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Biology: Genetics and Evolution: Genetic Basis of Inheritance

Topics are arranged as per SYLLABI published by Maharashtra State Board of Secondary and Higher Secondary Education.

The term genetics is derived from genesis - to grow / descent.

The term genetics was coined by William Bateson in 1906.

  • Gregor Johann Mendel (1822-1884) is known as "Father of Genetics".
  • He made systematic research on the principles of inheritance and gave first scientific explanation regarding the mechanisms involved in the transmission of characters (even before discovery of gene and chromosomes).
  • He conducted experiments on pea plant - pisum sativum. He presented the data and conclusions of his experiments in the paper under the title "Experiments in Plant Hybridisation".
  • In 1900, three scientists: Hugo de Varies (from Holland); Karl Correns (from Germany) and Eric von Tcshemark (from Austria) working independently arrived at the same conclusions as those derived by Mendel. Based on Mendel’s findings, Correns postulated the three Mendel’s Laws, known as Mendel’s Laws of Inheritance.
  • Mendel’s epoch-making work on Genetics is called as Mendelism.

  • Mendel chose to study peas (Pisum Sativum) because they were easy to grow and available with seven pair of contrasting characters. Seven pairs of contrasting characters / traits studied by Mendel are given below:

    Trait (Character) Contrasting Form
    Dominant Recessive
    Seed Color Yellow (YY) Green (yy)
    Seed Shape Round (RR) Wrinkled (rr)
    Flower Color Purple Coloured (CC) White (cc)
    Flower Position Axial (AA) Terminal (aa)
    Pod Color Green (GG) Yellow (gg)
    Pod Shape Inflated (II) Constricted (ii)
    Steam Length / Height Tall (TT) Dwarf (tt)

  • Pea plant is a common annual plant with a very short life cycle of 3-4 months that made it possible to study several generations within a relative short period. It is easy to cultivate perfectly bisexual flowers containing male and female reproductive organs, hence naturally predominantly self-pollinating.
  • Due to self-pollination, the plants are homozygous and therefore were easy to obtain pure breed for several generations.
  • Flowers were large enough for cross-pollination hence pollens from one flower can be introduced to the stigma of another flower for several generations.
  • He carried out his experiments upto several generations that is F2 and F3 generations.
  • He always started with pure breeding variety for a pair of contrasting characters.
  • He confined attention to one character at a time.
  • He kept an accurate pedigree record of several generations and made quantitative studies of each progeny (applying laws of probability) that resulted from a cross.
  • He always selected plants that where genetically pure, which bred true to species for at least three generations.
  • Crossing was carried out between parents of pure lines having sharp contrasting characters.
  • He grew pure lines in separate garden plots, preventing chance of their mingling.
  • His experiments did not show incomplete dominance.

Result of monohybrid cross experiment is summarized below:

Sr. Cross F1 F2 Ratio
1 Tall X dwarf Tall 787 Tall, 277 Dwarf 2.84 : 1
2 Yellow X Green Seeds Yellow Seed 6022 Yellow, 2001 Green 3.01 : 1
3 Round X Wrinkled seeds Round Seed 5474 Round, 1850 Wrinkled 2.96 : 1
4 Green X Yellow Pods Green Pods 428 Green, 152 Yellow 2.82 : 1
5 Inflated X Constricted pods Inflated pods 882 Inflated, 299 constricted 2.95 : 1
6 Axial X Terminal Flower Axial Flower 651 Axial, 207 Terminal 3.14 : 1
7 Violet X White Flower Violet Flower 705 Violet, 244 white 3.15 : 1
8 Grey X white seed coat Grey Seed Coat 705 Grey, 224 white 3.15 : 1

Monohybrid Phenotypic Ratio = 3:1

Monohybrid Genotypic Ratio = 1:2:1

Dihybrid Phenotypic Ratio = 9:3:3:1

Dihybrid Genotypic Ratio = 1:2:2:4:1:2:1:2:1

Back Cross = F1 Hybrid X parent (Dominant / Recessive)

