HEREDITY
CLASS 10
BIOLOGY
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INTRODUCTION
Heredity refers to the passing of characteristics from one generation to the next. Evolution is defined as the gradual process by which a simple life form leads to the development of complex organisms over a period of time, spanning several generations.
Here in this chapter, we will learn about the mechanism by which variations are created, the rules of heredity determining their pattern of inheritance, and how the accumulation of these variations leads to evolution.
Together, heredity and evolution help us to understand the continuity of life, the origin of variations, and the survival of the fittest. This chapter connects genetics with evolution and shows how small changes over generations can lead to big differences in living beings.
Reproduction is a process that gives rise to new offspring that are similar to the parents but with some variations between them due to the process of DNA copying.
It may be noted that these differences are slightly visible in asexual reproduction as in plants, but they are greater variations and diversity in the case of sexual reproduction which involves the fusion of two different gametes.
This leads to the variations that are seen in each generation.
These variations may be advantageous or disadvantageous to the individuals and may or may not enable them to cope with the changing environmental conditions. For example, the bacteria that can tolerate higher temperatures would survive the heat waves.
These variations accumulate over generations in the evolutionary process owing to the environmental factors and lead to the formation of new species and are equally important for their survival.
2. Heredity:
The biological process that maintains or passes on the characteristics and traits of the parents to their offspring is termed heredity. Example, colour of skin or hair, eyes, height etc.
This is responsible for maintaining the variations in generations and thus the evolution of species over a period of time.
2.1 Inherited Traits:
The characteristics that are inherited from the parents are termed as traits, like eye colour or skin colour in humans. If the parent has brown and black eyes, then the offspring may inherit a brown or black or a combination of both.
This depends on the set of genes responsible for that trait.
The traits that are inherited in this manner are termed inherited traits and these are the cause of the variations in the population, though everyone has a similar basic feature.
2.2 Rules for the Inheritance of Traits- Mendel’s Contributions:
The rules of inheritance come from the fact that both the parents contribute equally to the development of the traits in the offspring.
Gregor Johann Mendel, often referred to as the ‘Father of genetics’ was a pioneer who used his science and mathematics knowledge to frame these laws of inheritance. He did so using pea plants for his experiments as he found them easy to grow and they had a greater number of visible characteristics like tall/short, white/violet flowers, round/wrinkled seeds.
Mendel found during his experiments that there were some factors controlling the traits, which are now known as genes.
The genes are present as a pair for a specific trait and then they are termed alleles.
Depending on the genes the expression of the traits could be either dominant or recessive. If we take the tallness in a plant as a dominant trait, then it can be denoted by ‘T’ and shortness in the plant would be a recessive trait, denoted by ‘t’. Thus the plant will be tall if its alleles are ‘TT’ or ‘Tt’.
The condition is when the alleles of the genes have the same allele for a trait, they are termed as homozygous. Example - TT or tt.
The condition when the alleles of the genes have the different alleles for a trait, they are termed as heterozygous. Example - Tt.
The morphological expression of a single character is termed as phenotype. Example - tallness or shortness, round or wrinkled seeds of the plant.
The genetic constitution or the allele pair for a specific trait is termed as the genotype. Example - Tt or t tot TT.
Mendel was a mathematician so; he found the statistics of the traits in each generation by using a statistical method known as Punnett square for predicting the possible genotypes and phenotypes of the offspring.
He conducted his experiments to find two types of inheritance namely:
Monohybrid Inheritance:
Mendel took a tall (TT) and short (tt) pea plant, crossed it to get the offspring. The first generation or F1 were all found to be tall, showing only one of the traits of the parent. T
Then he self-pollinated the parent and the F1 plants. It was interesting to see that the offspring of the parent plants were all tall, but the offspring of the F1 plants, the F2 generation did not have all tall plants. A quarter of them was short, which indicated that both traits of the parent were inherited by the F1 generation.
The traits of tallness were dominant and so it was expressed even with the genotype of TT, Tt but the shortness was a recessive trait that could be expressed only with a genotype of tt.
So, the genotype - 1:2:1 And the phenotype - 3:1
The inheritance of a trait by a pair of single alleles of a gene is termed monohybrid inheritance.
Dihybrid Inheritance:
When a pea plant with two different traits; a plant with a round green colour seed (RRyy) is crossed with a wrinkled yellow seed (rrYY) plant.
The F1 generation turned out to have round yellow seeds, RrYy, which showed the dominant traits to be the round shape and yellow colour.
When the F1 generation were self-pollinated, the F2 generation had greater variations than the parent and new combination also came up.
So, the genotype - 9 different combinations and the phenotype - 9:3:3:1.
The inheritance of different traits by two pairs of alleles for it is termed as dihybrid inheritance. And here the two traits of round/wrinkled seeds and green/yellow colour were inherited independently.
Laws of Mendel
Law of Dominance says that a gene has two contrasting alleles and one always expresses itself in the organism.
It is called the dominant gene, and it expresses in any possible combination.
Law of Segregation says that traits get segregated completely during the formation of gametes without any mixing of alleles.
Law of Independent Assortment says that the traits can segregate independently of different characters during gamete formation.
Sex Determination:
There is a various mechanism that determines the sex of a new born organism. It may be based on the temperature where the fertilised eggs is kept as in few reptiles, or they may changes ex as in snails. Humans on the other hand the sex of a newborn child is determined predominantly by the genes inherited from the parents.
All the chromosomes in humans are not paired. There are generally a pair of 22 chromosomes, with one of the pair being from each of the parents. These generally determine all the traits.
There is a pair of chromosomes known as the sex chromosomes that differ in males and females. The females have a correct pair and they are termed as X chromosomes, but men have an X chromosome and its pair as Y. So the genotype of women is XX and men is XY.
If we look at the inheritance pattern of a male and female, we can see that X is inherited by the child from their mother by default and the sex of the child depends on which pair of the sex chromosome is being inherited from the father.
If it X, then the pair becomes XX and the child is a girl and if Y is inherited, then it becomes XY and thus the child is a boy.
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