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Inherited characteristics that are conditioned by the sex of the individual. These traits are determined by genes that act differently in the two sexes. The usual result is that a given trait appears preponderantly in one sex. The reasons for such inheritance are neither the chromosomal location of the responsible genes, which may therefore be autosomal as well as X-linked, nor the normal sex-determining mechanism of the species. See Sex determination
A special class of sex-influenced genes can be called sex-limited. These are manifested in only one sex because the other lacks the requisite organs. Some examples are milk production in cows and egg production in chickens.
Some traits are sex-influenced because of genes that interact with a substance that is not produced equally in males and females. An example is early pattern baldness. Since male-to-male transmission occurs, the responsible gene must be autosomal. There is a preponderance of this trait in males because the action of the gene depends on the level of male hormones (androgens) present. See Gene action
Some sex differences in expression of inherited traits may result from genetic imprinting. Genetic imprinting refers to different expression of chromosomes, parts of chromosomes, or single genes, depending on which of the two sexes they are inherited from. To achieve imprinting, some genetic materials can be modified during gamete production or early embryonic development in one of the two sexes, so the traits determined by the imprinted genes are expressed differently than would be expected under typical mendelian inheritance. A growing body of evidence points to methylation of cytosine residues in the context of cytosine-guanine (CpG) dinucleotides as the mechanism of imprinting. Such methylation, especially if it occurs in the promoter regions of genes, can nullify the ability of the genes to be transcribed. Certain genes that can be imprinted will be methylated in the production of sperm, others in the production of ova, and they can be reactivated by demethylation when they pass through gametogenesis in the opposite sex. It is still not known why certain alleles are subject to imprinting while others are not, and why they are more likely to be imprinted in one sex than the other.
Imprinting can involve amplification of genes rather than inactivation; that is, as the gene passes through gametogenesis in one of the sexes, sections of it become duplicated and the gene thereby is enlarged. This is seen in neuroblastoma, where one commonly finds an increased number of DNA segments containing the N-myc protooncogene: such amplification is correlated with poor prognosis of the disease. In an overwhelming proportion of cases, it is the paternal N-myc gene that is amplified, suggesting that imprinting is responsible. A similar phenomenon occurs in Huntington's chorea, an autosomal dominant condition that usually does not become manifested until middle age or beyond. Earlier manifestation, often even in childhood, is associated with amplification of certain segments of DNA in the gene; but the amplifications, if they occur, are only in HC genes inherited from fathers. See Deoxyribonucleic acid (DNA), Genetics, Human genetics, Oogenesis, Sex determination, Sex-linked inheritance, Spermatogenesis