Sex limited and sex influenced characters

Sex-Linked Characters

Sex-linked characters are traits determined by genes located on the sex chromosomes – X and Y chromosomes. In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Because males possess only one X chromosome, they express whatever allele is present on that chromosome, even if it's recessive. Females, with two X chromosomes, can be either homozygous (two identical alleles) or heterozygous (two different alleles) for a sex-linked gene.

Inheritance Patterns of Sex-Linked Characters

Sex-linked traits are broadly categorized into X-linked and Y-linked. X-linked traits are much more common due to the larger size and gene content of the X chromosome.

• X-linked Recessive: Males are more likely to express these traits because they only have one X chromosome. Females must inherit two copies of the recessive allele to express the trait. Examples include Hemophilia A and Red-Green Color Blindness.
• X-linked Dominant: Females are more likely to express these traits. A single copy of the dominant allele on the X chromosome is sufficient for expression. Hypophosphatemic rickets is an example.
• Y-linked: These traits are exclusively passed from father to son because the Y chromosome is only present in males. They are relatively rare. An example is SRY gene, responsible for male sex determination.

Sex-Influenced Characters

Sex-influenced characters are autosomal traits – meaning they are determined by genes located on non-sex chromosomes – but their expression is influenced by the sex of the individual. This influence arises because of differences in hormone levels or the presence of sex-specific regulatory factors.

Mechanism of Sex Influence

The expression of sex-influenced traits depends on the interaction between the autosomal gene and sex hormones, primarily testosterone. In males, the presence of testosterone can override the effect of a recessive allele, causing the dominant phenotype to be expressed. In females, the absence or lower levels of testosterone allow the recessive allele to manifest.

Examples of Sex-Influenced Characters

• Pattern Baldness: This is controlled by an autosomal gene. In males, testosterone promotes baldness. Females are less likely to experience pattern baldness due to lower testosterone levels.
• Hairy Ears: Similarly, hairy ears are an autosomal trait that is more common in males.
• Milk Production: While primarily influenced by hormones, the genes governing milk production can also be sex-influenced, with males typically producing less milk.

Comparing Sex-Linked and Sex-Influenced Characters

Feature	Sex-Linked Characters	Sex-Influenced Characters
Gene Location	Genes located on sex chromosomes (X or Y)	Genes located on autosomes (non-sex chromosomes)
Influence Factor	Presence/absence of sex chromosomes	Sex hormones (primarily testosterone)
Inheritance Pattern	Follows specific X-linked or Y-linked patterns	Inheritance pattern is autosomal but expression is sex-dependent
Examples	Hemophilia, Color Blindness, SRY gene	Pattern Baldness, Hairy Ears

Complexities and Considerations

The interaction between genes and the environment can further complicate inheritance patterns. Epigenetic modifications, where gene expression is altered without changes to the DNA sequence, can also influence the phenotype. Moreover, dosage compensation mechanisms in females ensure that X-linked genes are expressed at similar levels in males and females.