Test Cross = F1 Hybrid X parent (Recessive)

Mendel proposed three laws based on his experiments on pea plant. It is recommended to read text book for detail explanation of these laws, key important points for each law is mentioned below:

Law of Dominance:

  • When two homozygous individual with sharp contrasting characters are crossed, the F1 generation resembles only one parent, which is dominant. Ex: A cross between homozygous tall pea plant and a homozygous dwarf pea plant produces the F1 generation, which where all tall.
  • Each of the seven characters of pea plant studied by Mendel are inherited independently without influenced by other characters (unit character).
  • This law is applicable to plants and animals. [Ex: in guinea pigs black color is dominant over white color; In man curly hair is dominant over long straight hair, etc].
  • Exceptions to principle of dominance are "incomplete dominance" and "codominance".

Law of Segregation OR Law of Purity of Gametes :

  • A cross between homozygous tall pea plant and a homozygous dwarf pea plant produces the F1 generation, which are all tall. But F1 carry some hereditary factor for dwarfness which is unaffected by tallness.
  • This law states that both parental alleles (recessive and dominant) separate and are expressed phenotypically in F2 generation.
  • When F2 generation was produced by allowing F1 hybrid to self pollinate, to find out segregation or separation it was observed that both dominant and recessive plants appeared in 3:1 ratio.

Law of Independent Assortment:

  • The law of independent assortment states that when inheritance of two or more genes occur at one time, their distribution in the gametes and in the progeny of subsequent generations is independent of each other.
  • To prove this, he did a dihybrid cross. He crossed homozygous dominant smooth and yellow seeded (YYRR) with homozygous recessive wrinkled and green seeded (yyrr) plants.
  • The F1 hybrid was self pollinated and F2 generation was obtained with the phenotypic ratio of 9:3:3:1 and genotypic ratio of 1:2:1:2:4:2:1:2:1.

In post-Mendelian genetics, many experiments where done on other plants and animals and exceptions of Mendelism was observed. Based on these observations, different patterns of inheritance or gene interactions where discovered.

Gene Interaction: It was observed that phenotypic expression of a gene can be modified or influenced by the other gene. These gene interactions could be intragenic or intergenic.

Intragenic (interallelie) Intergenic (non-allelle)
Occurs between allele of the same gene. Occurs between allele of different gene on the same or different chromosome.
Includes : codomiance, Incomplete dominance and multiple alleles. Includes : polygenes, Epistasis, supplementary and complimentary gene, etc.

Incomplete Dominance: In incomplete dominance, the gene of an allelomorphic pair are not expressed as dominant and recessive but express themselves partially when present together in hybrid (heterozygous). Here one gene cannot suppress the expression of the other completely. Hence F1 hybrid show characters intermediate to the effect of two genes of the parents.
This is observed in four O’clock plant (Mirabilis Jalapa) and Snapdragon or dog flower (Antirrhinum majus) where there are two types of flower colours. i.e. Red and white. When pure red flowers (RR) is corsed with pure white flowers (rr) the gamete produced are R and r respectively. Hence the F1 hybrid (Rr) bears pink flowers.
If F1 hybrid is selfed to raise F2 generation, they produce red (RR) , Pink (Rr) and white (rr) flowers in the ratio 1:2:1.
Here both phenotypic and genotypic ratio will be same because the F1 hybrid is phenotypically intermediate between two homozygous types.

Co-Dominance: When the dominant character is not able to suppress even incompletely the recessive character and both the characters appear side by side in F1 hybrids, the phenomenon is called co-dominance.

  • Both the genes of an allelomorphic pair express themselves equally in the F1 hybrid (heterozygous).
  • Since the alleles are able to express themselves independently when present together are called co-dominant alleles.
  • Even in co-dominance ; in F2 generations genotypic and phenotypic ratios are identical i.e. 1:2:1

In Mendelism each character is concerned with two alleles. But now it is observed that more than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue. This is known as multiple alleles.

