blue eyes from one parent, and either a gene for blue eyes or for the dominant trait brown eyes from the other parent. Each child therefore has a 1 in 2 chance of having brown eyes.

Mixed and mixed

Each child has a 3 in 4 chance of inheriting at least one gene for the dominant trait brown eyes from a parent and having brown eyes. Each child has a 1 in 4 chance of inheriting two genes for the recessive trait and having blue eyes.

Dominant and recessive

Each child inherits a gene for the recessive trait blue eyes from one parent and a gene for the dominant trait brown eyes from the other parent. As all the children have a gene for the dominant trait, all will have brown eyes.

Sex-linked inheritance

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.

Colour-blind father and normal mother

A colour-blind father passes the altered gene on his X chromosome to all his daughters. These daughters are not colour blind, although they can pass the altered gene on to the next generation. Sons inherit the X chromosome from their mother and have normal colour vision.

Carrier mother and normal father

Each child of a carrier mother has a 1 in 2 chance of inheriting the altered gene. Sons inheriting the gene are colour blind as they have no other X chromosome. Daughters who inherit the gene are carriers.

X and Y chromosomes

The X chromosome carries several genes that can cause disorders. The Y chromosome carries genes that determine male sex, but appears to have few other important genes.

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.

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