Heredity

"Heredity is the passing of characters from parents to offsprings or one generation to next".

1.0Introduction

If we see the offspring of any organism, it looks like its parent only, like a pup produced from bitch, kitten produced from a cat and bunny from a rabbit, similarly all plants also produce their own kind. Asexually reproducing organisms look very much similar to their parents like all plants in a sugarcane field look very much similar to each other.

In asexually reproducing organisms, variations are very little (brought only because of small changes created in DNA at the time of their replication). Sexually reproducing organisms show great variation as biology which deals with the study of heredity and variations. compared to asexually reproducing organisms, because of involvement of two parents. So, their characters are varying between two parents.

Accumulation of variations during reproduction:

Small variations are produced in each generation. These small variations accumulate generation after generation. These variations may enable or disable the organisms to cope with changes in the environment. Advantageous variations are selected by environmental factor. e.g., Bacteria that tolerate low temperature can survive in a cold wave. Such heritable variations lead to the evolution and formation of new species. The offsprings are slightly different from each other. Some differences are unique and other are inherited from their respective parents.

Sexually reproducing organisms show greater variations which may lead to increased survival of individual. So greater diversity will be generated by sexual reproduction.

The above figure represents the situation if a single individual reproduces, as happens in asexual reproduction. If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying. However, if sexual reproduction is involved, even greater diversity will be generated,

Why do bacteria look alike?

• Explanation: Because they reproduce by asexual mode of reproduction which has less chances of variations.

2.0Terminology used

Alternative forms of a gene which are located on same position (loci) on the homologous chromosome are called alleles.

Chromosome is a thread like structure in the nucleus of a cell. It is formed of DNA which carries the genes. Unit of heredity which transfers characters from parents to their offsprings during reproduction is called genes.

The genetic constitution of an organism or the description of genes present in an organism e.g., TT, Tt , tt is called genotype.

External and morphological appearance of an organism for a particular character is called phenotype. e.g., Tall, Dwarf.

In homozygous condition, two factors of a character are same (TT). In hybrid/heterozygous condition, two factors of a character are different ( Tt ).

Dominant factor is one which able to express its effect in the presence of its contrasting factor in heterozygous state.

Recessive factor or allele is one which is unable to express its effect in the presence of its contrasting factor in heterozygous state. Its effect can be seen in pure or homozygous state only.

3.0Gregor Johann Mendel (1822-1884):

Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna. Mendel was an Austrian monk and scientist who first worked on heredity experiments and gave theory in 1866 but people of that time couldn't understand him and rejected his theory. Many others had studied the inheritance of traits in peas and other organisms earlier, but Mendel blended his knowledge of science and mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arrive at the laws of inheritance. Later in 1900 three different scientists from 3 different places also found the same result. They were Hugo de Vries, Tschermak and Correns. They rediscovered the theory of heredity proposed by Mendel. So, Mendel is called "Father of genetics".

4.0Mendelian experiments

In 1856-57, he started his historical experiments of heredity on pea (Pisum sativum) plant.

Mendel said that each character is controlled by a pair of factor which is now known as gene.

5.0Advantages of Pisum sativum

Pisum sativum is a unique plant selected by Mendel for his experiments which had following importance:

Before starting his experiments, he obtained the pure breed. A pure breed is the one in which a plant is crossed with same allelic type and results in all of its offspring of same kind. Tall plant only results in tall plant for any number of generations and so does other characters too.

Pairs of allelic characters found in garden pea plant:

6.0Monohybrid Cross

It is a cross in which only one character is studied at a time. In his first monohybrid cross, Mendel crossed a pure breed tall plant (TT) and a pure breed dwarf plant ( tt ). And he found that in ${\mathrm{F}}_1$ generation only tall plant were born out of these seeds, no intermediate character or mixing or blending was observed by him. Mendel termed tallness as dominant and dwarfness as a recessive character. He repeated the same experiment taking other characters too and always found same results.

7.0Mendel's postulates

To explain the experiment, he said that each character is controlled by a pair of factors. During gamete formation, the paired factors got segregated and moved to different gametes. When these gametes fused together, they used to restore paired state again.

In heterozygous conditions out of the two alleles only one allele is able to express its effect and it is called dominant allele and other which does not show its effect in heterozygous condition is called recessive allele.

When ${\mathrm{F}}_1$ plants are grown and allowed to self-pollinate he got many seeds and when they were grown, he found that the ratio of tall plants and dwarf plants was 3:1 respectively. The dwarfness which was not visible in $F_1$ generation reappeared in $F_2$ generation.

