From genetics to ecology, Biology Topics cover a vast array of life sciences.
Evolution is a Change in the Inherited Traits of a Population through Successive Generations
Recognizable features of a human being (or any other organism) like height, complexion, the shape of the hair, the color of eyes, and the shape of nose and chin, etc, are called ‘characters’ or ‘traits’. The transmission of characters (or traits) from the parents to their offspring is called heredity. In most simple terms, heredity means the continuity of features from one generation to the next.
Two parents, a male, and a female, are involved in sexual reproduction. The sexually reproducing organisms produce sex cells or gametes. The male gamete called sperm fuses with a female gamete called ovum (or egg) to form a zygote which gradually develops into a young one (or offspring), showing some similarities with the parents. Actually, hereditary information is present in the sex cells (or gametes) of the parents. Thus, gametes constitute the link between one generation and the next and pass on the paternal (father’s) and maternal (mother’s) characteristics or traits to the offspring. This relation that continues to exist between successive generations is referred to as heredity.
The heredity information is present in the gametes (or sex cells) of the parents. So, gametes constitute the link between one generation and the next, passing on parents’ traits to their children.
Variations in Human Beings
Although the offsprings inherit the parents’ characters (or traits) and resemble them very closely, the resemblance is not complete in all respects. The offsprings are never a true copy of the parents. In fact, no two individuals are exactly alike and the members of any one species differ from one another in some characters (or traits) or the other. These differences are known as variations. So, from the biological point of view, variation is the occurrence of differences among the individuals of a species.
For example, people have different heights. Their complexion, type of hair, colour of eyes, the shape of the nose, and the shape of the chin also show differences. The differences in the characters (or traits) among the individuals of a species are called variation. For example, human height is a trait that shows variation. This is because some people are very tall, some are less tall, some have a medium height, some have a short height and others are concise.
Here is another example of variations in human beings that involves our ears. The lowest part of our ear is called the earlobe. In most people, the earlobe is ‘hanging’ and it is called the free earlobe [see Figure (a)]. In some people, however, the earlobe is closely attached to the side of the head and it is called the attached earlobe [see Figure (b)], Thus, most people have free earlobes whereas some people have attached earlobes. So, the free earlobes and attached earlobes are the two variations found in the human population.
A free earlobe or attached earlobe is a variation found in the human population.
Some amount of variations is produced even during asexual reproduction but it is tiny. The number of variations produced during sexual reproduction is, however, very large. For example, sugar cane plants reproduce by the process of asexual reproduction, so if we observe a field of sugar cane, we will find very little variation in various sugar cane plants. All the sugar cane plants almost look alike. But in animals (including human beings) which reproduce by the process of sexual reproduction, a large number of variations are produced. It is due to these variations that no two human beings look alike (except identical twins). From this discussion, we conclude that the number of successful variations is maximized by the process of sexual reproduction. The variation is a necessity for organic evolution.
Accumulation of Variations
The reproduction of organisms produces variations. The variations produced in organisms during successive generations get accumulated in the organisms. The significance of a variation shows up only if it continues to be inherited by the offspring for several generations. This will become clear from the following example. Suppose a bacterium produces two bacteria by asexual reproduction. Again suppose that one of the offspring bacteria has a variation due to which it can tolerate a little higher temperature (or a little more heat) than the other one. Now, this variation of little more heat resistance will go on accumulating in the offspring of successive generations of this bacterium. And this will ultimately give rise to a variant of bacteria that will be highly heat resistant and able to survive even at very high temperatures.
The great advantage of variation to a species is that it increases the chances of its survival in a changing environment. For example, the accumulation of ‘heat resistant’ variation (or trait) in some bacteria will ensure their survival even when the temperature in their environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die. Before we describe Mendel’s experiments for explaining the transmission of characteristics (or traits) from parents to their offspring or progeny, we should know the meaning of some terms such as chromosome, gene, dominant gene, recessive gene, genotype, phenotype, F1 generation, and F2 generation. These are described on the next page.
A chromosome is a thread-like structure in the nucleus of a cell formed of DNA which carries the genes (see Figure). Different organisms have different numbers of chromosomes in their nuclei. A gene is a unit of DNA on a chromosome that governs the synthesis of one protein that controls a specific characteristic (or trait) of an organism (see Figure). There are thousands of genes on a chromosome that control various characteristics of an organism. Genes are actually units of heredity that transfer characteristics (or traits) from parents to their offspring during reproduction. Genes work in pairs. In diagrams and in explanations of heredity, genes are represented by letters. Genes controlling the same characteristics are given the same letters.
