Sex linkage refers to the inheritance of traits
determined by genes located on the sex chromosomes (X and Y). Unlike
autosomal traits, sex-linked traits exhibit distinct patterns of
inheritance due to the difference in chromosome composition between males (XY)
and females (XX).
In the fruit fly Drosophila
melanogaster, sex-linked inheritance was first extensively studied by Thomas
Hunt Morgan, who discovered a mutation affecting eye color. His
groundbreaking research laid the foundation for understanding X-linked
inheritance patterns in genetics.
Morgan’s Experiment:
The Discovery of X-Linked Inheritance
1.
Wild Type vs. Mutant Drosophila
- Wild-type Drosophila: Possess normal, bright red
eyes.
- Mutant Drosophila: Exhibit white eyes, a
rare variation discovered in male flies.
2.
The Crossbreeding Experiment
Morgan and his
colleague Calvin Bridges conducted controlled breeding experiments to
study inheritance patterns of eye color.
First
Cross (Parental Generation - P₀)
- A red-eyed female (XR XR)
was crossed with a white-eyed male (Xr Y).
- All F₁ offspring (both males and females) had red
eyes, indicating red was dominant over white.
Second
Cross (F₁ Interbreeding)
- When F₁ red-eyed females (XR Xr)
were crossed with red-eyed males (XRY), the F₂ generation exhibited a 3:1 phenotypic
ratio.
- ¾ (75%) of flies had red eyes, while ¼ (25%) had white
eyes.
- Notably, all white-eyed
flies were male, leading to a breakthrough in understanding sex-linked
inheritance.
Morgan’s Hypothesis
on X-Linked Inheritance
Based on these
results, Morgan proposed:
- The alleles for eye color
are located on the X chromosome.
- The Y chromosome lacks a
gene for eye color, meaning males inherit eye color from their
single X chromosome.
Genetic
Notation:
- XR = Dominant allele for red eyes
- Xr = Recessive allele for white
eyes
- Y = No allele for eye color
Genotypes
and Phenotypes:
|
Genotype |
Sex |
Phenotype (Eye Color) |
|
XR Y |
Male |
Red-eyed |
|
Xr Y |
Male |
White-eyed |
|
XR XR |
Female |
Red-eyed |
|
XR Xr |
Female |
Red-eyed (Carrier) |
|
Xr Xr |
Female |
White-eyed |
Patterns of
Inheritance in Sex-Linked Traits
1.
Cross Between a Homozygous Red-Eyed Female and a White-Eyed Male
(XR XR × Xr Y)
- All F₁ offspring have red eyes (XR Xr females and XR Y
males).
- F₁ females are carriers but do not express the
white-eyed trait.
2.
Cross Between a Carrier Female and a Red-Eyed Male
(XR Xr × XR Y)
- 50% of males are white-eyed (Xr Y),
and 50% are red-eyed (XR Y).
- All females are red-eyed, but 50%
are carriers (XR Xr).
3.
Test Cross (Carrier Female × White-Eyed Male)
(XR Xr × Xr Y)
- Males: 50% white-eyed (Xr Y),
50% red-eyed (XR Y).
- Females: 50% red-eyed (XR Xr),
50% white-eyed (Xr Xr).
4.
Confirmatory Test (White-Eyed Female × Red-Eyed Male)
(Xr Xr × XR Y)
- All males are white-eyed (Xr Y).
- All females are red-eyed (XR
Xr) but carriers.
Understanding
Sex-Linked Traits
A sex-linked
trait is determined by genes located on the X or Y chromosome. These
genes are classified as:
- X-linked genes – Present on the X
chromosome and inherited differently by males and females.
- Y-linked genes – Found only on the Y
chromosome and inherited exclusively by males.
Pseudoautosomal
Genes: Some genes on
the X and Y chromosomes behave like autosomal genes, exhibiting equal
inheritance patterns in both sexes.
Final Words
Morgan’s
experiments with Drosophila provided definitive proof of X-linked
inheritance, revolutionizing the field of genetics. Sex-linked traits
play a crucial role in heredity, influencing disorders such as color
blindness, hemophilia, and Duchenne muscular dystrophy in humans.
Understanding their patterns helps scientists predict genetic probabilities
and develop new insights into hereditary diseases.
No comments:
Post a Comment