Genetics is a branch of biology that deals with inheritance as well as variation of characters from parents to offsprings. The term genetics was firstly used by W Bateson in 1905. Inheritance is the process by which characters are passed on from parents to progeny and is the basis of heredity. It is due to heredity that, the born offsprings resemble their parents. All organisms such as human beings, plants or animals, exhibit various characteristics. Each character is carried forward to offsprings by the genes ( core component of chromosomes). Variation is the degree by which progeny differ from their parents.
Gregor Johann Mendel (1822-84) of Austria, was the first who started research on heredity. thus known as father of genetics. Mendel gained posthumous fame as the founder of the new science of genetics. During the period of 1856-1863, he conducted experiments on garden pea (Pisum sativum) and proposed the laws of inheritance in living organisms.
Mendel’s Experimental Material
Mendel grew pea plant (Pisum sativum) in his garden and selected seven pairs of species varieties, i.e., those plants which had shown same characters for atleast three successive generations.
He selected two varieties of pure plants (one tall and one dwarf) and crossed them. The resulting plants of next generation were all tall like one of the their parents. The offsprings obtained by crossing two plants with contrasting characters were called hybrids and their generations were F1 and F2 .
Mendel also coined two words for designating result of his experiment, i.e., dominant and recessive. Out of which, the dominant factor was denoted by the capital letter like ‘T’ (for tallness) while, the recessive factor was represented by a small letter like ť (for dwarfness). He also deduced from his experiments that there are two factors which express the same inherited characteristic in every reproductive cell. If these two factors are same then it is called homozygous (i.e., TT) while if these two factors are opposites, it is called heterozygous (i.e., Tt).
Mendel’s Law of Inheritance
By the experiments done by him and by the process of crossing and pollination, he obtained monohybrid and dihybrid crosses. And accordingly on the basis of the conclusion drawn through these crosses, he propounded the following three laws.
- 1. Law of dominance
- 2.Law of segregation or purity of gametes
- 3. Law of independent assortment
Mendel’s First Law (Law of Dominance)
According to this law, when two alternative forms of a trait or character (genes) are present in an organism, only one factor expresses itself in F1 progeny and is called dominant while, the other that remain hidden or masked, is called recessive. e.g, in garden pea plant ,stem length tallness is a dominant character and dwarfness is a recessive character.
Mendel’s Second Law (Law of Segregation)
According to this law, the alleles do not show any blending and both the characters are recovered as such in the F2 generation, though one of these is not seen in the F1 generation. Thus, factors or alleles of a pair segregate from each other such that a gamete recieves only one of the two factors. This law is also known as law of purity of gametes.
These two laws can be easily proved by the cross given below
- Mendel took pea plants with different characteristics such as tall plant and a short plant.
- The progeny produced from them (Fı generation plants) were all tall. Mendel then allowed F1 progeny plants for self-pollination.
- In the F2, generation, he found that not all plants were tall but some of them were short as well. This observation indicated that both the traits of shortness and tallness were inherited in F1 generation but only the tallness trait was expressed in F1 generation. (This proves the law of dominance).
- Two copies of the traits are inherited in each sexually reproducing organism.
- TT and Tt are phenotypically tall plants whereas tt is short plant. Thus, for a plant to be tall, a single copy of T is enough. Therefore, in traits Tt, ‘T’ is dominant trait while ‘t’ is recessive trait.
Mendel’s Third Law (Law of Independent Assortment)
Law of independent assortment is also known as ‘inheritance law’. It states that separate genes for separate traits are passed independently from parents to offspring. i.e., when two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of character at the time of gamete formation.
This law is mainly formulated by Mendel after performing dihybrid cross between plants that differ in two traits.
This law can be easily proved by the cross given below
Pure YYRR is crossed with yyrr and the hybrid obtained in F1 were all YyRr. But at the time of gamete formation, these genes segregate at random, none of the factor is influenced by the other.
9 Yellow round ( RRYY,RRYy , RrYY , RrYy )
9 Green round ( RRyy , Rryy )
3 Yellow wrinkled ( rrYY , rrYY )
1 Green Wrinkled ( rryy )
Thus the phenotypic ratio obtained was 9:3:3:1
Exceptions of Mendelism
Mendel’s laws are not universal in nature, they have some exceptions. Some of them are given below
1. Incomplete Dominance
It is the phenomenon in which phenotype of the F hybrid offspring does not resemble any of the parents but it is intermediate between the expression of the two alleles in their homozygous state. e.g., snapdragon (dog flower, Antirrhinum majus) and four O’clock plant (Mirabilis jalapa), are pink coloured flowers that are obtained as a result of cross pollination between the two types of pure breeding plants i.e., red flower and white flower.
