X-linked Inheritance : Morgon's work on fruit fly

 Mendel described factors of inheritance called genes, but he did this before anyone knew about the existence of the chromosomes that contain them, or the DNA they are made of.

  The early 20th century saw incredible progress in contextualising Mendel’s work within new discoveries in molecular biology, as it was then possible to see chromosomes in a microscope, and watch them replicate and redistribute themselves during cell division, which led to the chromosome theory of inheritance.

It was at this point that studies regarding mitosis and meiosis were able to explain Mendel’s laws of segregation and independent assortment, the two alleles for a gene separate during gamete formation, and that alleles for two different genes separate in random combinations, so long as they are on different chromosomes, as the chromosomes themselves, which contain the genes at specific loci, will undergo segregation and independent assortment, during gamete formation.

  But more evidence was still needed to corroborate the chromosome theory of inheritance. Some very compelling evidence for the chromosome theory of inheritance came from the work of Thomas Hunt Morgan.

  Morgan studied fruit flies, which have only four pairs of chromosomes, those being three pairs of autosomes and one pair of sex chromosomes, or an X and Y chromosome, just like humans. Two X’s for a female, and an X and a Y for a male.

  In studying the flies, he noticed a rare characteristic that sometimes appeared in males, and that was white eyes instead of the typical red.
- If considering phenotypes, red eyes would be referred to as the wild type, and
- white eyes would be considered the mutant, or non-wild type.

Morgan did some experiments that were similar to Mendel and the pea plants. He bred a red-eyed female with a white-eyed male.
- The entire F1 generation had red eyes, suggesting that red eyes are dominant, which is consistent with Mendelian genetics.
- Then he bred the F1 generation amongst it self, and he observed the expected 3 : 1 ratio for red to white eye color in the F2 generation, with an additional result.White eyes showed up only in males.
- All the F2 females had red eyes, and half the F2 males had red while half had white.

This was firm evidence that eye color was linked to sex, which makes perfect sense considering the discrepancy in sex chromosomes for males and females. It was suggested that the gene for eye color in fruit flies was an X-linked gene.

The reasoning went as follows.
- The original parental generation was a red-eyed female, with two X chromosomes, each with the red allele, which is represented by the letter W with a plus in superscript, and a white-eyed male, with an X chromsome containing the white allele, represented by the letter W, and a Y chromosome, which does not contain the gene in question.

In producing gametes, all the female eggs would have an X chromosome with the red allele, while for the male, half would have an X chromosome with with the white allele, and half would have the Y chromosome, without this gene.

  This means that all of the F1 generation would have precisely one red allele, and thus red eyes, since this is the dominant allele. But then,
  For the F2 generation,
25% would be homozygous dominant,
50% would be heterozygous, with the presence of the white allele or the Y chromosome being irrelevant to the phenotype, and 25% would have the white allele and the Y chromosome, thus missing the red allele, and possessing white eyes. But all of these flies will necessarily be male, which is indeed what was observed.

This constituted tremendous evidence for the chromosome theory of inheritance, because it demonstrated that information regarding the inheritance of a particular phenotype was associated with a particular chromosome, in this case, the X chromosome.
  Beyond corroborating the chromosome theory of inheritance, it was therefore realised that sex-linked genes have interesting patterns of inheritance. It is worth noting that not all organisms that reproduce sexually have the same X/Y system of sex determination that humans and flies do.

Chromosomal System of Sex Determination
1) X-0 System
The X-0 system is found in certain insects like grasshoppers and cockroaches. Here there is only one sex chromosome, the X chromosome. Females have two, and males have one.
  Their sperm may contain an X chromosome, or no sex chromosome.

2) Z-W System
In various birds, fishes, and certain insects, a Z-W system is utilised. Females are ZW, and males are ZZ.

3) haplo-diploid system
  A bit stranger is the haplo-diploid system, found with bees and ants. Here there is no sex chromosome, it is simply that eggs that are unfertilized develop into males, while eggs that are fertilized develop into females.
   This means that male bees and ants, having no fathers, are haploid organisms. All of their cells have only one set of chromosomes, just like our gametes, while females are diploid, each cell containing two sets of chromosomes, like most other animals.

Color-blindness: an X linked disorder

 With sex-linked genes better understood, we can examine the inheritance of certain traits like color blindness, which is an X-linked disorder. The normal allele is represented by the letter X with a capital N in superscript (XN), while the recessive allele has a lower case n in superscript(Xn).

Homozygous dominant female reproduces with a color blind male, meaning one recessive allele, and a Y chromosome, lacking the gene entirely. Off spring have a 50% probability of being female and heterozygous, and 50% probability of being male, with the dominant allele, a situation we would call hemizygous dominant, where hemizygous means that only one allele is present. Thus, there is a zero probability of passing on color blindness, but a 50% probability of producing a carrier.

  Now if the mother is heterozygous, and the father has the dominant has the dominant allele, there is a
25% probability of offspring being homozygous dominant,
25% heterozygous and therefore a carrier,
25% hemizygous dominant, and 25%hemizygous recessive, and therefore color blind.

if the mother is a carrier and the father is color blind is color blind, there is a
25% percent chance offspring will be a carrier,
25% hemizygous dominant,and  
50% chance of being color blind, with the possibility of producing a color blind female, which is far more rare, given that both parents must possess the rare recessive gene.

There are a large number of X-linked disorders, given that the X chromosome is so much larger than the Y chromosome, containing some 1100 genes, whereas there are far fewer Y-linked disorders, since it has only 78 genes, some of which are duplicates, although these can exist as well.