Heredity and Evolution: Definition, Mendel's Experiments and Concepts

Heredity and Evolution talks about resemblance which exists among individuals like a child who resembles his/her parent, grandparents or great grandparents. If we talk about resemblance in elephants, you will find that elephants have a great resemblance with other elephants and the same goes with the plants.

The rose plant will look exactly similar to other rose plants. The resemblance among individuals or species implies the continuity of life. However, the origin of the first life on earth is still not revealed to us due to evidence that is hidden in the distant past.

Key Terms: Theory on human evolution, Heredity, Heredity and variations. sex variations

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Introduction

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The chapter talks about many theories which tell us about the existence of plants and animals in this universe today. The term evolution is coined by Herbert Spencer and the word has been derived from the Latin word evolve which means unfold or unroll. Evolution is generally defined as a naturally occurring slow continuous and irreversible process of change. In simple terms, it refers to how members of species evolve generations after generations following environmental requirements and turn them into a better and organized form of species. Then we move a step ahead and the chapter talks about Organic Evolution which states “ Descent with modification” which means present-day living organisms have evolved from earlier simple organisms but with gradual changes which have occurred millions of years.

Fig: Evolution In Mankind

The plants and animals that have their existence in the past in the form of remains of rocks are known as fossils. The study of fossils is known as Paleontology. Leonardo Da-Vinci is known to be the Father of Paleontology. The Founder of Modern Paleontology is George Curvier. The Fossil is further Classified into three broad categories as Petrified, Mould and Casts and Prints. Petrified is the process of replacement of organic parts into minerals whereas Mould is hardened and fossilized mud which surrounds the dead organisms and when mould is found with petrified fossils of organisms, it is known as casts. Footprints or prints of wings, skin and leaves etc made in soft mud which subsequently become fossilised. 

Read Also: How do Organisms Reproduce?

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Important Theories On Evolution

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• Theory of Special Creation: This theory states that different forms of life which occur on Planet Earth have been created by God and this idea is found in the ancient scriptures in almost all religions. This theory is completely based on religious concepts and is based on faith. This theory has no scientific faith.
• Cosmozoic theory[theory of Panspermia]: As indicated by this hypothesis, life has arrived at this planet Earth from superb bodies and this thought was proposed by Richter in 1865 and upheld by Arrhenius.
• Oparin-Haldane theory: This theory is also known as Materialistic theory or physicochemical Theory. According to this theory, the origin of life on earth is due to the slow and gradual process of chemical evolution that occurred 3.8 billion years ago.

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Heredity And Variations

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The term Heredity refers to the passing of traits to offspring either from parents or from ancestors whereas Variations refers to differences between the individual of the same species and between offspring of the same parents. The Variations are further classified into two broad categories:

• Somatic Variation: Mutations which occur in the somatic cell are known as somatic variations. New mutations occur twice in sperms as in eggs because cell divisions are required to make sperms than eggs.
• Germinal Variation: These are further divided into two broad categories as Continuous Variations which is based on Darwin theory of selection and Discontinuous Variations which are large distinct differences from parents. They are generally termed as mutations or sports. 

Heredity and Variations

The study of heredity and variations is known as Genetic which was first used by William Bateson in 1905.

Heredity And Variations in Sexual and Asexual Reproduction

In sexually reproducing organisms, the new individuals receive genetic information through their mother in the form of eggs and half of it through their father in the form of sperm. They resemble their parents but they are not similar to them. In Asexual Reproduction, where individuals are carbon copies of one another or their parents are called ramparts and a group of individuals are known as a clone. Gregor Johann Mendel proposed a theory which states that inheritance is controlled by paired germinal units or factors known as Genes. They are present in all cells of the body and transferred to the future generation through gametes.

