In genetics, organisms that exhibit typical characteristics
of their species are known as wild types. Those that show unusual or
altered traits due to genetic changes are called mutants. A classic
example comes from the fruit fly (Drosophila melanogaster), a model
organism widely used in genetics.
Wild-type Drosophila have bright red eyes, while
mutants may show different eye colors, such as white. This specific trait was
the focus of one of the most groundbreaking genetic experiments conducted by Thomas
Hunt Morgan and his team in the early 1900s.
Discovery of the White-Eyed Mutant
While studying fruit fly genetics, Morgan’s colleague Calvin
Bridges noticed an unusual white-eyed male fly among a population of
red-eyed wild types. To investigate this trait’s inheritance, a red-eyed female
was crossed with this white-eyed male.
Key Findings from the Cross:
- All
F1 offspring had red eyes.
- In
the F2 generation, about 75% had red eyes, and 25% had white
eyes.
- Interestingly,
all white-eyed flies were males.
This unusual pattern suggested a connection between the
trait and the sex chromosomes, leading Morgan to propose a new
hypothesis.
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| Sex Linkage in Dorsophila |
Morgan’s Hypothesis on Eye Color
Inheritance
Based on his findings, Morgan proposed two key ideas:
- The
gene responsible for red and white eye color is located on the X
chromosome.
- The
Y chromosome lacks any gene for eye color.
In this model:
- R
= dominant red-eye allele
- r
= recessive white-eye allele
Because these alleles are located on the X chromosome, they
are written as superscripts:
- XᴿY
= red-eyed male
- XʳY
= white-eyed male
- XᴿXᴿ
or XᴿXʳ
= red-eyed female
- XʳXʳ
= white-eyed female
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| Sex Link Inheritence |
Inheritance Patterns in Different
Crosses
(a) Cross: Red-Eyed Female (XᴿXᴿ)
×
White-Eyed Male (XʳY)
- All
offspring have red eyes.
- All
females are carriers (XᴿXʳ).
- All
males are red-eyed (XᴿY).
(b) Cross: Carrier Female (XᴿXʳ)
×
Red-Eyed Male (XᴿY)
- All
females have red eyes (50% XᴿXᴿ, 50% XᴿXʳ).
- Half
of the males are red-eyed, half are
white-eyed.
(c) Test Cross: Carrier Female (XᴿXʳ)
×
White-Eyed Male (XʳY)
- Offspring:
- Males:
50% red-eyed, 50% white-eyed
- Females:
50% red-eyed, 50% white-eyed
This confirms the presence and behavior of a recessive
X-linked trait.
Confirming the Hypothesis
To verify his results, Morgan performed a cross between a white-eyed
female (XʳXʳ)
and a red-eyed male (XᴿY).
- All
males were white-eyed, and
- All
females were red-eyed (XᴿXʳ).
This strongly supported the idea that the eye color gene
is X-linked and that the Y chromosome does not carry any counterpart
for this gene.
What Are Sex-Linked Traits?
Sex-linked traits are traits controlled by genes located
on the sex chromosomes—X and Y.
- X-linked
genes are found on the X chromosome.
- Y-linked
genes are found on the Y chromosome.
These genes can sometimes mimic autosomal inheritance
patterns and are thus referred to as pseudoautosomal genes. However,
their inheritance still depends heavily on an individual’s sex.
Key Takeaways for Curious Minds
- Fruit
flies helped unlock the mystery of sex-linked inheritance,
thanks to Morgan's white-eyed mutant experiment.
- X-linked
recessive traits, like white eyes or color
blindness in humans, are more common in males because they only have one X
chromosome.
- Carrier
females play a crucial role in passing on
X-linked traits, even if they don’t show symptoms.
- Y-linked
traits are passed exclusively from
father to son and are relatively rare.
- Understanding
sex-linked inheritance is key to diagnosing and
predicting the spread of certain genetic disorders across generations.
If you're fascinated by how one tiny fly changed our
understanding of genetics forever, keep exploring the world of molecular
biology—where even the smallest mutation tells a powerful story.