Sex-Linked, Sex-Influenced and Sex-Limited Characters | Agriculture Optional Notes for UPSC PDF Download

Introduction

Genetics is a fascinating field that seeks to unravel the mysteries of inheritance, and it has uncovered various aspects of how traits are passed from one generation to another. In this article, we will delve into the intriguing world of sex-linked inheritance, sex-influenced inheritance, and sex-limited characters, all of which play pivotal roles in shaping an organism's traits.

The Basics: Autosomes and Sex Chromosomes

To comprehend these phenomena, one must first understand the distinction between autosomes and sex chromosomes. Autosomes are non-sex-determining chromosomes, while sex chromosomes dictate an individual's sex. In humans, males have an XY chromosome configuration, with X and Y representing the sex chromosomes. In contrast, females possess an XX configuration. The same principle applies to other species, with variations like ZZ for males and ZW for females in birds.

Sex Linkage: When Traits Are Tied to Sex

One of the fundamental concepts in genetics is sex linkage, which refers to the association of specific traits with sex chromosomes. Most sex-linked traits are found on the X chromosome, with the Y chromosome having a limited number of genes. This unique linkage gives rise to sex-linked traits, which are controlled by sex-linked genes, leading to sex-linked inheritance.

Sex-Linked Inheritance: More Prevalent in Males

Sex-linked inheritance can manifest as X-linked dominant, X-linked recessive, or X-linked codominant. Notably, in humans, the concept of dominance and recessiveness is relevant only in homogametic individuals, such as females. They can have two alleles at the sex-linked locus, making them capable of being either homozygous or heterozygous. In contrast, heterogametic individuals, like human males and female birds, do not display a dominance-recessive relationship due to the presence of only one sex chromosome.

Pseudo-Dominance: The Phenomenon of Recessive Alleles

Pseudo-dominance occurs when a single copy of a recessive allele is phenotypically expressed because the second copy of the gene is absent. This phenomenon is observed in male humans and female birds due to their hemizygous nature.

Criss-Cross Inheritance: A Unique Pattern

Inheritance of sex-linked characters does not follow the typical Mendelian segregation pattern. In humans and drosophila flies, males pass their sex-linked genes to all their daughters, and these daughters, in turn, transmit the gene to half of their male offspring. This pattern is referred to as criss-cross inheritance.

Examples of Sex-Linked Inheritance

Several well-known traits follow sex-linked inheritance, including barred plumage in poultry, eye color in drosophila, color blindness in humans, and hemophilia. These examples illustrate how the presence or absence of specific genes on sex chromosomes can result in different inheritance patterns.

Holandric Genes and Y-Linked Traits

The Y chromosome contains non-homologous portions with a few genes, known as holandric genes. One example is excessive ear hair in men, known as hypertrichosis, which is a Y-linked character. Y-linked genes are directly transmitted from father to son and never appear in females.

Features of Sex-Linked Diseases

Sex-linked recessive diseases are more frequent in heterogametic individuals. They are transmitted from affected men through normal daughters to half of their grandsons. Sex-linked dominant diseases are more common in females, and all female offspring of an affected male will inherit the trait.

Sexing of Day-Old Chicks: Auto-Sexing

Sexing day-old chicks is essential in poultry farming. Auto-sexing, a method based on the chicks' appearances at hatching, has revolutionized this process, offering a more accurate and convenient alternative to traditional vent sexing.

Sex-Influenced Traits: Dominance Depends on Sex

Sex-influenced traits are those that exhibit dominance in one sex but recessiveness in the other. The expression of these traits is influenced by an individual's sex, and the dominance or recessiveness depends on whether the individual is male or female.

Sex-Limited Traits: Express in One Sex Only

Sex-limited traits, as the name suggests, express in one sex but not in the other. These genes can be located on either sex chromosomes or autosomes and control primary and secondary sex characteristics.

Conclusion

Understanding the intricate world of sex-linked inheritance, sex-influenced inheritance, and sex-limited characters is crucial for comprehending the diversity of genetic traits in different species. These phenomena offer fascinating insights into how genes and chromosomes shape the characteristics of living organisms, and they continue to captivate geneticists and researchers worldwide.