The sex chromosomes in birds are opposite to those in mammals. I.e. females have a Z chromosome and a female-specific W chromosome, whereas there are two Z chromosomes in males. The W chromosome (like the Y chromosome in humans) is very much smaller and gene-deficient. Researchers have discovered how Z and W chromosomes have parted for their evolutionary paths (through sequencing the female songbird genomes), and also determined what features decree the outcomes of the genes on the W chromosome.
For most of the sections, sex chromosomes do not have a genetic interchange. I.e. they develop along distinct evolutionary courses; so sex-defining genes will not be combined again, from one sex chromosome to the other, and then appear on the contrary sex. The study discovered that such suppression of recombination has happened at 4-time points amongst the songbird sex chromosomes. In spite of the radically varied phenotypes of all existing 5,000 songbird types. One family of repetitive features (the ‘CR1 transposon’), has held the attention of researchers. Apparently, non-functional DNA sequences have largely gathered at a mutation hotspot situated amidst the 2adjacent evolutionary bands. This fetched the theory that scrap DNAs might have prompted the loss of recombination among sex chromosomes and exposed them to separate evolutionary trajectories.
As the recombination is lost on the W chromosome, genes cannot struggle against the assault of damaging mutations, as they can usually be efficiently removed by recombination. This is the price that the sex chromosome must pay. Presently, only a couple of genes are retained useful on the songbird W chromosomes because of these long-lasting genetic destruction. The research discovered that the recollected genes tended to be more largely or greatly expressed than any other genes that have vanished in non-avian species, where both sets of genes continue to exist. This shows that the maintained genes have more significant purposes than others, and being deprived of them, even while the Z-linked gene continues to exist in females, is too high a price for the species to allow a decreased dosage.