Heredity and Evolution: Genetics

Introduction

In biology, any differences between cells, individual organisms, or groups of organisms of any species either by genetic difference or by the effect of environmental factors on the expression of the genetic potential, are termed as variations. Variations may be shown in physical appearance, metabolism, fertility, mode of reproduction, behaviour, or any other measurable character. Passing on these variations from one generation to another is called heredity.

Accumulation of variation during reproduction

• Variation is defined as the “differences in the trait or characters among the individuals of species”.
  • Variations are inherited and accumulated.
  • In the long term, they lead to the evolution and development of a new species.
• Variations are generated due to changes in environmental factors and due to reproduction. All the variations in a species do not have equal chances of survival in the environment in which they find themselves.
• Hereditary variations may involve a change in the genotype of organisms. A sudden random change in the genetic material of a cell can alter the appearance or behaviour (i.e., in phenotype) from the normal. This change is called mutation and can be heritable.
• Some amount of variation is produced even during asexual reproduction due to small inaccuracies in DNA copying, for example, bacteria. If one bacterium divides and then the resultant 2 bacteria divide again, the 4 individual bacteria generated would be very similar. There would be only very minor differences between them generated due to small inaccuracies in DNA copying.
• The process of sexual reproduction maximises the number of successful variations because of the exchange of DNA segments during crossing over in gamete formation and the union of traits from two different parents during fertilization.
• Inheritance from the previous generation provides both a common basic body design and subtle changes in it for the next generation.
• The selection of variants by environmental factors forms the basis for evolutionary processes.

Heredity

• Heredity is defined as process of transmission/ inheritance of characters from one generation to the next generation.
• Characters that are seen in an individual, (for example height, body colour, eye colour, free and attached ear lobe, etc), are called traits.

Inherited traits

• Traits that are transferred from one generation to another are called inherited traits. These are determined by the genes of an individual.

The rules for the inheritance of Traits – Mendel’s contributions.

Mendel’s experiment.

• Gregor Johann Mendel carried out several experiments on pea plant. Mendel selected garden pea (Pisum sativum) for series of hybridisation experiments because it has some special features. Special features of garden pea plants are as follows:
  • It is easy to grow.
  • It has a short life span and therefore it is possible to study number of generations in less time.
  • Garden pea plant has distinct, easily detectable contrasting variants of features. Mendel in fact noted 7 pairs of such contrasting characters in Garden Pea Plant. The 7 contrasting characters/ traits are: Round/wrinkled seeds; Tall/Short plants; White/Violet flowers; Yellow/Green seed colour; Constricted/Inflated pod shape; Green/Yellow pod colour and Axial/Terminal flower position.
  • The plant has bisexual flowers, i.e., male and female parts are in same flower and also they mature at same time. The flower encloses reproductive parts, which ensures self-pollination. Self-pollination helps to maintain purity of gametes.
  • Artificial cross pollination, i.e., removal of stamens and dusting the pistil of flowers with pollen grains of desired plants, can be easily achieved as reproductive organs are large enough to be seen with naked eyes.
  • Each pea plant produces many seeds in one generation.

• Genotype: It is the genetic composition of an individual (for example, pure tall – TT, Hybrid tall – Tt and pure dwarf – tt.
• Phenotype: It is the visible trait of an individual (for example, Tallness or dwarfness).
• F1 or first filial generation: The generation of hybrid produced from a cross between genetically different individuals (parents).
• F2 or Second filial generation: Individuals arise from a cross amongst the individuals of F1 generation.
• Homozygous Organism: An Organism that contains identical alleles of a character on homologous chromosomes, example pea plants with TT alleles are homozygous for height.
• Heterozygous Organism: An Organism that contains two different alleles for a character on its homologous chromosomes, Example pea plants with alleles Tt are heterozygous for height.

• He carried out a large number of monohybrid crosses (crosses that focus on one pair of contrasting characters) and dihybrid crosses (crosses that focus on two pairs of contrasting characters simultaneously) and put forward several important conclusions.

Monohybrid cross.

• Mendel performed a cross between the two pea plants by collecting pollen grains of a flower from a tall plant and dusted over the pistils of the emasculated flower of a dwarf plant or vice versa.
• After pollination, flowers were covered with bags. The plants grown from the seeds of the parental plants were hybrid plants belonging to the F1 generation or first failure generation. All plants of the F1 generation were tall.
• The plants of F1 generation were self-pollinated and the seeds were collected. The plants raised from these seeds belonged to the F2 generation or second filial generation. The plants of F2 generation were 75% tall and 25% dwarf, i.e., in the ratio of 3:1.

