In this chapter, we will discuss Heredity, Genetics and Mendel’s Law of Hybridization.
Introduction
The process by which characters or genes of parent organisms get transferred to the offsprings is called heredity or inheritance. The characters that get transferred from parents to the offsprings are called hereditary characters.
Gene
Gene is a sub-microscopic unit found in chromosome which carries hereditary characters. It determines the physical, anatomical and physiological characteristics of organisms. It is located in the chromosome.
Humans have 23 pairs of chromosomes.
Functions:
- Its main function is to transfer hereditary characters from parents to the off-springs.
- It helps in evolution by mutation and genetic recombination.
- They code for different types of RNAs for protein synthesis.
- They control protein synthesis and metabolism.
Basics of Genetics
Allele
An allele is one of the two or more forms of genes controlling a pair of contrasting characters. For example: tallness and dwarfness are a pair of contrasting characters. Gene for talleness (T) and gene for dwarfness (t) are alleles.
Homozygous organism
An organism which consists of similar alleles for a pair of contrasting characters is called homozygous organism. For example: pure tall (TT), pure dwarf (tt), pure black (BB), pure white (bb), etc. It is also called pure organism.
Heterzygous organism
An organism which consists of both alleles for a pair of contrasting characters is called heterzygous organism. For example: hydrib tall (Tt), hybrid black (Bb), etc. It is also called hybrid organism.
Dominant characteristics (characters)
The characteristics which seems to be expressed in heterozygous forms is called dominant characteristics. In a cross between a tall and a dwarf organism, tall is a dominant character.
Recessive characteristics (characters)
The characteristics which seems not be expressed in heterzygous forms is called recessive characteristics. In a cross between a tall and a dwarf organism, dwarf is a recessive character.
Phenotype
The external appearance of an organism is called phenotype. The pea plants having genes TT and Tt are phenotypically similar.
Genotype
The genetic makeup of an organism for particular character is called genotype. The pea plants having genes TT and Tt are genotypically different.
Monohybrid cross
The cross involving only one pair of contrasting characters at a time is called monohybrid cross. For example: cross between tall and dwarf pea plant, cross between white and black guinea pigs, etc.
Dihybrid cross
The cross involving two pairs of contrasting characters at a time is called dihybrid cross. For example: cross between red flowered tall plant and white flowered tall plant, cross between round-yellow pea and wrinkled-green pea.
Mendel’s Experiment: Hybridization in Pea Plants
A pea plant has short life span. It is easy to cultivate. It produces fertile offspring. It is easy to do self and cross pollination in pea plant, it has many pairs of contrasting characters and they are easy to handle. So, Mendel chose pea plant for his experiment.
The conclusion of Mendel’s experiment are:
- The pure tall plants gave only tall offspring.
- The pure dwarf plants produced only dwarf offspring.
- The hybrid tall plants produced both tall and dwarf plants in the ratio 3:1.
Mendel’s First Law: Law of Dominance
Mendel’s law of dominance states that,” In the crossing between the pure parents having pair of contrasting characters, only one character appears in the F1 generation as a dominant character while other character remains hidden as a recessive character.”
Mendel’s Second Law: Law of Segregation
Mendel’s law of segregation states that two members of a pair of alleles remain together in hybrids of F1 generation but they segregate (or separate) at the time of gametes formation. Thus, each gamete receives only one type of allele. A gamete in F2 generation is always pure.
Monohybrid Cross
The cross involving one pair of contrasting characteristics is called monohybrid cross.
Variation
Variation is the morphological and physiological difference seen in offsprings due to genetic or environmental reasons. It gives unique identity to the offsprings and separates them from the rest of the population.
The variation which gets transmitted from one generation to another is called continuous variation. For example: different fingerprints, etc.
The sudden or drastic changes seen in the characteristics of offpsrings is called discontinuous variation. It is also known as mutation. For example: presence of six fingers in foot and hand, etc.
Mutation
Mutation is defined as the drastic change in the organisms due to change in DNA or genes that alters the genotype or phenotype of an organism caused by the factors like UV rays, X-rays, Infra-red rays, different chemicals, etc.