Theory:

Variations in sexual reproduction:
In higher organisms, sexual reproduction is the primary mode of reproduction. Two parents are involved in sexual reproduction because gametes from two different individuals are fused. Due to this, their genes become mixed up, resulting in many variants in their progeny. As a result, offspring differ from their parents in appearance. This degree of dissimilarity between the young ones and parents is called variation.
 
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Fusion of haploid gametes to form a diploid zygote
 
The genetic continuity is maintained by passing on the variation accrued in children from generation to generation. These differences can be seen from generation to generation.
 
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Creation of diversity over succeeding generation
 
The above picture is an illustration of how variations are accumulated across generations. The picture is a generation tree diagram of an organism represented in triangular form. The top portion of the triangle has one parent with some specific set of characteristic features. This parent reproduces asexually and produces two offspring, which are represented in the mid-level of the triangle. The offspring has certain features inherited from its parent and have some variations in their features, which are mentioned as pink and green.

At the bottom level of the triangle, there are four offspring. Two offspring are produced by each parent at the mid-level of the triangle. These four offspring inherit some features from their parents and also vary in some features from their parents. You can see the four organisms at the bottom level differ from each other and are unique in their features. This uniqueness in each organism is due to variations across generations, as mentioned in the picture
Heredity:
Heredity is the term for the phenomena of showing inheritance. There are certain similarities and differences between parents and their offspring as a function of heredity.
 
Inherited traits:
Characteristics that are passed down through generations are known as inherited traits.
Inheritance refers to the passing down of traits and variations from one generation to the next.
Example:
Skin colour, eye colour, hair colour, height, ear lobes, and eyebrows are among the most widely noticed inherited characteristics in humans.
As previously mentioned, the transfer of characters from parents to offspring has happened through the inheritance of genesHumans inherit our characters from our parents, such as eye colour, hair colour, blood type, height, etc. But one of the keen inherited traits that can be observed in all among us is the shape of the ear lobe. You might have observed the two prominent types of ear lobes, viz., Free lobe and attached lobe. See the below picture.
 
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Left to right: Attached and  free earlobe
 
Activity for inherited traits:
Consider doing an exercise in your classroom. Check out the earlobes of all your classmates and note down how many of them have free earlobe and attached earlobes. Ask your classmates to check their parents' ear lobes and note down all the information. Calculate the percentage of classmates with the free earlobe and attached earlobe.
Example:
If you carefully see the data, free ear lobe classmates will have at least one parent with a free earlobe. But attached earlobe classmates' parents will have both or anyone with free and attached earlobes. Also, if both the parents have attached earlobes, then your classmate must have attached ear lobe. They won't have free ear lobes.
 
If characteristic features are inherited from parents, why do some of the attached earlobe classmates' have both their parents with free earlobes? How do they get the attached earlobes?
 
Because their parents have the attached earlobe trait in their genes, but it is not expressed in their bodies. We are not precisely similar to our parents because we could inherit some of our parents' characters and some of the characters they don't exhibit. These characters that are not exhibited in the parents but exhibited in their offspring are called recessive characters, while other characters expressed in the parent are called dominant characters.
In this example, the free ear lobe is a dominant character, and the attached ear lobe is a recessive character. I.e., if both the free earlobe gene and attached earlobe gene are present in a person, then the free ear lobe character will get expressed.
For this reason, the percentage of classmates with free ear lobes is more than that of the percentage of classmates with attached ear lobes.
Traits found in humans:
Characteristics
Dominant
Recessive
Hair
  • Presence
  • Absence
Ear lobes
  • Free
  • Attached
Tongue
  • Rolled
  • Not rolled
Tongue
  • Folding
  • Not folding
 
Apart from these visible characteristics, offspring inherit anatomical characteristics from their parents, such as blood grouping. Some behavioural traits, such as hand use, walking patterns, and intelligence, appear to be passed down from generation to generation.
 
Dominant and recessive traits in human beings
  
Advantages of sexual reproduction:
  • It promotes variation in the progeny.
  • Because of their diversity, the species can adapt to different habitats, giving them a survival advantage.
  • Individuals in a population are less likely to be affected by the disease.
Disadvantages of sexual reproduction:
  • Finding a spouse takes time and energy.
  • An isolated individual will not be able to do so.
Why is variation needed for an organism?
Each organism reproduces to maintain its life on earth. Do you think all of these species variants have an equal chance of survival in the environment?
 
The answer is no. we understand that the environment is not the same everywhere. We must adapt to any adversity; variation is required as environmental conditions such as temperature, climate, humidity resources, and so on change. Evolutionary processes are based on the selection of variations by environmental factors.
 
If we take an example, unicellular organisms like Archaebacteria can withstand heat and survive better in a heatwave.
As we studied in previous classes, meiosis forms a DNA different from the parent cell due to the crossing over and recombination process. A change in DNA information indicates the initiation of variation.
 
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Illustration of genetic variations in species
Advantages of variations:
We can determine the similarities and differences in an individual offspring since the variation is a complex process. Variations have a wide range of benefits.
 
The following are some of the reasons why it is essential to demonstrate variations.
  • Evolution happens because of variation in an individual.
  • We can distinguish offspring of single parents based on differences, whether they reproduce sexually or asexually.
  • Due to variations, hybrids are produced: different animal and plant varieties can be formed.
  • From generation to generation, variations lead to the base of healthy genes.
  • With the help of variations, it is possible to adapt to any situation.
  • It aids in the selection of the most desirable trait.
Genetic variation causes chromosome linkages to disintegrate, resulting in mutations. Natural selection can use genetic variation to raise or decrease the frequency of alleles already present in the population, an influential factor in evolution. Mutations cause genetic variation by altering genes and alleles in a population. They could affect a single gene or an entire chromosome.
Reference:
https://commons.wikimedia.org/wiki/File:Figure_11_02_01.jpg
https://upload.wikimedia.org/wikipedia/commons/2/2c/Human_genetic_variation.jpg