Heredity &Variations | Zoology Optional Notes for UPSC PDF Download

Introduction

Reproductive process gives rise to new individuals that are similar but with some variations. Thus the similarities between parents and offspring are due to heredity and the dissimilarities are due to variation. Continuity of life is maintained through heredity and evolution. Heredity explains that the organisms resemble each other because they arise from a common ancestor. Evolution is the orderly change of various forms through a slow but continuous process. It gives rise to a new complex body design even while the simpler body designs continue to flourish.

Heredity

The transfer of characters or traits from the parents to their offspring is called heredity.

Variation

The differences between the characters or traits among the individuals of the same species are called variations.

Importance of Variation

• They enable the organism to adapt to changing environments.
• Variation forms the basis of heredity.
• They provide the raw material for evolution and the development of new species.

Variations may or may not help organisms to survive

• Some variations help organisms to survive: For example, some beetles living in green bushes increase their numbers through reproduction. Crows can easily see the red beetles and eat them. However, due to some variation during reproduction, some green beetles are produced instead of red beetles. The green beetles are not visible to crows and are not eaten by them. This variation has helped the organisms to survive.
• Some variations do not help organisms to survive: For example, during sexual reproduction, a color variation occurs in red beetles and some blue beetles are produced instead of red beetles. Both the red and blue beetles are visible to crows and are eaten by them. This variation has not helped the organisms to survive.
• Acquired traits cannot be passed from one generation to the next: For example, if the population of beetles increases and plants are affected by diseases, then the food available for the beetles decreases and their body weight also decreases. If after a few years the availability of food increases, then the body weight of the beetles also increases. This acquired trait cannot be passed from one generation to the next because there is no change in their genetic composition.

Accumulation of Variations during Reproduction

When organisms reproduce, the offspring show minor variations due to inaccuracies in DNA copying. These variations are less in asexual reproduction and more in sexual reproduction. Since DNA copying always involves some variations, it will bring a subtle change in the next generation. If this reproduces, the newborn will have differences that they inherit from the first generation, as well as some newly created differences. Therefore, variations get accumulated generation after generation, resulting in speciation, i.e., evolution. Some variations are useful and help the organism adjust to changes in the environment, while some variations do not help the organism adapt and may lead to extinction. The selection of variants by environmental factors forms the basis of evolutionary processes.

Heredity and Variation in Asexual Reproduction

In asexual reproduction, organisms are exact copies of their parents. They tend to preserve the similarities among all individuals belonging to a given line of descent/species. They exhibit very little variation due to some environmental factors or mutations, which are sudden changes in genes. Out of these two factors, only mutations are inheritable.

Heredity and Variation in Sexual Reproduction

In sexual reproduction, two parents are involved, and there is the formation and fusion of gametes. The offspring show variations from their parents due to crossing over and exchange of gene segments. They are not carbon copies of their parents due to the recombination of parental genes; thus, inheritable variations occur.

Inherited Traits

Inherited traits are traits/characters that are transferred from parents to offspring generation after generation.

Acquired Traits

Acquired traits are particular characteristics that are developed during the lifetime of an individual. Such characteristics are not genetically controlled and cannot be passed on to the next generation.

Rules for Inheritance of Traits

Characters are transferred through genes present in the DNA molecules in the chromosomes present in the nucleus of the cell. The inheritance of characters is due to the fact that both the father and mother contribute an equal amount of genetic material to the child. So for each trait, there are two factors, one from the father and one from the mother. Gregor Johann Mendel conducted experiments with garden pea plants and determined the rules for the inheritance of traits.

Gregor Johann Mendel

Gregor Johann Mendel was an Austrian geneticist and is regarded as the "father of Genetics." He performed his experiments on garden pea plants (Pisum sativum) during 1856 to 1865 and published his results in a book called "Experiments on Plant Hybridization." Mendel's work remained unrecognized until his death in 1884. His work was rediscovered in 1900 by three scientists: Hugo de Vries, Carl Correns, and Erich von Tschermak.

