The basis of chromosomal sex determination is fact that one chromosome or pair of non-homologous chromosomes doesn't occur in same numbers in both sexes. These chromosomes are thus, named sex chromosomes and symbolized as X and Y in some organisms or Z and W in others. The term linkage is used to explain occurrence of genes on same chromosome. By convention though, sex linkage refers to genes on X- or Z chromosome, although Y- and W- chromosomes are also sex chromosomes. One of the most definite pieces of proof showing sex-linked inheritance was reported by Morgan in 1910 from crosses with Drosophila melanogaster.
From the F1 of the original cross, one would conclude that white-eye phenotype is recessive to wild type that is red eye. Evidence that eye-color characteristic is possibly handled by one locus is fact that there is only one pair of alternative phenotypes - red or white. If characteristic were an autosomal characteristic we would have expected both male and female F1 in original cross to be heterozygous for pair of alleles, as both sexes have same numbers of each autosome. Furthermore, one half of white are females and other half males. Same is true for red-eyed class. These ratios of 1 red : 1 white and equal numbers of males and females in two classes are like what one would expect with autosomal characteristic. Though, fact that result of original crosses was so different from what would be expected for autosomal feature made Morgan look other explanations. The result of P-generation cross indicates that white eye phenotype is recessive. If this were autosomal feature white-eye flies would have to be homozygous. If locus of gene were on Xchromosome only white-eyed female need be homozygous for recessive allele as they have two X-chromosomes. By same token red-eyed females would be either homozygous dominant or heterozygous.
Heterozygous female would generate two kinds of eggs - one having X-chromosome with dominant allele and other containing X chromosome with recessive allele. Both kinds of eggs can be fertilized by Y-bearing sperm. If egg with dominant allele is fertilized eye color of resulting male offspring is of course red as dominant allele is present. Though, one can only work out basis of phenotype in the case of egg with recessive allele fertilized by Y-sperm. Phenotype has to be white as there isn't other X-chromosome present that could perhaps carry dominant allele. Although X and Y segregate like pair of homologues they are not homologous chromosomes. Thus, male fly has one set of sex-linked genes.
Morgan's postulate has the following implications:
1) A cross of white-eyed females with white-eyed males will give only white-eyed progeny.
2) Heterozygous red females will always generate and expected proportion of 50% red-eyed and 50% white-eyed male progeny in spite of type of males to which they are mated. Though, only matings of these females to white males will generate white daughters in the proportion of 50% red-eyed and 50% white-eyed.
3) F1 males from white x red (pure breeding) will generate only red-eyed females; but all sons from cross will be white-eyed.
4) F2 females from P cross of pure-breeding red x white will all be red but of two genetypes : 1/2 W+/W
5) Reciprocal crosses for sex-linked genes will give different results, in contrast to what is found with autosomal genes:
Another demonstration of sex-linked inheritance in Drosophila is as follows.
There is X-chromosome which is ring-shaped instead of rod-shaped. In early cleavage stages in development there is tendency for chromosome not to be included in some nuclei. Loss of ring X chromosome from some nuclei in XX embryo results in condition in which some cells keep their original X condition and other cells would be XO. Latter kinds of cells would be hemizygous for genes on remaining rod X-chromosomes.
With respect to sex-linkage, there is recessive allele that makes for yellow body chitin and bristles. Another recessive allele makes for twisted bristles. Embryo could be made heterozygous for two loci such that normal X-chromosomes holds two recessive alleles where as ring X chromosomes carries dominant alleles for normal color and straight bristles. Such phenotypic differences between XO male parts and XX female parts bear testimony to facts that genes are on X-chromosome and that male is hemizygous for X-linked genes. XO phenotypes for genes are no different from phenotypes in XY moles.
With respect to X-linked features sons are never like their fathers, in its place they are more like their mothers. It is only daughters won inherit their fathers' X-chromosome. It is evenly interesting to note here, that sex of child is determined by which of father's two kinds of sperm fertilizes egg. It is matter of possibility, 50%, which kind of sperm fertilizes egg.
Most of known sex-linked characteristics in man are recessive. These traits are rare in population but males are more often affected than females. Reason in that male is hemizygous, hence any male who inherits recessive allele from mother will be affected. Conversely female who is affected should have affected father and mother who is either heterozygous (known as a "carrier") or affected. Put in a different way, if she inherited abnormal recessive allele from one parent she might inherit dominant normal allele from other parent. A few examples of recessive x-linked features are night blindness, hemophilia in which affected can't differentiate between red and green, deficiency for enzyme glucose-6-phosphate dehydrogenase and ocular albinism.
One interesting feature of sex-linked inheritance in mammals is related with presence of dark staining body - the Barr body - found in interphase nucleus of most female somatic cells. Barr body is one of two x-chromosomes in female. Also it is not same X-chromosomes that happen as Barr body in all cells. Very simply hypothesis states that genes in X chromosome forming Barr body are inactive. Thus, every female somatic cell is technically hemizygous. As it is not same X chromosome that is inactive over whole body, female who is heterozygous for a trait would have two phenotypes of her body. She would be mosaic.
Y-chromosome doesn't occur in mammalian females or happens only under special situations in Drosophila females. As a result, any features present on Y-chromosome will be transmitted from father to son only. The genes and their corresponding traits will happen in only one sex. Such genes hence would be explained as holandric genes. Occurrence of unusual amount of hair on ear rims of some men was once thought to be holandric but absence of expected hairs in some males has cast some strong doubts on Y-linked explanation. In man only two features - a testis - determining factor and Y-histocompatibility - are known definitely to be Y-linked.
Sex - Limited Traits:
Sex-limited traits are also referred to as sex-limited genes. One example which is of agricultural significance is production of milk in cattle. Bull chosen to sire dairy cattle is just as significant as cows as genes controlling high milk production also happen in males but males don't produce milk as they are not equipped (differentiate) to do so. Other examples are size and shape of penis in males, breast development in women, heavy beard development in men and testicular feminization.
Sex - Influenced Genes:
Sex-influenced genes are also explained as sex-controlled or sex-modified genes/traits. These are genes that are expressed in both sexes, unlike genes for sex-limited traits. Though, in this case the kind of expression is different in sexes given same genotype. One common example is pattern baldness that has genetic basis. In population there are several more males than female affected with this trait. The reason is that affected males are either homozygous or heterozygous that is BB or Bb but only homozygous BB females are bald. In other words though trait is dominant men, it is recessive in females. Another sex-influenced trait in man is singing voice in which low bass males and high soprano females appear to have same genotype.
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