  • They are due to mutation of the wild type of gene.
  • A gene can mutate many times producing a series of alterative expression. Hence different alleles in a series show co-dominance or incomplete dominance among themselves.
  • Multiple alleles do not undergo crossing over due to their location on same locus.
  • Wild type is dominant overall other mutants.

One of example of multiple alleles is blood group. A person may be A type, B type, AB type or O type depends upon presence and absence of specific substance on the red blood cells. A blood group character is controlled by a set of three alleles.

  • A gene which produces antigen A - IA
  • A gene which produces antigen B - IB
  • A gene for absence of both the antigens as I

Gene IA and IB are dominant over I, but not over each other. IA and IB, if present together indicates co-dominance.

There are six different genotype and only four phenotypes.

Phenotype Genotype Antigen
Type - A IAIA or IA i A
Type - B IBIB or IB i B
Type - AB IAIB A & B
Type - O Ii None
Blood groups are inherited in simple Mendelian pattern with all the four type of blood groups are possible in cross between two persons heterozygous for blood group A & B.

Blood Group and its inheritance can be summarized as mentioned in the table below:

Father Mother Children
Phenotype Genotype Phenotype Genotype Phenotype
A IAIA
IAIO
A IAIA
IAIO
A,O
B IBIB
IBIO
B IBIB
IBIO
B,O
A IAIA
IAIO
B IBIB
IBIO
A,B,AB,O
A IAIA
IAIO
O IOIO A,O
B IBIB
IBIO
A IOIO B,O
AB IAIB A IAIA
IAIO
A,AB,B
AB IAIB B IBIB
IBIO
B,AB,A
O IOIO O IOIO O

When single gene controls two or more different traits it is called pleiotropic gene and the phenomenon is called pleiotropism.
Ex: The mutant gene in Drosophila affects the enzyme that give colour to the eyes, so the eyes are white, but it also alters the shape of the sperm storage organ in females.

Polygenic Inheritance:

  • Polygenic inheritance describes an inherited characteristic that is the result of the action of genes at two or more loci.
  • Polygenic characteristics usually show quantitative phenotypic variation.
  • When each of a number of genes contributes to the expression of trait, they are collectively known as polygenes or multiple factors.

Wheat kernel color was discovered by H. Nilsson- Ehlie (Swedish Geneticist). He crossed red kernelled variety of wheat with white kernellel variety. In F1 generation all plants had grains with intermediate color between red and white and in F2 generation five different phenotypic expression appeared in the ratio 1:4:6:4:1

Different phenotypic expression appeared in F2 Ratio
The Darkest Red 1/16
Medium Red 4/16
Intermediate Red (F1 hybrid) 6/16
Light Red 4/16
White 1/16

  • Davenport studied inheritance of skin colour in negro and white population in USA. Population derived from marriages between negro and white show intermediate skin colour and are called Mulattoes.
  • Melanin pigment in the skin determines skin colour.
  • Emasculation is a removal of stamens before anthesis. It is done in bud condition. Pea flowers have papilionaceous corolla with 10 stamens in 9+1 arrangement can be removed by forceps.
  • Selfing is a type of interbreeding where in both the parents possess same genetic constitution.
  • Reciprocal cross involves 2 crosses concerning the same character but with reversed sexes i.e. in 1st cross A is used as male parent and B as female parent, then in 2nd cross A is used as female parent and B as male parent.
  • Isoalleles are alleles which produce similar phenotypes but posses different optima.
  • Pseudoalleles are nonallelic genes which form related phenotypes. They are known only through rare crossing over. Nonalleles factors found on different loci.
  • Checker Board / Punnet Square is a square divided into smaller squares which show the mathematical result of cross, both phenotypic and genotypic. It is of three types- gametic, phenotypic and genotypic.
  • Forked Line or Branching system is also used to know phenotypic and genotypic probabilities.