Sex-Influenced and Sex-Limited Characters in Inheritance
Sex-influenced and sex-limited inheritance refer to
genetic traits that are expressed differently in males and females. Though not
located on the sex chromosomes, these traits are influenced by sex hormones,
leading to distinct patterns in inheritance.
1. Sex-Influenced Traits
Sex-influenced traits are autosomal (located on
non-sex chromosomes) and are expressed differently in males and females due to
the influence of sex hormones. These traits are not restricted to one sex, but
the likelihood of their expression varies between the sexes. A common example
is male-pattern baldness in humans.
- Example
- Male Pattern Baldness: The allele for baldness behaves as dominant
in males and recessive in females. If a male inherits one allele for
baldness, he will typically express the trait due to the effect of
testosterone. In females, two copies of the allele are usually required
for baldness to be expressed, as estrogen can mask its effects.
- Mechanism: Hormonal differences
between males and females affect gene expression. In the case of baldness,
testosterone influences hair follicle patterns, making the trait more
likely to appear in men.
2. Sex-Limited Traits
Sex-limited traits are also autosomal, but their
expression is restricted to one sex due to anatomical or physiological
differences. This restriction occurs because certain traits are only useful or
functional in one sex, so they are expressed exclusively in males or females.
- Example
- Milk Production in Mammals: Genes that influence milk production are
present in both males and females, but they are only expressed in females
due to the structure of the mammary glands.
- Mechanism: Only females have the physiological
environment necessary for milk production, so the genes related to this
trait remain inactive in males despite being present in their genome.
Differences between Sex-Influenced and Sex-Limited
Traits
|
Aspect |
Sex-Influenced Traits |
Sex-Limited Traits |
|
Chromosomal
Location |
Autosomal |
Autosomal |
|
Expression
Pattern |
Can
appear in both sexes but is influenced by sex |
Restricted
to one sex due to physiological factors |
|
Example |
Male-pattern
baldness |
Milk
production in female mammals |
