Heredity and Evolution Class 10 Notes – Science Chapter 9 Best Notes

Here, we have provided Heredity and Evolution class 10 notes – science chapter 9 notes. From Biology’s point of view, heredity and evolution are some of the most important chapters.

A certain amount of variations is produced in sexually reproducing organisms. This helps them to survive in the prevailing environmental conditions. Passing on these variations from one generation to other is called heredity.

The long-term accumulation of variations leads to evolving of a new species. This is called evolution which is a complex process that occurs over a long period.

Heredity, Mendel’s Contribution

The passage or transmission of characters (from parents to offspring) is known as inheritance or heredity. The process by which traits and characters pass from one genesis to the coming is called inheritance. Still, all characters present in progeny aren’t matching their parents. The rules of heredity decide the procedure by which traits are inherited.

Accumulation of Variations During Reproduction

Inheritance from the former genesis provides both a commonplace body arrangement and subtly changes in it, for the coming origination. Now, when the new generation reproduces, the alternate generation produced will have variations that they inherit from the first generation, as well as recently created differences.

Heredity and Evolution

Inheritance of Traits

Traits or characteristics, which are passed on from parents to their offspring from generation to generation are controlled by genes. A section of DNA that provides data for one protein is known as a gene for that protein. Due to the differences in genetic makeup, human populations show a lot of variations in the expression of various traits, e.g. height, skin color, eye! color, the shape of the nose, lips, ears, blood groups, etc.

It has been observed that attached and free earlobes are two variants found in human populations. The smallest part of the ear is called the earlobe.

In free earlobes, the earlobe is not attached to the side of the head. Whereas in an attached earlobe, the earlobe is closely attached to the side of the head. This particular trait is hereditary.

Mendel’s Contribution Towards the Inheritance of Traits

The rules for inheritance of traits in human beings are related to the fact that both father and mother contribute an equal amount of genetic material to the child. This implies that each trait can be impacted by both paternal and motherly DNA. Thus, for each trait, there will be two versions in the child.

Mendel worked out the main rules of such inheritances. The heredity in most living organisms is found to be regulated by certain definite principles. Mendel chose garden pea (Pisum sativum) to conduct his trials. His experiments with garden pea along with the inferences are drawn together constitute, the foundation of modern genetics.

Inheritance of Traits

Mendel’s contributions were unique because of the use of distinct variables and the application of mathematics to the problem. He kept the record of each generation independently and studied the heritage of only one pair of characters at a time.

He performed the following two experiments:

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Inheritance of Traits for One Contrasting Character:

  1. Mendel held pea plants with dissimilar characteristics such as height ( tall and short plants).
  2. The progeny delivered from them (F1- generation plants) were all tall. Mendel then allowed F1, progeny plants to undergo self-pollination.
  3. Two clones of the traits are inherited in each sexually reproducing organism.
  4. TT and Tt are phenotypically tall plants, whereas tt is a little plant. For a plant to be high, the single replica of ‘T’ is enough. Thus, in traits Tt, ‘T’ is a dominant trait while ‘t‘ is a recessive trait.
  5. In-generation, both the characters are recovered, though one of these isn’t seen in the F1, stage. During gamete conformation, the factor or allele of a pair segregates from each other.

Inheritance of Traits for Two Visible

  1. Mendel took pea plants with two different characters, i.e. one with a green ring seed and the distinct one with a yellow wrinkled seed.
  2. When the F1 progeny was attained they had round and yellow seeds, therefore establishing that round and yellow are dominant traits.
  3. Mendel also allowed the F1 progeny to be self- crossed( self-pollination) to obtain F2 progeny. He constitute that seeds were round yellow, round green, wrinkled yellow and some were wrinkled green.
  4. The ratio of plants with the former characteristics was 9:3:3:1, independently.
  5. In F2- generation, all the four characters were varied out independent of the others. Thus, he said that a pair of alternating or differing characters behaves independently of the other pair. For illustration, seed color is independent of the seed coat.

Expression of Trait

Cellular DNA is the source of data for forming proteins in the cell. A section of DNA that provides facts for one individual protein is called a gene for that protein.

As we know that plants have hormones that can spark growth. Thus, the amount of plant hormone formed will decide the plant’s height. And the amount of plant hormone formed will depend on the effectiveness of the process for making it.

Sex Determination

In human beings, the sex of the being is determined genetically. There are 23 pairs of chromosomes of which 22 are matching in male and female and are understood as autosomes. The remaining one is the sex chromosome which is XY in males and XX in females.