It is the phenomenon in which two alleles express themselves independently when present together in an organism. Offsprings show resemblance to both the parents as neither of the allele of gene is being dominant or recessive to the other.
- e.g., ABO blood group in humans (controlled by gene I ).
A total of six different genotypes of the human ABO blood typespresent are given below
3. Multiple Allelism
Diploid organisms generally have a maximum of two alleles for each gene expressing a particular characteristic deriving one from each parent. But in some cases, there also occur multiple forms of a Mendelian factor or gene distributed in different organisms in the gene pool. eg the ABO blood grouping is a good example of multiple alleles.
In this case, more than two ie three alleles are present governing the same character. Multiple alleles can be found only when population studies are made.
Chromosomal Theory of Inheritance
In 1900, de Vries, Correns and Von Tschermak were the three scientists who independently rediscovered Mendel’s result on the inheritance of characters. They also observed thread-like structures in the nucleus and named them chromosomes.
Walter Sutton and Theodor Boveri mentioned that the behaviour of chromosome was parallel to the behaviour of genes and they used chromosome movement to explain Mendel’s laws.
Chromosomes as well as genes occur in pairs. The two alleles of a gene pair are located on homologous sites of homologous chromosomes. Sutton and Boveri stated that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Thomas Hunt Morgan had experimentally verified this theory.
Linkage and Recombination
Linkage is the phenomenon of physical association of genes on a chromosome and the term recombination describes the generation of non-parental gene combinations. Morgan and his group found that even when genes were grouped on the same chromosome, some genes remain tightly linked. Linkage is stronger between the two genes. if the frequency of recombination is low. Whereas, the frequency of recombination is higher, if the genes are loosely linked, i.e., linkage is weak between two genes.
Sex Determination in Human Beings
The establishment of sex through differential development in an individual at the time of zygote formation, is called sex determination. The sex determining mechanism in humans is XX-XY type. In humans, among 23 pairs of chromosomes present, 22 pairs are exactly the same in both male and female known as autosomes (responsible for somatic characters) while the 23rd pair is known as sex chromosome (responsible for sex determination). The sexual reproduction involves two individuals, male and female which have different structural characters in terms of chromosomes to decide the sex of the offspring.
These pairs of chromosomes that govern sex are called ‘X’ and ‘Y’ chromosomes. Males have only one X chromosome and one Y chromosome with autosomes whereas, females have two X chromosomes with autosomes. Females have no Y chromosome.
The German biologist Henking in 1891 observed a specific nuclear structure through spermatogenesis in few insects and found that 50 % receive this structure whereas, rest 50 % did not and named these structures as ‘X’ body. In large number of insects, sex determination follows XO type i.e., all eggs have an additional X chromosome besides autosomes.
When sperm bearing X chromosome fertilises an egg the resulting offspring become females and if the egg is fertilised by the sperm bearing no X chromosome the male offsprings are produced.
Mutation is defined as the phenomenon in which DNA sequences are altered and consequently result in changes in the genotype and the phenotype of an organism. The factors that cause mutations are known as mutagens. T
he mutagens can both be physical or chemical Mutation can occur in two forms
1. Gene Mutation
This mutation occurs due to alteration in the sequence of bases of DNA. It is further divided as point mutation and frame-shift mutation. When change arises in a single base pair of DNA, the mutation is called point mutation, (e.g., sickle-cell anaemia) while deletion and insertion of base pairs of DNA cause frame-shift mutations.
2. Chromosomal Mutation
As one strand of DNA helix runs continuously from one end to the other in each chromatid, in a supercoiled form, any alteration in the number and functioning of chromosome leads to abnormalities. This is further divided as
- Structural alteration in chromosome occurs due to the loss or gain of a segment of DNA. e.g., chromosomal alteration in cancer cells.
- Numerical alteration can occur due to aneuploidy or polyploidy.
Failure of segregation of chromatids during cell division provides gain or loss of chromosomes and is known as aneuploidy whereas, failure of cyotkinesis after telophase stage of cell division provides an increase in whole set of chromosomes, in an organism is known as polyploidy. This condition usually occur in plants.