mendelian inheritance

In the 1860s, an Austrian scientist named Gregor Mendel introduced a new theory of inheritance.

Earlier, it was believed that parental ‘essences’ blend together, just like how red and yellow color blend to give an orange to result in inheritance. Based on his experimental work with pea plants, Mendel introduced three principles of inheritance:

1. Law of segregation which states that during reproduction the inherited factors (called alleles) separate into reproductive cells by a process called meiosis. These segregated alleles then randomly reunite during fertilization.
2. Law of assortment states that different pairs of alleles located on different chromosomes will be inherited independently of each other. Therefore, the inheritance of genes at one location in a genome does not influence the inheritance of genes at another location.
3. Law of dominance states that when mating occurs between two organisms of different traits, each offspring exhibit the trait of one parent only. If the dominant factor is present in an individual, the dominant trait will result. The recessive trait will only result if both factors are recessive. 

According to the Mendelian concept of inheritance, the inheritance of a trait depends on the passing-on of alleles. For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes, one of which may be dominant to the other.

If the two alleles that form the pair for a trait are identical, then the individual is said to be homozygous. A homozygous trait is written as BB or bb.

If the two genes are different, then the individual is heterozygous for the trait. A heterozygous trait is written as Bb

The allele that expresses itself at the expense of an alternate allele is called the Dominant allele. Dominant alleles are represented by upper case letters.

The allele whose expression is suppressed in the presence of a dominant allele is called the Recessive allele. Recessive alleles are represented by lower case letters.

Phenotype and genotype

An organism’s phenotype is all of its observable characteristics. Phenotype is influenced by both genotype and environment. An organism’s genotype is the specific allelic combination for a certain gene or set of genes that it carries.

Autosomal traits are controlled by genes on one of the 22 human autosomes. Widow’s peak, hitchhiker’s thumb, and earlobe attachment are examples of autosomal traits.

In contrast to the autosomal traits, there are traits controlled by genes on the sex chromosomes. These are called sex-linked traits or X-linked traits in the case of the X chromosome.

Males have just one X-chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. They always inherit it from their mothers and they pass it on to all their daughters but none to their sons.

Females have two X chromosomes; they have two alleles for any X-linked trait. Therefore, they must inherit two copies of the recessive allele to express the recessive trait. This explains why X-linked recessive traits are less common in females than males. For example, red-green color blindness is a recessive X-linked trait. More than one recessive gene on the X-chromosome codes for this trait, which is fairly common in males but relatively rare in females.