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Have you ever been told that you have your mother's eyes or grandfather's nose? Have you ever wondered why traits run in families and how they are passed down through generations? These questions give rise to an interesting field of Biology called Genetics. 

It all started with an Austrian monk by the name of Gregor Mendel. Based on consistent patterns he observed in animals and plants, Mendel correctly guessed that a kind of unit of heredity was transferred from parents to offspring. This assumption eventually led the scientific community to discover that unit of heredity, the gene. Understanding the gene is the key to understanding genetics.

All inherited traits in plants and animals are controlled by genes. A gene is a fundamental unit of heredity, which is transferred from a parent to offspring and determines some characteristic of the offspring.  Additionally, a gene is a locus (or region) of DNA, which is made up of nucleotides and is the molecular unit of heredity.

Heredity is the genetic information passing for traits from parents to their offspring and inherited traits are controlled by genes.

The complete set of genes within an organism's genome is called its genotype, whereas the complete set of observable traits of an organism as a result of the interaction of its genotype with the environment is called phenotype. As a result, many aspects of an organism's phenotype are not inherited. For example, sun-tanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to the next generation.

An allele is the variant form of a given gene. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, most genetic variations result in little or no observable variation.

Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve through the natural selection of specific phenotypic traits. In this chapter, we shall look at Mendel's model of inheritance and get a sneak preview of the modern concept of a gene.

Trait

Any characteristic that is transferred from parent to offspring. Example: height and colour.

Heredity

The transmission or passing of genetic characteristics or traits from the parents to their offspring either through sexual or asexual reproduction is called heredity. This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. For instance, in humans, eye colour is an example of an inherited trait. An individual might inherit the 'blue-eye trait' from one of the parents. Inherited traits are controlled by genes.

Mendel’s Contribution

In the 1850s, Gregor Mendel carried out experiments that laid the foundation of modern genetics. He formulated his ideas after conducting simple hybridization experiments with pea plants (Pisum sativum) that he had planted in the garden of his monastery. In these experiments, Mendel was able to selectively cross-pollinate pure-bred plants with particular traits and observe the outcome over many generations. This was the basis for his conclusions about the nature of genetic inheritance. 

From these experiments, Mendel established the basic rules of heredity, now referred to as the Laws of Mendelian Inheritance. Consequently, Mendel came to be known as the Father of Genetics. Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced much later. However, he did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.

Mendel’s Studies

In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations. This was the basis for his conclusions about the nature of genetic inheritance and following are his observations:

• Alleles can be dominant or recessive. 
• Homozygous: If an organism has two copies of the same allele, for example TT or tt, it is homozygous for that trait. 
• Heterozygous: If the organism has one copy of two different alleles, for example Tt, it is heterozygous. The dominant allele influences the outward appearance of plants in the filial generation. 
• Homozygous recessive: Both the alleles are recessive and expressed as ‘tt’. The recessive allele can express itself only in the presence of another identical recessive allele. 
• Homozygous dominant: Both the alleles are dominant and expressed as TT. It expresses itself when present.

Mendel's Law of Segregation and Monohybrid Cross

Monohybrid Cross

A cross between two plants with one (mono) character and different (hybrid) traits or two plants of contrasting characters is called a monohybrid cross. It is the inheritance of a single characteristic controlled by different alleles of the same gene. Mendel placed pollen from true-breeding pea plants with purple flowers (one trait) on the stigmas of true-breeding plants with white flowers (another trait).

• It is the inheritance of a single characteristic controlled by different alleles of the same gene.
• F 1  generation is the first filial generation offspring produced by crossing two parental strains.
• All the progeny of F 1  generation were tall i.e. the traits of only one parent were visible.
• F 2  generation is the second filial generation offspring produced by crossing F 1's.
• The F 2  progeny were not all tall. Instead, one-quarter of them was short indicating both the traits – that of tallness and shortness were inherited in the F 2  plants.
• Genotypic ratio – 1:2:1, Phenotypic ratio – 3:1.

Dihybrid Cross

A cross between two plants having two pairs of contrasting characters is called a dihybrid cross. It is the simultaneous inheritance of two characters.

• For instance, dihybrid inheritance involves a plant producing round and yellow seeds (RR and YY) crossing with a plant producing wrinkled green seeds (rr and yy). 
• F 1  progeny produces round and yellow seeds (R and r, and Y and y) in which round and yellow are dominant traits.
• F 2  progeny were similar to their parents and produced round yellow seeds, while some of them produced wrinkled green seeds. However, some plants of the F 2 progeny even showed new combinations, like round-green seeds and wrinkled-yellow seeds.
• The dihybrid ratio establishes that all traits are inherited independently of each other. It also indicates that each organism will have 2 sets of genes, one from each parent and that the gametes will have only one set.

