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Figure 1. Morgan's two experiments on fruit fly eye color. In the first experiment, a white-eyed male is crossed with a purebred red-eyed female; in the second experiment, a red-eyed male is crossed with a white-eyed female. The results of Morgan's expermients demonstrate that eye color must be encoded by a recessive allele on the X chromosome.
In “Independent Segregation of Chromosomes”, we discussed how chromosomes in diploid organisms form pairs of homologs.
It turns out that this is not the case for one pair of chromosomes in animals.
In 1905, Nettie Stevens and Edmund Wilson independently discovered that male animals
possess one chromosome that is shorter than its partner, whereas female animals instead
have two long chromosomes. The shorter chromosome earned the title of Y chromosome
for its stunted shape, whereas the longer chromosome became known as the X chromosome.
These two chromosomes are aptly termed sex chromosomes, or allosomes, and we write the female sex chromosome
genotype as XX and the male genotype as XY.
The remaining homologous chromosome pairs are called autosomes.
Sex chromosomes are still passed on to gametes based on the outcome of a coin flip, but
egg cells (deriving from females) must always possess an X chromosome, so that the
sex of an individual is determined by whether it receives an X or a Y chromosome from its father's
sperm cell.
Fast-forward five years to 1910 and the lab of Thomas Hunt Morgan, who is often considered
the first modern geneticist because of his tireless work to place Mendel's work on sound footing.
One of Morgan's many experiments with fruit flies (genus Drosophila)
began as he noticed a number of white-eyed males.
When these white-eyed flies were crossed with purebred red-eyed females,
their progeny were all red-eyed, and yet crossing the second generation's red-eyed individuals
with each other produced some white-eyed males but exclusively red-eyed females. Strange results indeed.
Morgan's experiments are summarized in Figure 1,
after which he concluded that the trait for eye color in fruit flies must be sex linked,
or encoded on a sex chromosome. More specifically, the factor for white eye color
is encoded by a recessive allele on the X chromosome.
Because a male only has one copy of the X chromosome, having only one
recessive allele will cause the individual to exhibit white eyes, whereas a female fly
requires both copies of the recessive allele to possess white eyes.
X-linked recessive traits are manifested in males
much more often than in females, because a male only needs to receive a recessive allele
from his mother to exhibit the trait: in the case of genetic conditions, half of all male children born to
carrier mothers will inherit the condition.