Genetics and Evolution

Genetics is the branch of biology that studies how traits are passed from parents to offspring through genes, the units of heredity. These genes are made of DNA (deoxyribonucleic acid), which carries instructions for the development, functioning, growth, and reproduction of all known living organisms. Evolution is the process by which species change over time due to genetic variations and natural selection. It explains how all living organisms share common ancestors and have diversified through gradual changes.

1.0Genetics

• Heredity : Transmission of genetic characters from parents to progeny.  
• Variation : The degree by which progeny differ from their parents. 
• Inheritance : The process by which characters are passed on from parents to progeny. It is the basis of heredity. 
• Father of genetics : G.J. Mendel.  
• Character : Characteristics feature of an organism. e.g. Stem Height. 
• Trait : Variable form of character. e.g. Tall/Dwarf 
• Factor/Genes : Segment of DNA which is responsible for the appearance of characters and is stably passed down from parent to progeny through gametes.
• Mendel used the term – 'factor'. 
• Allele : Alternate forms of a gene which are located at homologous sites on homologous chromosomes. 
• Heredity : Transmission of genetic characters from parents to progeny. 
• Variation : The degree by which progeny differ from their parents. 
• Inheritance : The process by which characters are passed on from parents to progeny. It is the basis of heredity. 
• Father of genetics : G.J. Mendel. 
• Character : Characteristics feature of an organism. e.g. Stem Height. 
• Trait : Variable form of character. e.g. Tall/Dwarf 
• Factor/Genes : Segment of DNA which is responsible for the appearance of characters and is stably passed down from parent to progeny through gametes. 
• Mendel used the term – 'factor'. 
• Allele : Alternate forms of a gene which are located at homologous sites on homologous chromosomes. 

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2.0Mendelism

• Gregor Mendel (1822-1884) performed an experiment on Garden Pea (Pisum sativum) between 1856=1863.

Reasons for Mendel's success 

• Statistical and mathematical analysis of data. 
• Large sampling size. 
• Confirmation of his inferences from experiment on successive generation.

Why did Mendel 's work remain unrecognised for many years? 

• Communication was not easy at that time. 
• The mathematical approach used by Mendel was new and unacceptable to many biologists. 
• He could not provide any physical proof for the existence of factors.
• Use of mathematics in Biology.

In 1900, rediscovery of Mendel's work by three independent scientists 

• Carl Correns 
• Hugo de Vries 
• Erich Von Tschermak 

 Mendel studied 7 characters 

Mendel's experimental technique steps: 

I. Selection of pure plants. 

II. Hybridisation between pure plants. (a) Emasculation (b) Bagging (c) Tagging (d) Dusting 

III. Selfing of F₁ hybrids to obtain F₂ generation.

Monohybrid Cross 

• A cross done to study the inheritance of one character or one pair of contrasting traits.

Dihybrid Cross 

• A cross done to study the inheritance of two characters or two pairs of contrasting traits.

Laws Based On Monohybrid Cross

Law of Dominance

• Characters are controlled by discrete units called factors. 
• Factors occur in pairs. 
• In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive) 

Law of Segregation

• This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F₂ generation though one of these is not seen at the F₁ stage.

Law Based On Dihybrid Cross

Law of independent assortment

• The law states that "when two traits (2 different characters) are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters".

Incomplete Dominance 

• Phenotype of F₁ does not resemble either of the two parents and is in between the two. 
• Dominant allele is not completely dominant over recessive allele. 
• Eg. (i) Flower colour in snapdragon/Dog flower (Antirrhinum sp.) (ii) Size of starch grain in a pea plant.

Co-Dominance 

• In heterozygous condition both alleles are equally dominant. 
• In case of co-dominance F₁ resembles both parents. 
• Eg. AB blood group in humans

Multiple Alleles 

• More than two alternative forms of same gene are called multiple alleles 
• e.g. ABO blood group. 
• e.g. ABO blood group are controlled by gene-I

Chromosomal Theory Of Inheritance

• 1902 - chromosome (colored bodies, as visualised by staining) movement during meiosis had been worked out. 
• Walter Sutto and Theodore Boveri – noted that behaviour of chromosomes was parallel to the behaviour of genes.

Sex Determination 

• Cytological observation in a number of insects → concept of genetic/chromosomal basis of se-determination.
• Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, 
• He gave a name to this structure as the X body but he could not explain its significance. The ‘X body’ of Henking was X-chromosome. 
• Male honeybees (Drones) do not have fathers and thus cannot have sons, but have a grandfather and can have grandsons.

Genetic Disorders

• Thalassemia
• Down’s Syndrome
• Klinefelter’s Syndrome
• Turner’s Syndrome

3.0Evolution

• Evolutionary biology is the study of the history of life forms on earth.

4.0Theories for Origin of Life

Evidence in favour of chemical evolution

S.L. Miller's experiment (1953) 

• Created similar conditions as on primitive earth in a laboratory scale. 

Analysis of meteorite content 

• Revealed similar compounds 
• Similar process occurred elsewhere in space 
• With this limited evidence, chemical evolution was more or less accepted. 

5.0Evidences Of Evolution

1. Paleontological Evidences 

• Study of fossils in different sedimentary layers indicates the geological period in which they existed. 
• Different aged rock sediments contain fossils of different life forms, who probably died during formation of the particular sediments. 
• Life forms varied over a period. 
• Some Represent extinct organisms [e.g. Dinosaur]

2. Evidence from comparative anatomy and morphology

• Homologous Organ
• Analogous Organ

3. Evidence from biochemical similarities 
4. Embryological Evidences

Evolution of Life Forms A Theory

Lamarck Theory Proposed by – French Naturalist - Lamarck

• Inheritance of acquired characters
• Use and disuse of organs

Darwin's Theory [Darwinism] Proposed by – Charles Darwin

• Branching Descent
• Natural Selection

Mutation Theory Proposed by – Hugo de Vries

•  Evolution is caused by mutations and not by minor variations.

6.0Natural Selection

Natural selection can act on populations in different ways depending on environmental pressures. The three main types of natural selection are:

Directional Selection

• Definition: Favors individuals at one extreme of a trait distribution.
• Effect: The population shifts toward that extreme over time.
• Example: In a population of birds, if larger beaks are better for cracking seeds during a drought, birds with larger beaks will survive and reproduce more.

Stabilizing Selection

• Definition: Favors the average or intermediate traits, and selects against the extremes.
• Effect: Reduces variation and maintains the status quo.
• Example: Human birth weight — very low or very high birth weights have higher mortality, so average weight is favored.

Disruptive Selection

• Definition: Favors both extremes of a trait, but not the average.
• Effect: Can lead to the formation of two distinct groups or even speciation.
• Example: In an environment with both large and small seeds but no medium ones, birds with very small or very large beaks may do better than those with medium-sized beaks.

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