Sex Determination in Humans

Sex determination is a mechanism which determines whether an individual is to be a male or a female based on the sex chromosomes present in it. Sex is determined by special chromosomes called sex chromosomes and the human body has one pair of sex chromosomes.

• A non-identical pair of sex chromosomes, consisting of one X and one Y, determines the offspring to be a male. Here, the child inherits the X chromosome from his mother and Y from his father.
• An identical pair of sex chromosomes, consisting of two X chromosomes (X and X), determines the offspring to be a female. Here, the child inherits an X chromosome from her father and another X from her mother.
• A child who inherits an X chromosome from the father will be a girl, and one who inherits a Y chromosome from him will be a boy.

 

2. Evolutionary Relationships I

Just like you can build a family tree to show the relationships of your ancestors and their descendants, scientists can build trees to show the evolutionary relationships of species. All of the species of organisms that are alive today have descended from ancestral species. This is due to evolution, or simply, change over time.

The evolutionary relationships of ancestral species and their descendants can be diagrammed using branching evolutionary trees. According to the modern theory of life evolution, all existing species originated from one common ancestor and are therefore related to each other.

The diversity of living organisms on earth is truly overwhelming. Understanding different types of living things, naming, identifying and classifying them began during Aristotle's time. But over the years, scientists have come up with numerous ways to organise and classify biological diversity according to a number of criteria, including overall similarities, colours, ecological functions, etc. However, it is generally agreed that the most useful way for scientists to organize biological diversity is to group organisms according to a shared evolutionary history. This way the grouping not only results in an organized classification, but it also conveys information about the evolutionary history of these groups.

In this chapter, we shall learn how living organisms can be classified into groups on the basis of either evolutionary relatedness or other features and the various factors influencing evolution. This chapter covers the evolutionary relationships between all living organisms.

Evolution and Classification

Evolution is the sequence of gradual changes over millions of years in which new species are produced. It involves finding similar characteristics among organisms to trace back their ancestors.

Classification also involves the same procedure of grouping together organisms with similar features. In fact, classification has helped in the study of evolution.

• Organisms are broadly classified based on body design and structure. More closely related species must have originated from a common ancestor and will have more similarities in genetic make-up. Further the separation in classification, more is the difference in DNA.
• Comparing the DNA of different species is the ideal way to find out evolutionary relationship. More the similarity in DNA, closer the organisms are on the evolutionary tree.
• Organisms are further classified into smaller groups on the basis of many characters (external, internal, development and physiology).

 

Factors Influencing Evolution

• Natural Selection: It is the change in heritable traits of a population over time. This is a key mechanism of evolution. This occurs partly because random mutations arise in the genome of an individual organism, and offspring can inherit such mutations. This is influenced by natural selection of features that give survival advantage to organisms. For example, green beetles can survive better in a forest than red ones because they are better camouflaged by leaves. Red ones are seen better by predators and eaten.
• Genetic Drift: Genetic Drift is the change in the frequency of a gene variant (allele) in a population due to random sampling of organisms. Members of a population with certain genetic features may be isolated by some accident, calamity or natural causes and can establish a new population in a new environment. While there is no specific adaptation, it causes diversity. For example, blue beetles may be carried by a bird and accidentally dropped in a new place where it breeds with local population creating a new strain. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.

Acquired and Inherited Traits

• An acquired trait is a non-heritable change in a function or structure of a living being caused after birth by disuse, misuse, or other environmental influences. Acquired traits are not passed on to offspring through reproduction alone.
  • Example: Learned skills like riding a cycle, swimming and reading.
• An inherited trait is one that is passed on from parents to their offspring, either through asexual reproduction or sexual reproduction. This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Inherited traits are controlled by genes.
  • Example: Received from biological parents like colour of the eyes, hair, height, blood group etc.

Speciation

• Speciation is the evolutionary process by which biological populations evolve to become distinct species.
• It is an event that splits a population into two independent species which cannot reproduce among them.
• Individuals from different species cannot interbreed.
• Formation of new species is dependent on two major factors: geographic isolation resulting in genetic drift and natural selection.
• Process of Speciation
  • Genetic drift: It occurs due to changes in the frequencies of particular genes by chance alone. For example, if a hurricane strikes the mainland, and bananas with beetle eggs on them are washed away to an island where the beetle breeds with local population creating a new strain. This is called a genetic drift.
  • Natural Selection: These are the variations caused in individuals due to natural selection which lead to the formation of a new species. For example, if the ecological conditions are slightly different on the island as compared to the mainland, it leads to a change in the morphology and food preferences in the organisms over the course of generations.
  • Splitting of Population: A population splits into different sub-populations due to geographical isolation that leads to the formation of a new species.