48. Among the following which is probable cause of unequal results during the reciprocal crosses?
(1)  X-linked inheritance
(2) polygenic inheritance
(3) Mendelian Inheritance
(4) Epitasis

Unequal results during reciprocal crosses are most likely caused by X-linked inheritance. This happens because the inheritance pattern and expression of traits on the X chromosome differ between males and females, causing the two reciprocal crosses (male mutant x female normal and vice versa) to produce different phenotypic ratios.

Explanation of Each Option

• X-linked inheritance: This involves genes located on the X chromosome. Since males have one X and one Y chromosome and females have two X chromosomes, crossing a mutant X-linked allele from male to female and female to male yields different outcomes. Thus, X-linked inheritance naturally leads to unequal results in reciprocal crosses due to the sex chromosome differences.​

• Polygenic inheritance: This refers to traits controlled by multiple genes and typically does not cause differences in reciprocal crosses. Polygenic traits show continuous variation rather than distinct reciprocal cross differences.​

• Mendelian inheritance: Mendelian inheritance typically predicts equal results in reciprocal crosses for autosomal genes because alleles segregate and assort independently regardless of the sex of the parents. So it does not explain unequal reciprocal crosses.​

• Epistasis: Epistasis is interaction between genes where one gene masks or modifies the expression of another. While it affects phenotype ratios, it usually does not cause differences in reciprocal cross results, which involve reversing the sexes of the parents.​

Introduction

Reciprocal crosses, where the genetic parents are swapped between male and female, sometimes produce unequal results in offspring phenotypes. Understanding the cause of these differences is essential in genetics. This article explains how X-linked inheritance causes such unequal results, distinguishing it from polygenic inheritance, Mendelian inheritance, and epistasis.

X-linked Inheritance and Reciprocal Crosses

X-linked genes are carried on the X chromosome. Males (XY) receive their single X chromosome from their mother, while females (XX) receive one X from each parent. Therefore, a mutant allele on the X chromosome produces different phenotypic outcomes depending on which parent contributes it and the sex of the offspring. This leads to unequal phenotypic ratios in reciprocal crosses, with one cross showing mutant traits not equally seen in the reciprocal cross.​

Why Other Genetic Mechanisms Don’t Cause Unequal Reciprocal Results

• Polygenic inheritance involves multiple genes contributing to a trait and usually results in continuous variation without unequal reciprocal cross results.

• Mendelian inheritance governs autosomal genes and predicts equal ratios from reciprocal crosses since autosomes are equally inherited from both parents.

• Epistasis refers to gene interactions affecting phenotypes but does not affect reciprocal cross outcomes since it is not sex-dependent.

Thus, the probable cause of unequal results in reciprocal crosses is X-linked inheritance, owing to the different chromosomal makeup of males and females and how X-linked genes express differently in them.

This explanation aligns with classical genetic principles and experimental evidence on reciprocal crosses in genetics.