• Conclusion: Out of these two characters of parents, only one expresses itself in hybrids. This character was termed as a dominant trait and the other a recessive trait. It also showed that the dominant trait (tallness) is expressed in both homozygous and heterozygous conditions. While recessive traits appear only in homozygous conditions.

Dihybrid cross.

• Mendel selected pure breeding plants for yellow and green colour of seeds and round and wrinkled shape of the seeds. He cross pollinated pure pea plants, having round yellow seeds (RRYY) and wrinkled green seeds (rryy)
• The plants of this generation were referred to as P generation or parent generation. The seeds produced belonged to F1 generation (First filial generation) and all were round and yellow. Plants of F1 generation were self pollinated. On self pollination, these produced different seeds in next generation (F2 Generation or Second field Generation) ss shown in the given dihybrid cross.

• The ratio resulted in the following conclusions:

Findings of Mendel’s dihybrid cross.

• F1 offspring always exhibited only one of the parental forms of a trait and not the other.
• The trait which was hidden in the F1 generation appeared in the F2 generation. In the F2 generation, both the parental traits appeared. Mendel termed the form of the trait expressed in the F1 generation as the dominant trait.
• Mendel observed a difference in the behaviour of plants raised from F2 offsprings with dominant trait. One-third were true breeding, rest two-third of plants were not true breeding and resembled the F1 hybrid plants in their behaviour.
• Mendel observed that one of the parental forms of the trait was always absent in F1 hybrid but reappeared unchanged in F2 generation. It proved that alternate forms of a trait can retain their identity in the hybrid and can reemerge unchanged in subsequent generations.
• In dihybrid crosses, Mendel observed 4 types of plants in F2 generation. He concluded that the factors of each of the 2 characters are sort independent to each other.
• 4 types of phenotypes. Round yellow.: Round green.: Wrinkled yellow: wrinkled green. 9331.

Mendel’s Principles of Inheritance.

With his experiments on pea plants, Mendel arrived at the following laws of inheritance.

• Principle of dominance Or Law of dominance: In heterozygous individuals or hybrids, a character is represented by 2 contrasting factors called alleles or allelomorphs. Out of the 2 alleles, only one is able to express its effect in the individual. It is called dominant factor or dominant allele. For example, trait like ‘T’ for height of plant. The other allele, which does not show its effects in the heterozygous individual is called recessive factor or recessive allele. For example, trait like ‘t’ for height.
• Principle of segregation or Law of segregation: The two factors (gene) of a character which remain together in an individual do not get mixed up but keep their identity distinct, separate at the time of gametogenesis or sporogenesis, get randomly distributed to different gametes and then get paired again in different offsprings as per the principle of probability.
• Principle of independent assortment or Law of independent assortment: According to this principle or law, the two factors of each character assort or separate independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring.

DNA

• Cellular DNA (Deoxyribose nucleic acid) is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein.
• Genes are the hereditary units which are transmitted from one generation to another.
• Genes control characteristics or traits, for example, plants have hormones that can trigger growth (tallness). Plant height can thus depend upon the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it. If an enzyme important for this process works efficiently, a lot of hormone will be made and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less and the plant will be short.
• Genes or factors can have 2 forms (alleles):  Dominant or recessive.
• Genes controlling a particular trait separate from each other during gamete formation. Hence, gamete is always pure as far as contrasting characters are considered. Each gamete will possess only one gene set. During fertilization, paired condition is restored when male and female gametes fuse.
• For a particular trait, the offspring receives one allele from the father and one allele from the mother.

Sex determination

• Gender in different organisms may be determined by environmental factors, as in some snails (individuals can change sex), turtles and lizards. In some animals, the temperature at which fertilised eggs are kept determines whether the animals developing from the eggs will be male or female. In human beings, fruit flies, etc., it is determined through chromosomes.
• In human beings, sex is determined by the sex chromosomes. Women have a perfect pair of sex chromosomes, both called X, but men have a mismatched pair in which one is normal-sized X while the other is a short one called Y. So, women are XX while men are XY. If a sperm carrying ‘X’ fuses with the ovum, female offspring are produced. If a sperm with ‘Y’ fuses with the ovum, male offspring is produced. Hence, the sex chromosomes of the male parent are involved in determining the sex of the offspring.