Heredity and Evolution

Variation & Trait

The branch of science which deals with the transmission of characters from one generation to another is known as heredity and it can be defined as the science which deals with the inheritance of characters from one generation to next generation or from parents to offspring.

Accumulation of Variation During Reproduction

Variation

Variation is the differences in the trait among the organisms of a species.

An offspring does not exactly resemble to either of its parents. It receives 50% of its characters from father and 50% from mother. Parents may differ in many characters like height, skin, color, eye color, types of hair, shape of nose, chin and so many other characters. All these differences are called variations.

These variations are also observed in animals and plants. The greatest advantage of variation in a species is that they increase the chances of survival in a changing environment.

The second generation organisms will differ from first generation organisms; for example: in an environment where temperature rise up, has a colony of bacteria and there are variations among the heat resisting character of these bacteria. Due to rise in environmental temperature, most of the bacteria will die and only a few variants will be left which will be able to withstand high environmental temperature. All these variations in a species do not have equal chances of survival in the environment in which they are generated. This selection of variants by environmental factors is from the basis for evolution.

Heredity

Heredity is the transmission of characters from parents to their offsprings. Heredity is not similar in Asexual and Sexual Reproduction.

Heredity in Asexual Reproduction

During asexual reproduction, an organism inherits all the characters of its parent because it involves a single parent so the genetic constitution of the offspring is exactly the same as that of the parents.

What is Asexual Reproduction?

Reproduction occurs without the fusion of gametes is called Asexual Reproduction. In asexual reproduction only one parent is involved, such as reproduction in amoeba, bacteria, fungi, etc.

What is Sexual reproduction?

Reproduction involving fusion of two gametes, which come from male and female is called Sexual Reproduction.

In sexual reproduction both male and female parents are involved and each of the parents produces gametes.

Male produces sperm and female produces eggs. During gametogenesis in testis and ovary of parents, meiotic cell division occurs where the numbers of chromosomes in gametes become half as compared to their somatic cells.

There are two types of cells

(a) Somatic cell

Somatic means body, it means body forming cells are called somatic cells. In somatic cells only Mitosis occurs.

(b) Gametic cells

[Means gamete producing cells in these cells only Meiosis is occur.]

During meiosis, crossing over occurs between the homologous chromosomes. Crossing over is the exchange of genetic fragments between the homologous chromosomes. It leads to genetic recombination which ultimately causes variation. When sperm and ova have different genetic characters of father and mother fuse to form Zygote, the zygote receives different genetic make-up which is quite different from either of the parents. Even sons and daughter of the same parents are genetically different because they are born as a fussion of different sets of sperm and ova.

Inherited Traits

The genetically controlled characters pass from parents to their offspring. A human child bears all the basic features of human beings, but at the same time they show remarkable difference from their parents. This is because every individual has its own genetic make-up, different from that of others.

There are a large number of traits which pass from generation to generation and can be observed in our day- to- day life. One of them is free and attached ear lobes in human beings.

Mendel's law of Heredity

Greorge Johann Mendal (1822-1844) an Austrain monk, was the first to formulate the laws of heredity. He is known as the father of the genetics because he explained the mechanism of transmission of the characters from parents to their offsprings. Mendel was the first to introduce the concept of genes as the basic unit of heredity.

Mendel's experiment

Mendel selected garden pea (edible or sweet pea- Pisum sativum) for his experiment. He conducted his experiments for eight years in his monastery garden.

Reasons for selecting garden pea plant for experiment.

Peas are available in many pure breeding varieties.

Pea plants show several well-defined, easily detectable contrasting traits.

Pea plants have a short life cycle, making it possible to study the transmission of characters in shorter period.

Flowers of pea are bisexual and are well protected from the influence of foreign pollen. They are predominantly self-pollinating forms.

A large number of seeds are produced per plant which helps in drawing authentic conclusion.

The plant can be grown easily and does not require after care.

