Sex linkage is defined as the phenotypic expression of a allele related to present on the sex chromosome of the individual. The inheritance of these alleles are in contrast to the inheritance of traits on autosomal chromosomes. The traits of the autosomal chromosomes are inherited to the both the sexes with same probability of inheritance. There are more genes on the X chromosomes than on the Y chromosome. Hence, there are more X-linked traits than Y-linked traits.
The genes that are present on the sex chromosomes are called as sex-linked genes. The sex-linked genes can be present on the X chromosome or the Y chromosome. The gene present on the X chromosome is known as the X linked gene and the gene present on the is known as the Y linked gene. The Y-linked genes can be inherited only by males as the males mostly have a genotype of (XY). Y chromosome is not present in the females. The present on the x chromosomes can be inherited by both the males and females. For a given trait there are two forms of alleles, one dominant and one recessive. The recessive trait character is masked by the dominant trait.
The recessive traits on the X chromosome are expressed in males. The males contain only one X chromosome and there is not allele for the same trait to mask the expression. In the females if the second X chromosome contains a normal gene for the same trait, the phenotype may be masked.
Sex-linked traits are the characters that are determined by the genes present on the sex chromosomes. Genes are sequences of DNA present on the chromosomes. The gene carry information for production of proteins and the genes are also responsible for inheritance of specific genetic traits. Genes are present in alternative forms known as alleles. One of the allele for each trait is inherited from each parent. The sex-linked traits, like the traits from the autosomes are passed on from the parents to the offspring through the process of sexual reproduction.
As the X chromosome is larger than the Y chromosome most of the genes that are sex-linked are present on the X chromosome. The X chromosomes also carries genes that determines the female characteristics and also non-sexual characters like the ability to see colors and the the blood clot mechanism. In the X linked traits, the allele which is recessive is more often expressed in males than in females because the males have only one allele for a gene on the single X chromosome while the females have two alleles, as they have two X chromosomes. As the female has two alleles for the same gene, the recessive allele will not affect her. AS in the case of the males, there is no dominant allele that can counteract with the recessive allele on the single X chromosome and his Y chromosome lacks the corresponding gene.
X-linked dominance is usually referred to as X-linked dominant inheritance. X-linked dominance is a mode of genetic inheritance, where the dominant gene is carried on the X chromosome.
In case of an affected father all of his daughters will be affected but none of his sons will be affected unless the mother is also affected. All the sons will be affected if the mother is affected, and there are 50% chances for the daughter to be affected. If a mother is affected every child of the affected mother has 50% chance of inheriting the disorder and thus being affected. AS the daughters inherit the X chromosome of their fathers, 100% of the female progeny will inherit the disorder. If the mother is unaffected, none of her sons will be affected, and the male child will inherit only the Y-chromosome from the father.
Examples of X-linked dominant inheritance are : fragile X syndrome, Vitamin D resistant rickets, Rett Syndrome, Aloport's Syndrome, Aarsog's syndrome.
The X-linked recessive inheritance is the mutation seen in the gene of the X chromosome and it causes the chromosome to be expressed in males who are hemizygous for the condition as they have only one X chromosome and the females who are homozygous for the mutation.
The females having the recessive X-linked gene are the carriers generally and they do not show clinical symptoms for the disorder. All the males having the X-linked recessive mutated gene are affected. All the offspring of the carrier mother have 50% chance of inheriting the mutation, though the father does not carry the recessive gene. If the father is affected all his daughters will be carriers as the daughters inherit one X chromosome from the father, assuming here that the mother is not affected nor a carrier. Male offspring of an affected father will not be affected, as they inherit only the Y chromosome from the father.
The Y linkage is known as holandric inheritance in mammals. The Y-linked trait is the mutation present on a gene of the Y chromosome. As the chromosome is small, and contains a few traits, the diseases linked with the Y chromosome are rare. Human males' posses only one Y chromosome, females do not have any Y chromosomes. The Y-linked traits are passed on from the father to son with no genetic recombination. The deletions in the Y chromosomes are the general cause of the male infertility.
Sex influenced traits are the traits where the phenotypic expression is affected by the sex of the organism. The genes that are concerned with the sex-influenced traits may be present on any of the autosomes or on the homologous parts of the sex chromosomes. The expression of the alleles of the sex-influenced loci, either dominant or recessive is reversed in males and females, as the difference is provided by the internal environment of the sex chromosomes. Example for sex influenced trait is baldness in humans.
Some genes in the autosomal chromosomes are expressed in one of the sexes. This happens either because of the differences in the internal environment of the hormones or due to the dissimilarities in the anatomy.
Examples of sex limited traits are: female sterility in Drosophila and polymorphic characters in insects in relation to mimicry.