Inheritance Health Dictionary

Inheritance: From 2 Different Sources


The transmission of characteristics and disorders from parents to their children through the influence of genes. Genes are the units of DNA (deoxyribonucleic acid) that are contained in a person’s cells; controls all growth and functioning of the body. Half of a person’s genes come from the mother, half from the father.

Genes are organized into chromosomes in the cell nucleus. Genes controlling most characteristics come in pairs, 1 from the father, the other from the mother. Everyone has 22 pairs of chromosomes (called autosomes) bearing these paired genes, in addition to 2 sex chromosomes. Females have 2 X chromosomes; males have an X and a Y chromosome.

Most physical characteristics, many disorders, and some mental abilitiesand aspects of personality are inherited. The inheritance of normal traits and disorders can be divided into those controlled by a single pair of genes on the autosomal chromosomes (unifactorial inheritance, such as eye colour); those controlled by genes on the sex chromosomes (sex-linked inheritance, such as haemophilia); and those controlled by the combination of many genes (multifactorial inheritance, such as height).

Either of the pair of genes controlling a trait may take any of several forms, known as alleles. For example, the genes controlling eye colour exist as 2 main alleles, coding for blue and brown eye colour. The brown allele is dominant over blue in that it “masks” the blue allele, which is called recessive to the brown allele. Only 1 of the pair of genes controlling a trait is passed to a child from each parent. For example, someone with the brown/blue combination for eye colour has a 50 per cent chance of passing on the blue gene, and a 50 per cent chance of passing on the brown gene, to any child. This factor is combined with the gene coming from the other parent, according to dominant or recessive relationships, to determine the child’s eye colour. Certain genetic disorders are also inherited in a unifactorial manner (for example, cystic fibrosis and achondroplasia).

Sex-linked inheritance depends on the 2 sex chromosomes, X and Y. The most obvious example is gender. Male gender is determined by genes on the Y chromosome, which is present only in males. Any faults in a male’s genes on the X chromosome tend to be expressed outwardly because such a fault cannot be masked by the presence of a normal gene on a 2nd X chromosome (as it can in females). Faults in the genes of the X chromosome include those responsible for colour vision deficiency, haemophilia, and other sex-linked inherited disorders, which almost exclusively affect males.

Multifactorial inheritance, along with the effects of environment, may play a part in causing certain disorders, such as diabetes mellitus and neural tube defects.

Health Source: BMA Medical Dictionary
Author: The British Medical Association
The transfer of characteristics, traits and disorders from parents to children by means of

GENES carried in the CHROMOSOMES of the germ cells. (See GERM CELL; GENETIC CODE; GENETIC DISORDERS.)

Health Source: Medical Dictionary
Author: Health Dictionary

Sex-linked Inheritance

The way in which a characteristic or an illness determined by the SEX CHROMOSOMES in an individual’s cells is passed on to the succeeding generation. Men have one X and one Y sex chromosome and women have two X chromosomes. Disorders that result from an abnormal number of sex chromosomes include KLINEFELTER’S SYNDROME, which affects only men, and TURNER’S SYNDROME, which affects mainly women. Recessive GENES on the X chromosome cause most other sex-linked characteristics; in women these may well be masked because one of their two X chromosomes carries a normal (dominant) gene. In men, who have just one X chromosome, no such masking occurs – so more men than women are affected by X-linked characteristics or diseases. (See also HEREDITY.)... sex-linked inheritance

Cytoplasmic Inheritance

the inheritance of characters controlled by genes present in the cell cytoplasm rather than by genes on the chromosomes in the cell nucleus. An example of cytoplasmic inheritance is that controlled by mitochondrial genes (see mitochondrion).... cytoplasmic inheritance



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