X-linked inheritance

X-linked inheritance,

a pattern of inheritance in which the transmission of traits varies according to the sex of the person, because the genes on the X chromosome have no counterparts on the Y chromosome. The inheritance pattern may be recessive or dominant. The trait determined by a gene on the X chromosome is always expressed in males. Transmission from father to son does not occur. Kinds of X-linked inheritance are X-linked-dominant inheritance and X-linked recessive inheritance. Compare autosomal inheritance. See also sex-linked.

---

inheritance

1. the acquisition of characters or qualities by transmission from parent to offspring.

2. that which is transmitted from parent to offspring. See also gene, deoxyribonucleic acid and heredity.

Mendelian inheritance is the basis of all genetic practice, but it has limitations in explaining the small differences that occur in a range of offspring of similar and related matings. Galtonian genetics deals specifically with this problem and is better fitted as a tool in population genetics and in dealing with characters that are dependent on a number of chromosomal loci rather than on a single locus.

---

autosomal inheritance

controlled by genes located on autosomes.

intermediate inheritance

inheritance in which the phenotype of the heterozygote falls between that of either homozygote.

maternal inheritance

the transmission of characters that are dependent on peculiarities of the egg cytoplasm produced, in turn, by nuclear genes.

X-linked inheritance

see x-linked.

---

inheritance

The acquisition of traits, characteristics and disorders from parents to their children by transmission of genetic information. Genes come in pairs: one originating from the father, the other from the mother. If an individual presents only the hereditary characteristics determined by one gene of the pair on an autosomal chromosome, that gene is called dominant. Conditions caused by such genes are said to show autosomal dominant inheritance. For instance, for a rare autosomal dominant disease, if one parent is affected, then on average about 50% of their children will also be affected, irrespective of the children's sex. Examples: Marfan's syndrome, congenital stationary night blindness, neurofibromatosis 1 and 2, von Hippel-Lindau disease. If the individual does not present the hereditary characteristics unless both genes in a pair are of the same type, then the gene is called recessive. Conditions caused by such genes are said to show autosomal recessive inheritance. For a rare autosomal recessive disease, if a child is affected, then on average about 25% of their siblings will also be affected, irrespective of their sex. Examples: Laurence-Moon-Biedl syndrome, Tay-Sachs disease, oculocutaneous albinism, galactokinase deficiency.Thirdly, inheritance may be controlled by genes on one of the sex chromosomes, most often the X chromosome. A recessive mutation on the single X chromosome carried by a male will cause a disease, whereas in the female, a recessive X chromosome mutation would have to be carried on both of her X chromosomes. Therefore in X-linked recessive inheritance (sex-linked recessive inheritance) males are affected more often than females. Examples: colour blindness, ocular albinism, choroideremia. A fourth type of inheritance considered in ophthalmic practice is mitochondrial (maternal) inheritance in which the inheritance of a trait encoded in the mitochondrial DNA is transmitted through the female line (mother to son or mother to daughter). Examples: Leber's hereditary optic atrophy; Kearns-Sayre syndrome. See acquired; chromosome; defective colour vision; gene; hereditary.

Table I5 Divisions of the infrared spectrum
IR-A (near)	780-1400 nm
IR-B (middle)	1400-3000 nm
IR-C (far)	3000-1 000 000 nm