The fundamental characters which easily transmit generation to generation from their parents are called genetic character and the processes of transmission of such heredity and the studies of their causes are called Genetics. Gregor John Mendel is known as ‘Father of Genetics’ who made the scientific study of heredity and genetic theory. His method was based on cross breeding of various kinds of the garden peas that had the opposing pairs of obvious traits. He established the principles of segregation, dominance and independent assortment which became the most fundamental basis of the science of genetics. Gene (factor by Mendel) is the core component of chromosome which carries heredity characteristics.
Gregor J. Mendel studied the various pea plants through the cross breeding and propounded a comprehensive theory on the basis of genetic heredity which is called Mendel’s law of Inheritance. He had randomly selected seven pairs of pea’s species in which it was seen the inheritant characteristic of the one pair suppressed the inheritant characteristic of another pair in his experiment. The first pair he called as Dominant written with a capital letter like for tallness ‘T’ and another pair as Recessive written with small letter as for Dwarfnes ‘t’ which are responsible for the heredity as genetic symbol.
|Characters||Dominant Characters||Recessive Characters|
|Shape of the seed.||Spherical smooth seed.||Wrinkled seed.|
|Colour of the cotyledon.||Yellow cotyledon.||Green cotyledon|
|Colour of the flower.||Red||White|
|Shape of the fruit.||Smooth||Shrinked or Wrinkled|
|Colour of the fruit.||Green||Yellow|
|Position of the flower.||Closed||Farthest|
|Length or height of the plant.||Tall||Dwarf|
According to Mendel there are two factors to express the same inheritant characteristic in every reproductive cell and whenever these two factors are same then it is called Homozygous but when these two are opposite then it is called Heterozygous. He studied one or two pair species of opposite characteristic to diagonise the hereditary characteristics of the cross breeded species. So, one pair cross is Monohybrid Cross and two pair is Dihybrid Cross.
1. Monohybrid Cross and law of segregation
Inheritance is the transmission of genetically controlled characteristics from one generation to the next. In this we will discuss about the inheritance of single characteristic or trait such as plant height.
(i) Mendel first crossed pure-breed tall pea plants with pure-breed dwarf pea plant and found that only tall plants were produced in the first generation or F1 generation. No dwarf pea plants were obtained in the first progeny.
(ii) Mendel then crossed the tall pea plant of F1 generation and found that tall and dwarf plants were obtained in the second generation ie F2 generation in the ratio of 3:1.
The ratio 3:1 is known as Monohybrid ratios i.e. 3 (tall) and 1(dwarf).
According to Mendel’s first law of inheritance is the characteristic of an organism are determined by internal ‘factors’ which occur in pairs. Only one of a pair of such factors can be present in a single gamete.
2. Dihybrid Cross and Law of Independent Assortment
This involves the inheritance of two pairs of contrasting characteristics chosen by Mendel were shape and colour of seeds: round-yellow seeds, and wrinkled – green seeds.
(i) He first crossed pure-breed pea plants having round-yellow seeds with wrinkled-green seeds and found that only round- yellow seeds were produced in F1 generation. No wrinkled green seeds were obtained.
(ii) When F1 generation having round- yellow seeds were cross breed by self pollination then four types of seeds having different combinations of shape and colour were obtained in F2 generation as explained below in the form of table.
The ratio of each phenotype (appearance) of the seeds in the F2 generation is 9:3:3:1. This is known as dihybrid ratio.
According to Mendel’s second law of inheritance: In the Inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.
How are characteristics or traits transmitted to Progeny?
The characteristics or traits of parents are transmitted to their progeny (offsprings) through genes present on their chromosomes during the process of sexual reproduction. As genes work in pairs one is dominant and other one is recessive and each parent possesses a pair of genes for each characteristic on a pair of chromosomes. Thus, the male and female gamete carries one gene for each characteristic from the gene pairs of parents. But when male and female gamete fuses during fertilization zygote is formed, which grows and develops to form a new organism having characteristics from both parents which it has inherited through genes.
Please note that though the progeny inherits two genes or a pair of genes for each trait from its parents but the trait shown by the progeny depends on which inherited gene is dominant of the two.
How do genes control the characteristics or Traits?
A gene is the section of DNA on a chromosome which codes for the formation of a protein controlling a specific characteristic of the organism. Suppose a plant progeny has gene for the characteristic called ‘tallness’. Now, the gene for tallness will give instructions to the plant cells to make a lot of plant growth hormones. And due to this, the plant will grow too much and hence become tall and if the plant has a set of gene for dwarfness, then less plant growth hormone will be produced and remain short and become dwarf. Just like plants the characteristics in animals are also transmitted from the parents through genes by the process of sexual reproduction.
How Blood groups are inherited?
There are four Blood Groups in a person: A, B, AB or O. This blood group system is controlled by a gene which has three different forms denoted by the symbols IA, IB and IO. The genes IA and IB show no dominance over each other. So, they are co-dominant, but dominant over the gene IO. In other words, the blood gene I0 is recessive in relation to genes IA and IB.
Although there are three gene forms (called alleles) for blood, but a person can have only two of them. So, the blood group of a person depends on which two forms of the genes he possesses.
(i) If the genotype (gene combination) is IAIA, then the blood group of the person is A. And if the genotype is IA I0 even then the blood group is A (because I0 is a recessive gene).
(ii) If the genotype is IBIB, then the blood group of the person is B. And if the genotype is IB IO even then the blood group is B (because IO is a recessive gene).
(iii) If the genotype is IAIB, then the blood group of the person is AB.
(iv) If the genotype is IOIO, then the blood group of the person is O.
Image courtesy: www.med.stanford.edu