Mendel selected 7 pairs of contrasting characters for his cross experiments but all the characters were not simultaneously considered. Initially he performed separate crosses involving traits of one contrasting character, two contrasting characters, three contrasting characters and so on. The crosses were respectively called a monohybrid, dihybrid, trihybrid and polyhybrid crosses.

Monohybrid Cross

Crosses which were made to study the inheritance of one pair of contrasting characters by Mendel are known as monohybrid crosses.

In one such cross, Mendel selected two sets of pea plants with contrasting characters for height. One set of pea plants was above 6 (six feet) in height and the other set was of short plants with an average height of 1 foot (one foot). Mendel called these plants homozygous tall and homozygous dwarf. These were called as pure strain.

Homozygous tall plants cross-pollinated with homozygous dwarf plants. These plants represented the parent generation.

The plant grow from the seeds of parental plants were hybrid plants, these belongs to the F₁ generation or first filial generation. All plants of F₁ generation were tall.

The plants of F₁generation were tall.

The plants of F₁generation were self- pollinated and seeds were collected. The plants raised from these seeds belonged to second filial generation or F₂generation. Mendel observed that among the plants of F₂ generation 75% were tall and 25% were dwarf the ratio of tall and dwarf is 3:1

F₂ plants were self- pollinated to produce F₃ generation. Mendel observed that dwarf plants 25% of F₂generation produced only dwarf plants. Of the F₁generation tall plants are 75%, only 25% produced tall plants on self ? pollination. These were called pure tall or homozygous tall plants.

Dihybrid Corss

These are Mendelian cross in which two pairs of contrasting characters are taken into consideration at a time.

Mendel's laws of inheritance

(1) Law of Dominance

In heterozygous condition out of two contrasting alleles one expresses itself morphologically and the other remains unexpressed. The allele which expresses itself phenotypically in the presence of its contrasting allele is called dominant and the other which remains unexpressed is called recessive.(Alleles: Genes or factors controlling contrasting traits, occupying same locus on the homologous chromosomes.)

(2) Law of Segregation

The two factors controlling one character segregate without influencing each other during the formation of gametes receives one factor for each character.

(3) Law of independent assortment

The factors controlling different characters assort independently without influencing each other during the formation of gametes.

Sex Determination

How is the sex of a new born individual determined? Different species use very different strategies for this. In human the genes inherited from our parents decide whether we will be boys or girls. All human chromosomes are not paired. In this way, female carries 44A + XX chromosomes and male carries 44A + XY chromosomes. Females are homogametic, i.e., they produce only one type of gamete(ova), each having 22A+Xchromosomes while males are heterogametic i.e. they produce two types of gametes (sperms), of which 50% contain 22A + X chromosomes and another 50% contain 22A + Y chromosomes.

During fertilization, if a spermatozoa (sperm) bearing X-chromosomes fertilizes the ovum, the zygote becomes female (44A+XX) because of having two X- chromosomes and if a sperm containing Y-chromosomes fertilizes the ovum, the zygote becomes male (44A + XY) because such a zygote receives Y chromosomes from his father. Thus, sex of a zygote depends on whether the ovum is fertilized by an X-carrying sperm or a Y- carrying sperm. There is equal chance of an ovum getting fertilized by the sperm carrying X or Y ?chromosome, under random mating system, there is always a possibility of 50% male and 50% female offspring. The sex in human beings is determined at the time of fertilization; this condition is referred to as syngamy.

Sex determination by environment

In some animals (not in humans) sex is determined by environmental factors and not by sex chromosomes.

Examples:

In Bonelia viridis, a marine invertebrate, if a free swimming larva comes accidently in contact with the proboscis of any adult female, it becomes male.

In Chrysoma picta, a variety of turtle, high incubation temperature results in the development of female sex.

In Agama agama, a lizard, high incubation temperature results in the development of male sex.