Males produce two kinds of sperms ‘X’ and ‘Y’, while female produces one kind of egg ‘X’. If an ‘X’ type of sperm fertilizes the egg then the sex of the baby will be female (XX). If ‘Y’ type of sperm fertilizes the egg then the sex of the baby will be male (XY).

Sex Determination


Evolution can be defined as a natural being’s slow, continuous and irreversible process of change. The gradational change of living organisms from pre-existing organisms since the beginning of life is called organic evolution. Whereas, gradational change in elements from one figure to another with time is named inorganic evolution.


Evolution by stages:

Complex organs like the eye have developed from rudimentary organs, (e.g., rudimentary eye in flatworm might be useful enough to give only a fitness advantage and the structure of the eye in different organisms is different indicating them to have different evolutionary origins) not by a single DNA revision but created bit-by-bit over geneses.

Heredity and Evolution

A change that’s useful for one property to start with can come useful afterward for quite a different function (e.g., Feathers might start as providing insulation in cold rainfall). But latterly, they might be useful for flight. Some massive birds and reptiles also have feathers but they do not glide.

Some very dissimilar-looking structures develop from a common ancestral blueprint, e.g. wild cabbage was cultivated as a food plant and multiple different vegetables were generated by selection over the last two thousand generations,

  • (a) Selection of very small lengths between the leaves gave rise to cabbage we eat.
  • (b) Selection for caught up flower development gave rise to broccoli,
  • (c) Selection for sterile blossoms gave creation to cauliflower
  • (d) Selection for blown parts gave rise to kohlrabi.
  • (e) Selection for larger leaves gave rise to green vegetable kale. It suggests that, if these selections weren’t done also there would have been only wild cabbage.
Heredity and Evolution

Homologous organs are organs having the same origin and introductory structure but they appear different and perform different functions in colorful organisms,

e.g. Forelimbs of horse and arms of a man, Wings of birds, and flippers of the whale.

Similarities in the introductory structure of (homologous) organs in different organisms, indifferent groups indicate common strain.

Analogous organs are organs, which look similar because they carry off identical functions, but they do not have the same origin and primary structure.

e.g. Wings of birds and wings of insects, Fins of fish, etc.


It means the origin of a new class from the existent ones. It happens when dissimilar populations of the same species evolve along dissimilar lines.

How does speciation occur?

It occurs when two populations are separated (both geographically and reproductively) leading to nearly no gene inflow between the two populations.

Over generations, inheritable drift will accumulate different changes in each sub-population. Natural selection may also operate else in these different locales.

Together natural selection and inheritable drift will cause similar changes ( severe changes in the DNA) that these two groups won’t be suitable to reproduce with each other indeed if they happen to meet.

When DNA changes come to a larger extent, it may lead to a change in the number of chromosomes or gene expression, ultimately, the origin cells of the two groups can not fuse. This leads to the emergence/formation of a new species.

Estimating Age of Fossil:

There are 2 methods :

Relative method: On digging, the fossils which are closer to the exterior are yet recent than the fossils found in deeper layers.

Dating fossils (carbon dating method): It is done by detecting the ratios of different isotopes of the same element (i.e., an isotope of C-14 which is radioactive) in the fossil material.


Reasons for the increase in population:

The various ways in which individuals with a particular trait may increase in a population: Differences in the population are responsible for the diversity such as the color of eyes, hair, shape of ear lobes. This occurs due to:

  • Sexual reproduction
  • Inaccuracies during DNA replication
  • Due to environmental changes. This diversity will expand with time as these variations can be handed on only through DNA/ genes during reproduction through reproductive tissue ( origin cells or gametes).

If these variations give a survival advantage, then such traits are selected in nature and such traits increase in a population.

Due to genetic drift. This occurs due to geographical or reproductive insulation. It results in the alteration in gene frequency in an individualized population.

Migration leads to gene flow in and out of the population. The mutation is caused due to particular type of environment.  Acquired traits due to a particular type of environment.

Evidence of evolution: Errors in DNA copying (mutation) and sexual reproduction lead to interpretations that form the base of evolution. Characteristics that are common in different kinds of living organisms provide evidence in favor of evolution.

Human Evolution

Conclusion – Heredity and Evolution Class 10 Notes

I hope these notes for heredity and evolution (class 10)  will help you understand the chapter better and you’ll perform well in your exams.

Thank You!

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