CBSE Class 10 Science Notes Chapter 8 - Heredity

Heredity and evolution are key biological concepts explaining traits are passed from generation to generation and how species gradually change over time. Heredity focuses on how genetic information is transmitted, while evolution studies the changes in species due to variations and natural selection. Together, they provide a comprehensive understanding of biological diversity and adaptation.

1.0Heredity: Passing Traits Across Generations

• Heredity is the process through which traits are passed from parents to offspring. 
• This involves the inheritance of genes. 
• These genes are located on chromosomes found in every organism's cell. 
• Sexual reproduction, in particular, creates new combinations of genes, leading to greater variation among offspring.

2.0Gregor Johann Mendel 

• 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. 
• Later in 1900 three different scientists from 3 different places also found the same result. They were Hugo deVries, Tschermak and Correns. 
• They rediscovered the theory of heredity proposed by Mendel. So Mendel is called "Father of genetics".

Advantages of Pisum sativum:

Pairs of allelic characters found in garden pea plant

Laws of Inheritance

1. Law of dominance : In crossing between organisms purely for contrasting characters of a pair, only one character of the pair appears in the F1 generation. This character is termed as dominant while the one which does not express itself in F1 generation is termed as recessive. 
2. 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. 
3. 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 color character. Rather, the two characters inherit independently of each other.

3.0Sex Determination

• In sexually reproducing species, determining the sex of offspring can vary between species. 
• In humans, sex is determined genetically by specific sex chromosomes. 
• When offspring are conceived, the mother always contributes an X chromosome, while the father can contribute either an X or a Y chromosome. 
• If the child inherits an X chromosome from the father, the child will be female; if the child inherits a Y chromosome, the child will be male.
• In other species, environmental factors such as temperature can determine the sex of offspring, as in certain reptiles. 
• Additionally, some species, like snails, can change their sex based on environmental conditions, demonstrating that sex determination mechanisms vary widely in nature.

4.0Evolution: The Process of Change Over Time

• It is the gradual change in species over generations, driven by genetic variation and natural selection. 
• Variations arise through mutations and gene recombination during reproduction. Some variations may offer an advantage to individuals in a population, helping them survive and reproduce more successfully than others. 
• Over time, these advantageous traits become more common, leading to evolutionary changes in the species.

5.0Natural Selection

• Natural selection, a concept introduced by Charles Darwin, is how organisms better suited to their environment tend to survive and pass on their traits to the next generation. 
• For example, birds with longer beaks may be more successful at accessing food in hard-to-reach places, giving them a survival advantage. 
• Over time, this trait will become more common in the population, as birds with shorter beaks are less likely to survive and reproduce.
• Natural selection gradually accumulates beneficial traits within a population, helping species adapt to changing environments. 
• This process explains how organisms evolve, resulting in the vast diversity of life we see today.

1. Genetic drift 

• The change in the frequency of certain genes in a population over generations or the random changes in the gene frequency occurring by chance alone. 
• The effect of genetic drift is very small in large populations and large in small populations. 
• Genetic drift provides diversity without any adaptation.

2. Speciation 

• This  occurs when populations of a species become reproductively isolated, leading to the emergence of new species. 
• Geographic isolation, such as the formation of mountains or rivers, can separate populations, allowing them to evolve independently. 
• Over time, genetic differences accumulate, and the populations can no longer interbreed, forming distinct species.

3. Adaptation

• Adaptation is how a species becomes better suited to its environment. 
• For example, polar bears have adapted to their cold environment by developing thick fur and fat layers to retain heat. 
• Similarly, desert plants have evolved mechanisms to store water and survive in arid conditions. 
• Adaptations can be physical, such as body structures, or behavioral, such as migration patterns.

4. Homologous organs 

• These are those organs having similar basic structure but have been modified to perform different functions. e.g. forelimbs of reptiles, frogs, lizards, birds and humans are homologous organs. 
• Such homologous characteristics help to identify an evolutionary relationship between apparently different species. 
• This relationship is called divergent evolution or homology.

5. Analogous organs: Those organs which have different origin and structural plan but appear similar and perform similar functions are called analogous organs, while this relationship is called convergent evolution or analogy. e.g. Wings of an insect, bird and bat.
6. Fossils : Fossils are remains or impressions of the hard parts of the extinct organism preserved in the sedimentary rock or other media. 

6.0The Role of Variation in Evolution

• Variation is essential for evolution. 
• In sexually reproducing organisms, variation arises from mixing genes from two parents. 
• Mutations, or random changes in DNA, can also introduce new genetic traits. 
• Variation allows populations to adapt to environmental changes. 
• For instance, if a population of insects is exposed to a new pesticide, some individuals may carry a genetic mutation that makes them resistant to the chemical. 
• These insects are more likely to survive and pass the resistance gene to their offspring. Over time, the population evolves to become resistant to the pesticide.