Sex-linked diseases in human beings

The genes for certain disease in human beings are situated on sex chromosomes, i.e. on X and Y chromosomes. The mode of transmission of these diseases to next generation follows the inheritance of sex chromosomes. Genes for such diseases to may remain present only on X- chromosome or y-chromosome or on both, and accordingly the diseases manifest themselves in the two sexes. For example,

Haemophilia and Red- Green colourblindness are X-linked diseases.

Icethysis and Hystrix gravis are Y- linked diseases.

Nephritis and spastic paraplegia are both x and y-linked diseases.

Evolution

The world Evolution means to unroll but this term is commonly used to denote changes towards development. But the evolution to which we are concerned here is biological evolution, which means evolution of living beings. Biological evolution deals with changes undergone by living things, i.e. plants and animals.

Biological evolution, nowadays, is also referred to as organic evolution, because living beings constitute mainly the organic substances. The rich diversity of living things that now inhabit our planet itself have generated the through that extensive changes in living organisms have taken place in past, are now taking place and continue to exist. It is the greatest general principal of biology.

Definition of evolution

Sequence of gradual changes which occur over millions of years and leads to the emergence of complex form of life from simpler form.

An Illustration

Variation is the key factor of the evolution without which long evolutionary processes are impossible. Living beings have an inbuilt tendency to produce variations during gametes formation in the form of crossing over and because of errors in DNA copying at the time of chromosomal replication. The other important factors causing variation are mutations of different types. Thus, genetic recombination during reproduction and mutations are the source of hereditary variations which act as raw material for evolution.

We can understand the consequences of variations and resulting phenomenon of evolution by the following illustration.

Let us consider a group of 12 red beetles living on green leaves of bushy plants. Since, these beetles reproduce by sexual reproduction they generate variations. Crows are natural predators and eats these beetles. Thus, crows regulate the population of beetles by eating them and, therefore, fewer beetles are available for sexual reproduction.

Now, imagine the different situation that can develop in beetle population.

Situation 1:

In one situation, let us assume that a colour variation occurs during reproduction. One beetle develops green body colour instead of normal red colour. This green beetle produces offspring with green body colour. It is difficult for crows to locate green coloured beetles on green leaves while red coloured beetles on green leaves are easily visible. As a result, red coloured beetles are often eaten away by crows and green coloured beetles escape the predator?s attack. Gradually. The number of green coloured beetles increases and the number of red coloured beetles decreases in the population.

Situation 2:

In the another situation. Another colour variation arises in beetles during sexual reproduction and one beetle develops blue body colour instead of natural red colour. This blue beetle produces offspring with blue body colour but still most of the beetles are red coloured. At this point, an elephant comes and stamps the bushes where the beetles live. As a result , most of the beetles get killed. By chance, the few beetles that have survived are blue. This population slowly expands and now most of the beetles population is of blue coloured beetles.

Situation 3:

In the third situation as the beetle population begins to expand, the bushes start suffering from a plant disease. The amount of leaf material for the beetles is reduced. The beetles are poorly nourished as a result. The average weight of adult beetles decreases from what it used to be when leaves were plentiful, but there is no genetic change the plant disease is eliminated. After few years the plant disease is eliminated, bushes flourish again and beetles regain their normal weight.

Acquired traits

The characters or traits that are acquired by living organisms during their lifetime are called acquired traits. We have seen in the above example that when food is not sufficiently available to beetles, their body weight decreases due to starvation and when the food is available in plenty, body weight increases again. Loss or gain in the body weight of beetles an acquired characters. These changes occur in somatic cells of beetles and not in the germ cells, hence it is not inherited to the next generation.

Inherited traits

The characters or traits which are controlled by specific genes and are passed on from one generation to the next are called inherited traits. Any alteration in the present in germ cells is likely to be passed to the next generation and is responsible to cause variations.

In this way, we can conclude that since evolution is based on the occurrence of heritable variations, understanding evolution is not possible without studying the principles of heredity and genetics. Mendel's law of heredity and the subsequent development of genetics that scientists could differentiate between heritable and non-heritable variations and their mode of transmission to the next generation.