principles of inheritence and variation laws of inheritance incomplete dominance ad codominance
Channel: ISHTA’s BIO ACADEMY
Date: 2021-11-22

Genetics is a branch of biology which deals with principles of inheritance
and its practices. Progeny resembling the parents in morphological and
physiological features has attracted the attention of many biologists.
Mendel was the first to study this phenomenon systematically. While
studying the pattern of inheritance in pea plants of contrasting
characters, Mendel proposed the principles of inheritance, which are
today referred to as ‘Mendel’s Laws of Inheritance’. He proposed that
the ‘factors’ (later named as genes) regulating the characters are found
in pairs known as alleles. He observed that the expression of the
characters in the offspring follow a definite pattern in different–first
generations (F1
), second (F2
) and so on. Some characters are dominant
over others. The dominant characters are expressed when factors are
in heterozygous condition (Law of Dominance). The recessive characters
are only expressed in homozygous conditions. The characters never
blend in heterozygous condition. A recessive character that was not
expressed in heterozygous conditon may be expressed again when it
becomes homozygous. Hence, characters segregate while formation of
gametes (Law of Segregation).
Not all characters show true dominance. Some characters show
incomplete, and some show co-dominance. When Mendel studied the
inheritance of two characters together, it was found that the factors
independently assort and combine in all permutations and
combinations (Law of Independent Assortment). Different combinations
of gametes are theoretically represented in a square tabular form known
as ‘Punnett Square’. The factors (now known as gene) on chromosomes
regulating the characters are called the genotype and the physical
expression of the chraracters is called phenotype.
After knowing that the genes are located on the chromosomes, a
good correlation was drawn between Mendel’s laws : segregation and
assortment of chromosomes during meiosis. The Mendel’s laws were
extended in the form of ‘Chromosomal Theory of Inheritance’. Later, it
was found that Mendel’s law of independent assortment does not hold
true for the genes that were located on the same chromosomes. These
genes were called as ‘linked genes’. Closely located genes assorted
together, and distantly located genes, due to recombination, assorted
independently. Linkage maps, therefore, corresponded to arrangement
of genes on a chromosome.
Many genes were linked t