8.0Dihybrid cross

It is a cross in which 2 characters are simultaneously studied. In Mendel's first dihybrid cross, he crossed a pure breed plant with yellow and round (YYRR) seeds with another pure breed plant having green and wrinkled (yyrr) seeds. ${\mathrm{F}}_1$ generation had plants with yellow and round seeds. It means yellow and round seed characters are dominant over green and wrinkled seeds. He allowed these plants produced in ${\mathrm{F}}_1$ generation to self-pollinate and he expected same 3 : 1 in $F_2$ generation. But to his surprise the ratio was all new $9:3:3:1$ in which beside 2 parental varieties, 2 new varieties were also produced. So, the ratio was 9 yellow and round; 3 yellow and wrinkled; 3 green and round; 1 green and wrinkled.

This could be explained only when beside the parental combination of factors (YR) and (yr) two new combinations of factors are also produced as follows (Yr) and (yR).

Phenotypic ratio: 9 round and yellow: 3 round and green: 3 wrinkled and yellow: 1 wrinkled and green.

The occurrence of four types of plants in the ${\mathrm{F}}_2$ generation of dihybrid cross shows that during gamete formation in ${\mathrm{F}}_2$ generation, the two factors of a character are independently assorted and get randomly rearranged in the offspring. So, four types of gametes are formed as RY, rY, Ry, ry from each parent. When they are randomly fused, they resulted following combinations. So, this is how the $9:3:3:1$ ratio could be understood.

9.0Mendel's laws of inheritance

Human beings were always interested to know more about the law of inheritance. On the basis of Mendel's work, 3 basic laws of inheritance were proposed. (i) Law of Dominance (ii) Law of Segregation (iii) Law of Independent Assortment

Law of dominance: In crossing between organisms, pure for contrasting characters of a pair, only one character of the pair appears in the ${\mathrm{F}}_1$ generation. This character is termed dominant while the one which does not express itself in $F_1$ generation is termed as recessive.

How a character is expressed?

• Explanation: A function is controlled by formation of a specific enzyme, this enzyme then leads to a specific chemical reaction to express its character. If we consider the hybrid condition obtained by Mendel in ${\mathrm{F}}_1$ generation, it has both " T " and " t ", yet it shows tallness not dwarfness. There are two possible reasons for it as follows - (a) Allele t may not be able to make its enzyme, so its character is not expressed. (b) Its enzymes are not in enough quantity to show its expression in presence of enzyme of " T ".

Law of segregation: Different alleles or genes of a character remain together in an individual and segregate randomly at the time of gamete formation. This is also known as the Law of purity of gametes.

Law of independent assortment: This law states that when individuals differing in two or more than two pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other. e.g., The inheritance of seed shape character is not related to the seed colour character. Rather, the two characters inherit independently of each other.

• The rules for inheritance of traits in human beings are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA. Thus, for each trait there will be two versions in each child.

10.0Sex Determination:

It is the phenomena of determination of sex of an offspring. Factors responsible for sex determination (i) Environmental: In some animals the temperature at which the fertilised eggs are kept decides the gender whether the animals developing in the eggs will be male or female. e.g. in turtle. In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined. (ii) Genetic: In some animals, like humans, gender of individual is determined by a pair of chromosomes called sex chromosome $\mathrm{XX}\to$ Human Female and XY $\to$ Human Male.

Sex determination in humans: Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.

During gamete formation the male produce two types of gametes i.e. one having X chromosome and other having Y chromosome while both gametes produced by females are alike i.e. each having $X$ chromosome.

When X chromosome of male fuses to X chromosome of female, girl child is born. When Y chromosome of male fuses to X chromosome of female, boy child is born. Two important things to be noticed here are - (a) Birth of male and female child has equal %, because X and Y chromosomes are produced in equal number and it all depends on either sperm with X or Y chromosome fuses with egg containing $X$ chromosome. (b) Birth of male and female child depends on Y or X chromosomes of male.

• Female always produce X chromosomes, so have no significant role in sex determination of offspring.
  
  Sex determination in human beings

Note: The lowest part of the ear, called the earlobe, is closely attached to the side of the head in some of us, and not in others. Free and attached earlobes are two variants found in human populations.

11.0Chapter At a Glance

12.0SOME BASIC TERMS

• Heredity: Passing of characters from parents to offsprings.
• Genetics: Branch of biology which deals with the study of heredity and variations.
• Allele: Alternative forms of a gene which have located on same position on the homologous chromosomes.
• Genotype: Genetic constitution of an organism.
• Phenotype: Morphological appearance of an organism.
• Gene: Unit of heredity which transfers characters from parents to offsprings.
• Homozygous: Two factors of the character are same.
• Heterozygous: Two factors of the character are different.
• Monohybrid cross: Cross in which only one character is studied at a time.
• Dihybrid cross: Cross in which two characters are simultaneously studied.