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.

1. 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.
2. 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.

1. 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.
2. 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