These and several related questions are dealt with, scientifically, in a branch of biology known as Genetics. This subject deals with the inheritance, as well as the variation of characters from parents to offspring.
Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity. Variation is the degree by which progeny differ from their parents.
Variation is the degree by which progeny differs from each other and with their parents. Humans knew from as early as 8000-1000 BC that one of the causes of variation was hidden in sexual reproduction.
Gregor Johann Mendel, for the first time conducted experiments to understand the pattern of inheritance of variation in living beings.
Gregor Mendel, conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.
He conducted experiments on garden pea plant (Pisum sativum) for seven years (1856-1863) and proposed the laws of inheritance in living organisms.
He selected garden pea plant as a sample for experiment because of:
Mendel selected 14 true-breeding (a breeding line which has undergone continuous self-pollination shows stable trait inheritance and expression for several generations) pea plant varieties, as pairs, which were similar except for one character with contrasting traits.
Mendel proposed two general rules on the basis of his observations on monohybrid crosses which came to be known as the Principles of Laws of Inheritance.
Law of dominance is known as the first law of inheritance. In this law, each character is controlled by distinct units called factors, which occur in pairs. If the pairs are heterozygous, one will always dominate the other.
Alleles do not exhibit any blending, both traits are recovered in F2 generation, one of these is although not observed at the F1 stage. During gamete formation, though parents possess two alleles, they split from each other in a way that a gamete obtains only one of the two factors.
Mendel proposed another set of general rules basis his observations on dihybrid crosses which came to be known as Mendel’s Law of Independent Assortment which states “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”. This can be effectively understood with the help of a Punnett square carried out with two pairs of genes during the stage of meiosis and egg-production & pollen in the F1 generation of RrYy plant.
Morgan coined ‘linkage’ term to brief about the physical association of genes on a chromosome or linkage of two genes and ‘recombination’ term to explain the generation of non-parental gene combinations
It results in the alteration of DNA sequences resulting in the changes in the phenotype and genotype of an entity. Along with recombination, mutation also leads to variations in DNA.
Chromosomal alteration causes aberrations or abnormalities. Typically, these aberrations are observed in cancer cells.
Point mutation – takes place as a result of changes in single base pair of DN. Example- sickle cell anemia while insertions and deletions of base pairs of DNA leads to frame-shift mutations.
Single gene disorders, like Huntington's disease and cystic fibrosis, actually follow Mendelian inheritance patterns. Mendel's studies of inheritance patterns in pea plants are a solid foundation for our current understanding of single-gene diseases in humans.
Some of the Mendelian disorders are: