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Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart.
Sex linked describes the sex-specific reading patterns of inheritance and presentation when a gene mutation is present on a sex chromosome (allosome) rather than a non-sex chromosome . In humans, these are termed X-linked recessive, X-linked dominant and Y-linked. The inheritance and presentation of all three differ depending on the sex of both ...
In the 1950s using human pedigrees, many genes were incorrectly determined to be Y-linked. [5] Later research adopted more advanced techniques and more sophisticated statistical analysis. [6] Hairy ears are an example of a gene once thought to be Y-linked in humans; however, that hypothesis was discredited. [5]
Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits. However, females carry two or more copies of the X chromosome, resulting in a potentially toxic dose of X-linked genes. [4] To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation.
X-linked recessive inheritance. X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation, see zygosity.
Genes that are carried by either sex chromosome are said to be sex linked. Sex linked diseases are passed down through families through one of the X or Y chromosomes. Since usually men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes. [21]
Non-human mammals use several genes on the Y chromosome. [citation needed] Not all male-specific genes are located on the Y chromosome. The platypus, a monotreme, use five pairs of different XY chromosomes with six groups of male-linked genes, AMH being the master switch. [8]
The human genome is a complete set of nucleic acid sequences for humans, encoded as the DNA within each of the 24 distinct chromosomes in the cell nucleus. A small DNA molecule is found within individual mitochondria.