The expression of a particular trait is determined by the presence of two alleles contributed by the father and the mother.The experiments proved that the dominant allele can be determined by observing the phenotypic traits in the offspring but the recessive allele that remains hidden can be passed to the offspring and expressed in later generations. He proposed three laws known as вЂMendel's laws of InheritanceвЂ™. Gregor Johann Mendel, widely known as the вЂFather of GeneticsвЂ™, conducted hybrid experiments on garden peas and published the results in the year 1860, thereby laying the foundation of genetics. When the alleles are dominant then the trait is expressed but if some traits are absent or not expressed in the child but the allele is present in the parent then it is called a recessive trait.

Each parent contributes equally for the expression of a particular trait.

are determined by genes that are inherited from the parents.

This essentially means that traits like the color of the eyes, hair, skin, etc. We know that a child doesnвЂ™t necessarily look like his/her parents but would have their features. We recommend that use of the terms X-linked recessive and dominant be discontinued, and that all such disorders be simply described as following 'X-linked' inheritance.When we hear the word traits, we think of the distinguishing characteristics of people. They do not capture the extraordinarily variable expressivity of X-linked disorders or take into account the multiple mechanisms that can result in disease expression in females, which include cell autonomous expression, skewed X-inactivation, clonal expansion, and somatic mosaicism. The high proportion of X-linked disorders with intermediate penetrance is difficult to reconcile with standard definitions of X-linked recessive and dominant inheritance. Our analysis classified penetrance as high in 28% of the disorders studied, intermediate in 31%, and low in 40%. Contrary to standard presentations of X-linked inheritance, penetrance was highly variable in females. As expected, penetrance and an index of severity of the phenotype (defined in our Methods) were both high in males, while the severity index was low in females. We reviewed the literature on 32 X-linked disorders and recorded information on penetrance and expressivity in both sexes. However, many X-linked disorders such as adrenoleukodystrophy, fragile X syndrome, and ornithine transcarbamylase deficiency do not fit these rules. The former was defined as vertical transmission in which carrier women pass the disorder to affected sons, while the latter was defined as vertical transmission in which daughters of affected males are always affected, transmitting the disorder to offspring of both sexes. Early concepts of dominance and recessiveness were first used in conjunction with autosomal traits, and then applied to 'sex'-linked traits to distinguish X-linked recessive and X-linked dominant inheritance. Our modern concepts of Mendelian (including X-linked) inheritance originated just after the turn of the last century. The existence of X-linked disorders in humans has been recognized for many centuries, based on lessons in religious texts and observations of specific human families (e.g., color blindness or Daltonism).

Recessive and dominant traits