Mendel's Experiment on Inheritance of Traits

Mendel used a number of contrasting visible characters of garden pea plants, such as height, color of flower, position of flower, shape of pod, color of pod, shape of seed, and color of cotyledons. He crossed plants with contrasting traits and observed the inheritance patterns in the offspring. He determined that traits are inherited in a predictable manner and follow certain ratios, such as the 3:1 ratio for the inheritance of a dominant and recessive trait in the second filial generation (F2).

Monohybrid Cross

The cross between plants having only one pair of contrasting characters, such as tall and short plants, is called a monohybrid cross. Mendel selected pure tall (TT) and pure short (tt) pea plants and cross-pollinated them. He obtained all tall plants (Tt) in the first generation (F1). When the first-generation plants were self-pollinated, he obtained tall and dwarf plants in a 3:1 ratio (phenotypic ratio) in the second generation (F2). The genotypic ratio was 1:2:1 for pure tall, hybrid tall, and pure dwarf.

Dihybrid Cross

The cross between plants having two pairs of contrasting characters, such as shape and color of seeds, is called a dihybrid cross. Mendel selected plants having round yellow seeds (RRYY) and wrinkled green seeds (rryy) and cross-pollinated them. He obtained plants with round yellow seeds (RrYy) in the F1 generation. When these plants were self-pollinated in the F2 generation, he obtained plants with different combinations of traits in a 9:3:3:1 phenotypic ratio.

How Traits Get Expressed/Mechanism of Inheritance

Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called a gene for that protein. Proteins control characteristics by regulating various processes in the cell. For example, the tallness of a plant depends on the amount of a particular plant hormone. The amount of plant hormone depends on the efficiency of the process of making it. If the gene for the enzyme involved in this process is altered, the efficiency of the process and the amount of hormone produced will change, resulting in a change in plant height.

Sex Determination in Human Beings

Human beings have 23 pairs of chromosomes in the nucleus of the cell. Out of these, two chromosomes are sex chromosomes, X and Y. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The sex of a child is determined by the combination of sex chromosomes received from the parents.

Speciation

The formation of new species from existing species is mainly due to the accumulation of variations, physical barriers, genetic drift, natural selection, and migration.

Origin of Life on Earth

Life is believed to have originated from simple inorganic molecules present on the early Earth. The first primitive organisms would have arisen from further chemical synthesis. This hypothesis is supported by the experiments of Stanley Miller and Harold Urey, who demonstrated that the building blocks of life, such as amino acids, could be produced from simple inorganic molecules.

Evidences of Evolution

There are several common features in different organisms that provide evidence of evolutionary relationships. The main evidences of evolution come from the study of homologous organs, analogous organs, and fossils.

Homologous Organs

Homologous organs are organs that are similar in structure but have different functions. For example, the forelimbs of amphibians, reptiles, birds, and mammals have similar structures but different functions. This indicates an evolutionary relationship between these organisms.

Analogous Organs

Analogous organs are organs that have different structures but perform similar functions. For example, the wings of birds and bats have different structures but both are used for flying. This is an example of convergent evolution.

Fossils

Fossils are the remains or impressions of organisms that lived long ago. They provide evidence of the structures and time periods in which they lived. The study of fossils, known as paleontology, shows the evolution of simpler forms into complex forms and provides evidence of evolutionary relationships between different organisms.

Evolution by Stages

Complex organisms and their organs have developed gradually from simpler organisms over generations. For example, the evolution of eyes from simple eye spots to complex organs in higher animals and the evolution of feathers from being used for protection from cold to being used for flying in birds.

Evolutionary Theories

There are several theories proposed to explain evolution, including Lamarck's theory, Darwin's theory of natural selection, the mutation theory, and neo-Darwinism. These theories explain the mechanisms of how evolution occurs and the factors that contribute to it.

Human Evolution

Human beings evolved in Africa and some populations migrated to different parts of the world. Due to genetic variations and environmental changes, different populations developed changes in their forms and features. There is a great diversity among human beings in terms of their form and features around the world.