ACCUMULATION OF VARIATION DURING REPRODUCTION
By virtue of being the progeny of the parent, the progeny individual, need not just be the replica of what its parents are. (Inheritance of characters from the parents to the progeny (i.e. , Heredity) ensures the passing of the parental characters to the progeny). The difference or change in the characteristics between the individuals is called Variation. Human population shows a great deal of variation.
Inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation. The second generation will have differences that they inherit from the first generation, as well as newly created differences.
The above figures shows Creation of diversity over succeeding generations. The original organism at the top will give rise to, say, two individuals, similar in body design, but with subtle differences. Each of them, in turn, will give rise to two individuals in the next generation. Each of the four individuals in the bottom row will be different from each other.
While some of these differences will be unique, others will be inherited from their respective parents, who were different from each other.
INTEXT QUESTIONS
Q1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Ans: Trait ‘B’.
Percentage of any gene in a population increases from generation to generation.
Q2. How does the creation of variations in a species promote survival?
Ans: During reproduction (also inaccuracies in DNA replications), many variations occur in the offspring. Some individuals have more favourable variations than the other. Such individuals survive and pass these variations on their progeny. For example, let us consider the population of beetles. Due to certain conditions, a colour arise during reproduction so that one beetle is green in colour (instead of red). This beetle can pass this colour to its progeny. Crows now cannot see these green-coloured beetles on green leaves and hence, their population become more than that of red-coloured beetles.
HEREDITY
The progeny produced through the reproductive process is similar to its parents, in body design, function etc., The rules of heredity determine the process by which the traits and the characteristics are relatively inherited.
“The inheritance of characteristics through generation is called heredity”
The inheritable characteristics may be morphological/anatomical/physiological/ reproductive and are also known as traits.
If we take a very close look at the rules of inheritance, both father and mother contribute equal amount of genetic material to the child. This means that each trait can be influenced by both paternal and maternal genetic material – i.e, DNA.
RULES FOR THE INHERITANCE OF TRAITS
MENDEL’S CONTRIBUTIONS
Gregor Johann Mendel (1822-1884) worked out the first ever scientific experimental study on heredity.
Mendel, an Austrian Augustinian monk observed variations in the characteristics of garden pea plant (Pisum sativum) which he had cultivated in his monastery garden.
Mendel was curious to find out the results of crossing of pea plants with the variation in traits.
The visible contrasting characters that Mendel observed in the garden pea plants were given below:
2. Seed colour - Yellow / Green
3. Flower colour - Violet / White
4. Pod shape - Full / Constricted
5. Pod colour - Green / Yellow
6. Flower position - Axillary / Terminal
7. Stem height - Tall / Dwarf
MENDEL’S MONOHYBRID CROSS
Mendel selected the garden pea plant, Pisum sativum for his experiments. He selected tall and dwarf plants and allowed them to grow naturally. As pea plants produce seeds only by self pollination, he observed that tall plants produced always tall plants generation after generation under natural condition.Similarly, dwarf plants produced always dwarf plants generation after generation. Hence, he termed the tall and dwarf plants as wild types or pure breeding varieties.
Then he crossed a tall plant with a dwarf plant, produced progeny and calculated the percentage of tallness and dwarfness in subsequent generations.
When a pure breeding tall plant was crossed with a pure breeding dwarf plant, all plants were tall in the first filial generation (F1) i.e., there was not any medium height plants or dwarf plants.
This means that only one of the parental traits were seen and not the mixture of the two. When such a F1 tall plant was allowed to have self pollination, both the tall and dwarf plants appeared in second filial generation (F2) in the ratio of 3:1.
This indicates that both tallness and dwarfness were inherited in the F1 plants but only tallness trait was expressed.
The first experiment of Mendel considering the inheritance of a single trait (Height of the plant Tall/Dwarf) is called Monohybrid Cross.
Expression of morphological characters as tall or dwarf plant, violet or white flower is called Phenotype.
The expression of gene (or Chromosomal make up) of an individual for a particular trait is called Genotype.
POINTS TO REMEMBER
1.Heredity : The passing of traits from the parents to offspring is called heredity.
2.Genotype : The complete set of genes in an organism’s genome is called genotype.
3.Phenotype: The observable characters in an organism make the phenotype. Phenotype is a result of genotype’s interaction with the environment. Due to this reason, many phenotypes are not inheritable.
4.Acquired Traits: Traits; which are acquired due to interaction with the environment are called acquired traits. Acquired traits are not inheritable.
5.Inheritable Traits: Traits; which can be expressed in subsequent generations are called inheritable traits. Such traits bring a change in the genotype of the organism and hence become inheritable.
MENDEL'S FIRST LAW
Law of Segregation: Every individual possesses a pair of alleles for a particular trait. During gamete formation, a gamete receives only one trait from the alleles. A particular trait can be dominant or recessive in a particular generation.
DIHYBRID CROSS
The cross in which two pairs of characters are studied is called dihybrid cross. In his second experiment, Mendel used dihybrid cross.
Let us take example between plants with round and green seeds and those with wrinkled and yellow seeds. The genotype of round and green seeds is shown by RRyy and that of wrinkled and yellow seeds is shown by rrYY.
In the F1 generation, all plants produced round and yellow seeds, which means that wrinkled texture was the recessive character and so was the green colour of seeds.
When plants of F1 generation were allowed to self pollinate, it was observed that most of the plants in F2 generation produced round and yellow seeds. Some plants produces round green seeds, some produced wrinkled yellow seeds and some produced wrinkled green seeds. The ratio was 9 : 3 : 3 : 1; as shown in the below figure.
MENDEL’S SECOND LAW OF HEREDITY
Law of Independent Assortment: Alleles of different characters separate independent from each other during gamete formation.
In the above example, alleles of texture were assorted independently from those of seed colour.
SEX DETERMINATION IN HUMANS
Somatic cells in human beings contain 23 pairs of chromosomes. Out of them the 23rd pair is composed of different types of chromosomes which are named as X and Y chromosomes.
The 23rd pair contains one X and one Y chromosome in a male. On the other hand, the 23rd pair in a female contains only X chromosomes.
This means that all the eggs would have X chromosome as the 23rd chromosome, while a sperm may have either X or Y chromosome as the 23rd chromosome.
When a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a female child. And when a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.
