Humans have diploid number of 46 chromosomes, of which 44 are autosomes and 2 are sex chromosomes. There are two kinds of sex chromosomes as X and Y, the individuals with XX sex chromosomes/ allosomes are females and XY allosomes are males. Content wise the X chromosome has more genetic material and number of genes compared to Y chromosome. Y chromosome has around 100 to 200 genes, while X chromosome has around 1000 to 1500 genes. X chromosome is a medium sized sub metacentric chromosome and Y chromosome is the shortest acrocentric chromosome. Y chromosome is exclusively transferred from father to son and hence similar to mitochondrial inheritance where mother is the contributor for all the progeny. In humans all chromosomes occurs in two copies (diploid) except for the Y chromosome. Due to diploid nature, all genes in humans have alleles including the genes that are present in the pseudoautosomal region of Y chromosome. In normal conditions, the expression of one allele is sufficient for the regular function of the cells. In most situations, the allele that is dominant gets expressed suppressing the recessive allele present in the other chromosome. This mechanism protects the organism in suppressing a recessive allele expression as majority of the diseases or abnormalities are observed due to expression of recessive alleles.
In nature the occurrence of Y linked diseases are very rare as most of the genes that are present in pseudoautosomal region have complementary genes on X chromosome or the fatal mutations in the Y chromosome specific genes might leads to non development of fetus. On the other hand, it is very common to have the diseases that are X linked. X chromosome from the father always passes to daughters, while the X chromosome from the mother passes to both sons and daughter equally. Hence, always the sons will have X chromosome coming from mother side or maternal side and never from paternal or father side.
The haploid condition of X chromosome in male causes the expression of the genes that are suppressed in mother. The recessive alleles that are suppressed in mother become expressed in sons and the diseases that are expressed in this manner are called X-Linked recessive diseases, as X chromosome is responsible for carrying such diseases. Mothers carrying a recessive and dominant allele are heterozygous and known as carriers since they transmit the alleles and never themselves express any disease symptoms. Examples of X-Linked diseases that are recessive are color blindness (especially red-green), haemophilia, and muscular dystrophy which are more common in males than females. Females are carriers for all of the X-linked recessive diseases and are protected in nature. The males on the other hand become victim due to the presence of only one X chromosome and express the diseases that are not present in the mother.
In the X-Linked diseases, always affected genes are passed from female to males and in next generation from male to all females and never to males. The carrier female of the defective gene will pass the gene into half of male and female progeny. The male which receives defective gene expresses the disease symptoms, while the female receiving defective gene becomes carrier once again. Therefore, X linked recessive diseases passes from grandfather to grandson with intermediate carrier mother. In some cases the females expresses the X-linked recessive allele either due to X chromosome inactivation or chromosome aberrations like deletion. X chromosome has many genes that are vital for the function for the brain, hence recessive mutations of genes on X chromosome passed from carrier mother might affects normal function of brain in sons. Mental retardation observed in males is also an example for the expression of X linked recessive gene.
X-Linked recessive diseases affect mainly males and very rarely females. Most of the diseases associated with X chromosomes are of recessive in nature and hence males have higher risk and females becomes carries of the affected genes without expressing any diseases. Though the presence of X chromosomes are required for female phenotype expression, the expression of the defective genes on X chromosomes are observed only in males. Therefore in nature, the females are covered from the defective recessive genes that are present on the X chromosomes and males are made vulnerable for the recessive genes that are present on X chromosomes.
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An enthuiastic author from India