GENETICS- PRINCIPLES OF INHERITANCE AND VARIATION
Genetics is the branch of biology that deals with inheritance and variations. Inheritance is
the process by which the characteristics of the parent are passed on to the progeny which is
the basis of hereditary. Variation is the degree by which the progeny differs from the parent.
MENDEL’S LAW OF INHERITANCE
Gregor Mendel during the mid-nineteen century conducted experiments on garden peas for
7 years and proposed law of inheritance in living organisms. His experiments were credible
due to large sampling size and mathematical logic, and the results were consistent.
Mendel conducted experiments by using artificial pollination on 7 pairs of pea plants
with contrasting traits, which are
Stem height – Tall / Dwarf
Flower colour – Violet / White
Flower position – Axial/ Terminal
Pod shape – Inflated / Constricted
Pod colour – Green/ Yellow
Seed shape – Round / Wrinkled
Seed colour – Yellow / Green
True breeding pea line – one that having undergone continuous self-pollination, showing
stable trait of inheritance and expression for several generations
Mendel selected 14 true breeding pea plants (7 pairs) which were similar except for one
character with contrasting trait.
First hybrid generation of plants formed by the cross pollination of the pair were called Filial
1 progeny or F1. Mendel observed that all the F1 progeny resembled either one of the
parents and the trait of the other parent were not seen. For example, F1 progeny of tall x
dwarf pair were all tall. Mendel then self-pollinated the tall F1 plants and found that in the
,Filial 2 generation some of the offspring were dwarf. The character that was not seen in F 1
was now expressed. 1/4th of the F2 were dwarf while 3/4th were tall. No blending of the traits
were found.
Parent Tall x Dwarf
F1 generation Tall x Tall
Selfing
F2 generation Tall Tall Tall Dwarf
Factors – Mendel concluded that something was stably passed down to the offspring
through gametes and called them “Factors”. It is now known as Genes.
GENES
Genes are the unit of inheritance. Genes carry information regarding a particular trait which
is expressed by the organism.
Alleles – it’s the pair of genes which codes for contrasting traits, ie., they are slightly
different forms of the same gene.
, Alphabetical symbols are used for each gene. For the trait that is expressed in the F 1
generation capital letter is used and for the other trait small letter is used. For example, T is
used for tall, and t is used for dwarf. So, the alleles for height can be either TT, Tt, or tt.
Homozygous – when the allelic pair of genes are identical, like in the true breeding pea
plant, it is called homozygous. Eg – TT, tt. If the pair is not identical it is called heterozygous
eg- Tt
Genotype and phenotype
Tt or tt are called genotype while the descriptive term tall or dwarf are called the
phenotype.
Mendel observed that that the phenotype of F1 generation (heterozygote Tt) is exactly
similar to the TT parent. Hence, he proposed that, in a dissimilar pair of factors one is
dominant and other is recessive. For example, T for tallness is dominant while t for
dwarfness is recessive.
Since there is no blending of traits in the F2 generation, we can infer that, when the
tall and dwarf plant produces gametes by meiosis, the alleles of parental pair separate or
segregate and only one allele is transmitted to the gamete. This process is random and the
chance of a gamete containing either allele is 50%. British geneticist Reginald C Punnett
developed a graphical representation to explain the monohybrid cross (study of inheritance
of one character) and to calculate the probability of all genotypes of offspring called Punnett
square.
Tall x Dwarf
TT tt
Genetics is the branch of biology that deals with inheritance and variations. Inheritance is
the process by which the characteristics of the parent are passed on to the progeny which is
the basis of hereditary. Variation is the degree by which the progeny differs from the parent.
MENDEL’S LAW OF INHERITANCE
Gregor Mendel during the mid-nineteen century conducted experiments on garden peas for
7 years and proposed law of inheritance in living organisms. His experiments were credible
due to large sampling size and mathematical logic, and the results were consistent.
Mendel conducted experiments by using artificial pollination on 7 pairs of pea plants
with contrasting traits, which are
Stem height – Tall / Dwarf
Flower colour – Violet / White
Flower position – Axial/ Terminal
Pod shape – Inflated / Constricted
Pod colour – Green/ Yellow
Seed shape – Round / Wrinkled
Seed colour – Yellow / Green
True breeding pea line – one that having undergone continuous self-pollination, showing
stable trait of inheritance and expression for several generations
Mendel selected 14 true breeding pea plants (7 pairs) which were similar except for one
character with contrasting trait.
First hybrid generation of plants formed by the cross pollination of the pair were called Filial
1 progeny or F1. Mendel observed that all the F1 progeny resembled either one of the
parents and the trait of the other parent were not seen. For example, F1 progeny of tall x
dwarf pair were all tall. Mendel then self-pollinated the tall F1 plants and found that in the
,Filial 2 generation some of the offspring were dwarf. The character that was not seen in F 1
was now expressed. 1/4th of the F2 were dwarf while 3/4th were tall. No blending of the traits
were found.
Parent Tall x Dwarf
F1 generation Tall x Tall
Selfing
F2 generation Tall Tall Tall Dwarf
Factors – Mendel concluded that something was stably passed down to the offspring
through gametes and called them “Factors”. It is now known as Genes.
GENES
Genes are the unit of inheritance. Genes carry information regarding a particular trait which
is expressed by the organism.
Alleles – it’s the pair of genes which codes for contrasting traits, ie., they are slightly
different forms of the same gene.
, Alphabetical symbols are used for each gene. For the trait that is expressed in the F 1
generation capital letter is used and for the other trait small letter is used. For example, T is
used for tall, and t is used for dwarf. So, the alleles for height can be either TT, Tt, or tt.
Homozygous – when the allelic pair of genes are identical, like in the true breeding pea
plant, it is called homozygous. Eg – TT, tt. If the pair is not identical it is called heterozygous
eg- Tt
Genotype and phenotype
Tt or tt are called genotype while the descriptive term tall or dwarf are called the
phenotype.
Mendel observed that that the phenotype of F1 generation (heterozygote Tt) is exactly
similar to the TT parent. Hence, he proposed that, in a dissimilar pair of factors one is
dominant and other is recessive. For example, T for tallness is dominant while t for
dwarfness is recessive.
Since there is no blending of traits in the F2 generation, we can infer that, when the
tall and dwarf plant produces gametes by meiosis, the alleles of parental pair separate or
segregate and only one allele is transmitted to the gamete. This process is random and the
chance of a gamete containing either allele is 50%. British geneticist Reginald C Punnett
developed a graphical representation to explain the monohybrid cross (study of inheritance
of one character) and to calculate the probability of all genotypes of offspring called Punnett
square.
Tall x Dwarf
TT tt