For example, the gene for tallness is represented by the letter T whereas the gene for dwarfness is represented by the letter t. The letters T and t actually represent two forms of the same gene (which controls the length of an organism, say the length of the stem of a plant). Please note that genes had not been discovered at the time when Mendel conducted his experiments on pea plants to study the inheritance of characteristics. The term ‘factors’ which was used by Mendel as carriers of heredity information are now known as ‘genes’.
This is a full set of chromosomes present in a human cell. A normal cell of the human body contains 23 pairs of chromosomes. 22 pairs of chromosomes match in males and females but 23rd pair is different. X and Y are the sex chromosomes in a male (as shown above) but a female has two X chromosomes.
Every chromosome has small parts called genes. Genes control the development of inherited characteristics such as hair colour, eye colour and skin colour, etc, in humans.
Genes for controlling the same characteristic of an organism can be of two types: dominant or recessive. The gene which decides the appearance of an organism even in the presence of an alternative gene is known as a dominant gene. It dominates the recessive gene for the same characteristic on the other chromosome of the pair. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. A single recessive gene cannot decide the appearance of an organism. The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfness is t. Thus, when we write the genetic cross for pea plants, then the capital ‘T’ represents ‘tall’ and the small ‘t’ represents ‘dwarf’.
Genotype shows the genetic constitution of an organism. In simple words, genotype is the description of genes present in an organism. Genotype is always a pair of letters such as TT, Tt, or tt (where T and t are the different forms of the same gene). Thus, the genotype of a tall plant could be TT or Tt whereas that of a dwarf plant is tt. The characteristic (or trait) which is visible in an organism is called its phenotype. For example, being ‘tali’ or ‘dwarf’ (short) are phenotypes of a plant because these traits can be seen by us or are visible to us. The phenotype of an organism is actually its physical characteristic which is determined by its genotype. For example, genotype TT or Tt results in a tall phenotype, and genotype tt results in a dwarf phenotype.
This picture shows a tall pea plant on the left side and a short pea plant on the right side. The genes present in the tall plant are TT or Tt whereas the genes present in the short plant (or dwarf plant) are tt.
This picture shows red and white flowered Busy Lizzie plants. Just like other characteristics of plants (like tallness or dwarfness, etc.) the colour of flowers is also controlled by genes.
When two parents cross (or breed) to produce progeny (or offsprings), then their progeny is called the first filial generation or Fx generation (where F stands for Filial which denotes progeny of a cross). When the first-generation progeny cross (or breed) among themselves to produce second-generation progeny, then this progeny is called second filial generation or F2 generation. In other words, the generation produced by crossing two F1 progeny is called the F2 generation. An example will make it more clear. Mother and father are parental generations. Their children are F1 generation, and the grandchildren are F2 generation.
Gregor Mendel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny (see Figure). He did this by using different varieties of pea plants (Pisum sativum) which he grew in his garden. Some of the characteristics (or traits) of the pea plants whose transmission to progeny was investigated by Mendel were the height of the pea plant or length of the stem of the pea plant (tall or dwarf), the shape of seeds (round or wrinkled) and colour of seeds (yellow or green) (see Figure). A yet another contrasting characteristics (or traits) investigated were colours of flowers (white or violet).
Gregor Mendel: The first scientist to make a systematic study of heredity. He is known as the Father of Genetics.
Mendel chose pea plants for studying inheritance because pea plants had a number of clear-cut differences which were easy to tell apart. For example, some pea plants were ‘tali’ (having long stems) whereas others were ‘dwarf (having short stems). Some pea plants produced round-yellow seeds whereas others produced wrinkled-green seeds, etc. Another reason for choosing pea plants was that they were self-pollinating (which enabled them to produce the next generation of plants easily). And finally, Mendel chose pea plants to study inheritance (and not animals including human beings) because many generations of pea plants can be produced in a comparatively short time span and their study is much simpler than that of animals.
Some of the characteristics (or traits) of pea plants were studied by Mendel.
A new form of plant resulting from a cross (or breeding) of different varieties of a plant is known as a hybrid. When we breed two pea plants having one contrasting characteristic each (or one trait each) to obtain new plants, then it is called a monohybrid cross. In monohybrid cross we will study the inheritance of one pair of contrasting characteristics ‘tallness’ and ‘dwarfness’ of the pea plants by their first-generation and second-generation progeny. On the other hand, if we breed two pea plants having two contrasting characteristics each (or two traits each) to obtain new plants, then it is called a dihybrid cross. In the dihybrid cross, we will study the inheritance of two pairs of contrasting characteristics of pea plants such as round-yellow seeds and wrinkled-green seeds.