Heredity and Variations in Sexual and Asexual Reproduction

The study of genetics explains how the characteristics are inherited by offspring from their parents. Gregor Johann Mendel, an Austrian priest, carried out a theory and worked on the pea plant. He studied the inheritance of several contrasting pairs of characters like tall or dwarf seeds for several generations. From his experiment, he concluded that contrasting pairs in pea plants are controlled by units called factors and each pair of contrasting pairs is controlled by one factor, thus, the character plant height[tall or dwarf] is constrained by one factor. The factors act as hereditary carriers that are passed from one generation to another. He for the first time explained the fundamentals of inheritance and these factors were later identified as genes by the later scientists.

Read More: Sexual and Asexual Reproduction

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Theories of Heredity

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1. Preformation Theory: This theory was proposed by Anton Van Leeuwenhoek who observed human sperms and called them animalcules. According to him, each sperm can develop into a new individual when introduced into a mother womb where it gets proper nourishment and warmth from the mother. However, this was rejected as it doesn’t explain the maternal character of the offsprings.
2. Particulate Theory: Mauperkius, a French biologist, explained that each animal produces minute particles at the time of reproduction and new offspring are born by the union of particles of two parents. Charles Darwin, another famous English Naturalist proposed the theory of pangenesis for the inheritance of the characters. According to him, tiny particles are known as pangenesis and they are formed in various parts of the body and combined with the reproductive cells and guide the offspring for the formation of the respective part. This theory is not rejected as it fails to explain the cause of inheritance. 

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Mendel’s Law of Inheritance

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According to Mendel, in human beings, the rules of inheritance of traits are contributed by both father and mother to produce an equal amount of genetic material in their child. Each trait in a child is influenced by father and mother DNA which means DeoxyriboNucleic Acid. The child will have two versions, one from the father and another from the mother. Gregor Johann Mendel was the first scientist who worked on the basic guidelines of inheritance of traits. 

Gregor Johann Mendel who is the father of Genetic conducted the following crosses:

1. Monohybrid Cross: Cross between 2 pea plants with one set of differentiating characters Tall/Short. The particular image which is given below will help you understand better about Monohybrid Cross.

2. Dihybrid Cross: A rearing experiment managing two characters simultaneously.The image which is given below will help you to comprehend how Dihybrid Cross takes place in the F1 generation and F2 generation.

Contrasting pairs of pea plant as noticed by Mendel are:

Plant Height- Tall/short

Flower Colour-Violet/white

Pod colour-Green/yellow

Pod shape- Smooth/ Wrinkled

Seed Colour-Green/Yellow

Seed shape-Round/ Wrinkled

Basic terms used by him in his experiment are:

Genotype: The internal genetic constitution of an organism is known as Genotype.

Filial Generation- The Generation of Offspring is known as Filial Generation.

First Filial Generation[F1]- The first generation of offspring produced by the parent generation.

Second Filial Generation[F2]- The second generation of offspring.

Phenotype: Externally Visible character of an organism.

Genome:A total arrangement of chromosomes acquired as a unit from one parent.

Gene: a unit of inheritance located at a fixed place of the chromosome.

Dominant Gene: Gene which expresses itself in front of a contrasting gene.

Recessive Gene: Gene whose expression is suppressed in the form of the dominant gene.

Alleles: Two alternative genes at the same locus of homologous chromosomes.

Heterozygous:An individual having two distinct alleles for a specific character.

Homozygous: An individual having two identical alleles.

Read More: Rules for the Inheritance of Traits - Mendel's Contributions

Mendel concluded three things from his experiments:

1. Law of Segregation: Genes[Factor] which got segregated during gamete formation.
2. Law of Dominance: Appearance of only one of two contrasting traits in the F1 generation.
3. Law independent Assortment: Inheritance of gene controlling a particular trait in an organism's independent of others.

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Sex Determination

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The mechanisms by which the sex of an individual is determined is known as Sex Determination. In Diploid organisms having separate sexes, a specific pair of chromosome in diploid cell tell us the sex of an individual. Human beings have 46 chromosomes. Out of these 44 pairs[22] are autosome and 2[ one pair] are sex chromosomes. The chromosomes in human beings are fruit-fly and are of two types- X and Y chromosomes.