Rajasthan Board RBSE Class 10 Science Notes Chapter 3 Genetics
- Heredity: Inheritance of characters from one generation to next generation is called heredity.
- Variation: The difference in traits between two individuals is called variation.
- Genetics: The branch of biology which deals with heredity and variations is called genetics.
- Mendelism: Laws of inheritance were proposed on the basis of Mendel’s findings. These laws are also termed as Mendelism.
Reasons for Mendel’s Success :
Some of the reasons for his success are as follows:
- Mendel studied inheritance of only one trait at a time rather than taking too many traits into account.
- Mendel did careful statistical analysis of all the data collected during the experiment. This gave a solid mathematical foundation to his findings.
- Mendel was also careful in selecting the plant for his experiments.
Selection of Pea Plant:
Scientists could make some assumptions on this topic which can be as follows:
- Pea is an annual plant and hence it is possible to study many generations of pea in a short time period.
- Pea plant produces bisexual flowers. Hence, it is possible to get a pure line of homozygous plant through self pollination.
- Artificial pollination can be easily done with the help of encapsulation of flowers.
- A pea plant shows many pairs of contrasting characters which are inheritable.
|Seven Pairs of Inheritable Traits as Selected by Mendel|
|Traits of plant||Dominant||Recessive|
|Height of plant||Tall||Dwarf|
|Position of flower||Axial||Terminal|
|Shape of mature pod||Inflated||Constricted|
|Colour of immature pod||Green||Yellow|
|Shape of seeds||Round||Wrinkled|
|Colour of seeds||Yellow||Green|
- Gene: A factor which controls a trait is called gene. Mendel used the term ‘factors’ which was later termed as ‘gene’ by Johannsen.
- Allelomorph or Allele: For any given trait, there are two independent inheritable forms. These are called allelomorph or allele. For example, the gene which controls the height of a plant has two alleles, i.e. T(tallness) and t(dwarfness).
- Homozygous: When both the genes in a pair of alleles are similar, the gene is called homozygous, e.g. TT or tt.
- Heterozygous: When the genes in a pair of alleles are dissimilar, the gene is called heterozygous, e.g. Tt.
- Phenotype: The apparent characters which can be easily recognized make the phenotype. For example, height, hair colour, eye colour, colour of flowers, etc.
- Genotype: The genetic constitution of an individual is called genotype. For example, a tall plant can be hozomozygous with genotype TT or heterozygous with genotype Tt.
- Monohybrid Cross: When inheritance of only one character is studied during hybridization, the hybridization is called monohybrid cross.
- Dihybrid Cross: When inheritance of two characters is studied during hybridization, the hybridization is called dihybrid cross.
- Trihybrid Cross: When inheritance of three characters is studied during hybridization, the hybridization is called trihybrid cross.
- Polyhybrid Cross: When inheritance of many characters is studied during hybridization, the hybridization is called polyhybrid cross.
A cross in which plant ‘A’ (TT) is used as male parent and plant ‘B’ (tt) is used as female parent and another cross in which plant ‘A’ (TT) is used as female parent and plant ‘B’ (tt) is used as male parent, then this case is called reciprocal cross.
- Parental Generation: The plants which are hybridized to obtain offspring is called parental generation.
- F1 or First Filial Generation: The first generation obtained after crossing the parents is called F1 generation or first filial generation.
- F2 Generation or Second Filial Generation: The generation obtained after cross of Ft generation is called F2 generation or second filial generation.
- Monohybrid Ratio: The ratio of offsprings with different traits obtained after monohybrid cross is called monohybrid ratio.
- Dihybrid Ratio: The ratio of offpsrinng with different traits obtained after dihybrid cross is called dihybrid ratio.
- Mendel’s Laws of Inheritance: These are laws proposed by Mendel which are also called laws of genetics.
Law of Dominance :
This law is based on the results of monohybrid cross. This law says that when homozygous plants are crossed for a pair of particular trait then only one of the traits expresses in Fa generation. This trait is called dominant trait. The trait which does not express in F1 generation is called recessive trait.
Law of Segregation or Law of Purity of Gametes:
This law is also based on monohybrid cross. This law says that during gamete formation of hybrid plants of Fa generation, both the alleles separate from each other and go to different gametes. As the genes from a pair segregate during gamete formation hence this law is called law of segregation. Due to presence of two alleles for each trait, this law is called the law of purity of gametes.
When F1 generation is crossed with either of the parents (TT or tt), it is called back cross. There are two types of back cross which are as follows:
- Out Cross: When plants of Fx generation (Tt) are crossed with the parent with dominant character (TT), it is called out cross. All offsprings obtained after this cross are tall plants. Half (50%) of them are homozygous tall (TT) and half (50%) of them are heterozygous tall (Tt).
- Test Cross: When plants of F] generation (Tt) are crossed with parent with recessive character (tt), it is called test cross. Half of the offspring are heterozygous tall (Tt) and half (50% are homozygous short (tt). Thus, both phenotype and genotype ratio among offsprings is 1:1.
Law of Independent Assortment:
This law is based on results of dihybrid cross. This law says that when plants with two or more characters are crossed then inheritance of one character has no effect on inheritance of another character. This means that alleles of each character are not only separated during gamete formation but also behave independently of each other. Because of freedom of combining with any other trait, this law is called the law of independent assortment. This means that tallness can go with any colour of flower, i.e. white or violet. Similarly, violet followers can go with any shape of pod.
Importance of Mendel’s Law of Inheritance
- The presence of dominance of a particular trait ensures that harmful traits are suppressed through generations. Thus, presence of dominant trait is very important for survival.
- Law of independent assortment confirms the gene concept.
- Appearance of new traits can be easily explained with these laws.
a Traits can be carefully selected for producing new varieties of plants with desirable characters.
- Knowledge of these laws can be utilized to produce plants and cattle with desirable trait. For example, large varieties of rice with higher yield have been produced by careful hybridization.
- The concept of Eugenics is based on Mendelism. Eugenics deals with improvement in the human race.