Inheritance Made Simple | O Level Biology (5090) | Best Notes

If you want a comprehensive guide on Inheritance, you will love this amazing resource I am about to share with you.

But first, let’s start with a simple definition.

Inheritance (heredity) is the passing (transfer) of traits from parents to their offspring. We will understand all this in detail. But before that, here are some important definitions you need to know:

Inheritance Notes:

Here are some important definitions you need to know about.

inheritance heredity
  • Chromosomes: In each cell, DNA molecules are packed in a thread-like structure known as the chromosomes.

The DNA molecules are tightly packed around proteins that support their structure.

Note that chromosomes are found inside the nucleus of a cell. They are NOT visible inside the nucleus even with the help of a microscope. They are only visible during cell division.

  • DNA (deoxyribonucleic acid): The DNA molecule is the hereditary material inside the animal cells.

In simple words, this is the information molecule because it contains instructions to make proteins.

  • Gene: It is made up of DNA and contains information to specify traits (characteristics).

Let me explain. Gene will store important information such as the eye colour of the offspring. For example, if both of your parents have brown eyes, you might inherit the trait of brown eyes from them.

Pretty simple, isn’t it?

With this, it is time to move on and talk about continuous and discontinuous variation.

Continuous and Discontinuous variation:

Before getting into this topic, there are two concepts you need to know.


Earlier, we discussed that a gene is a small segment of DNA. Now, let me introduce alleles to you.

In simple words, genes can have different forms. These alternative forms are known as alleles. Let me explain.

genotype phenotype inheritance

Let’s say that the gene for height may have two alleles: short and tall.

If an individual inherits two identical alleles (for example brown hair), their genotype is said to be homozygous. But if the alleles are different, then the genotype is said to be heterozygous.

This combination of alleles is known as genotype.

In other words, “Genotype” refers to a set of genes that determine the characteristics of an individual.

Now you might be wondering, what is phenotype?


In genetics, phenotype refers to the observable characteristics (traits) of an organism.

For example, height, hair colour and eye colour are some examples of phenotype. In simple words, what you see is the phenotype.

Simple, isn’t it?

So from all this, here is what we can say.

Continuous variation refers to an “unbroken range” of phenotypes.

Let me explain.

Taking height as an example, note that an individual can have ANY height: From the shortest person in this world to the tallest person.

So there is a range of values for this characteristic. This is known as continuous variation. Some other examples are:

  1. Weight
  1. Height
  1. Arm span

Moving on, let’s discuss discontinuous variation.

continuous and discontinuous variation inheritance

In this type of variation, we have a “list of values” instead of a range. This simply means that the phenotypes can be categorised easily.

Here is an example to better understand this topic.

In the ABO blood group system, only four types of blood groups are possible: A, B, AB and O. You cannot have any other blood group apart from these four. This is an example of discontinuous variation.

Some examples are:

  1. Blood group
  1. Eye colour

Here is a simple way to differentiate between the two:

Continuous: “Unlimited”

Discontinuous: “Limited”

With this, it is time to move on and talk about dominant and recessive alleles.

Dominant and Recessive alleles:

A dominant allele is always expressed. It is represented by a capital letter. On the other hand, a recessive allele is the one that is ONLY expressed if it has two copies.

Let me make this simple for you.

Let’s say that a gene of eye colour has two alleles: Black (B) and brown (b). And since black is dominant (represented by “B”), it will be expressed.

In other words, an allele of a gene is said to be dominant if it overrules the other allele (recessive).

You will understand all this in a better way after we study the test cross.

The Test Cross:

This is a simple technique to identify whether an individual will be homozygous or heterozygous. You can also say that a test cross allows us to identify the genotype of an organism.

Here is an example.

Question: In an experiment, a pure-bred black cow was crossed with a pure-bred white one. What will be the ratio of the offspring(s) if a homozygous black cow is crossed with a homozygous white one.

Note that black (B) is dominant to white (b).


The homozygous black cow represents this genotype: BB

The homozygous white cow represents this genotype: bb

If both are crossed, the resulting genotypes will be:

Bb, Bb, Bb and Bb

Since black is dominant (B), all the offspring will be black. This is because white is only possible if there are two recessive alleles (bb).

Further Reading:

How to Ace Biology | 5 Proven Tips

Eye Structure Made Simple | Best Notes

Homeostasis | Best Revision Guide

Here is a past paper question for you to better understand this topic.

Question (May June 2019 / 11):

Two heterozygous plants are crossed.

What is the likely ratio of homozygous genotypes to heterozygous genotypes of the offspring?


Here is the solution for this problem:

inheritance test cross

As you can see in the image below, we get the following genotypes when two heterozygous plants are crossed:

TT, Tt, Tt and tt

The homozygous genotypes are: TT and tt

The heterozygous genotypes are: Tt and Tt

So the ratio is: 2:2 = 1:1

Here is a simple technique that you can use:

Homozygous: “Same

Heterozygous: “Different

Gender determination:

In mammals, the Y chromosome determines gender.

Here is what I mean.

Note that the cells in females contain two X chromosomes. This can be represented as “XX”.

And, cells from males contain an X and a Y chromosome. This can be represented as “XY”.

Here is how you can remember this:

XY – Male

XX – Female

Natural and Artificial selection:

To suit the changes in the environment, living organisms also constantly adapt (change). This is known as natural selection.

Now you might be wondering, how do organisms adapt?

The answer is that there are variations (changes) in the genotypes that are passed from generation to generation. In other words, natural selection is the mechanism of evolution.

According to natural selection, the organisms that are best suited to the environment survive and reproduce, while the less adapted organisms die.

Here is an example.

Let’s say that a population of mice has moved into an area where the rocks are dark. Now due to genetic variation, some mice are white while some are black.

The white mice are more visible on the rocks than black mice. As a result, the white mice are eaten at a larger frequency. This means that the population of black mice would be comparatively greater (because they are consumed at a lower rate).

Since black mice have a higher chance of leaving offspring, the next generation would have a greater number of black mice.

Pretty simple, isn’t it?

Now, let’s talk about artificial selection.

Artificial selection:

In artificial selection, man selects the varieties of organisms that suit his needs. Similarly, these varieties are produced by selective breeding.

Let me explain.

This method is used for animals and plants to get desirable traits. Here is an example.

Let’s say that you want cows to produce more milk and provide better meat. What would happen is that cows that provide more milk will be used as parents for the next generation.

Moreover, selective breeding of chickens, sheep and cattle have allowed us to produce better meat as well.

Here is an example for plants.

Today, the wheat grown is more resistant to diseases, yield (production) is higher and offers comparatively offers more competition to weed. This is an example of artificial selection.

Production of human Insulin:

The technique of transferring genes from one organism to the other is known as genetic engineering.

Due to increasing cases of diabetes, humans decided to produce insulin through bacteria. This is because insulin is a hormone that controls the amount of sugar in the body.

Here is how it is done:

insulin production inheritance
  • The human insulin gene is isolated and cut using the restriction enzyme.

Note that a restriction enzyme is isolated from bacteria and cuts DNA molecules in a specific sequence.

  • The circular DNA (plasmid) from the bacterial cell is also cut using the restriction enzyme.

Due to the use of this enzyme, we have the “sticky ends”. This simply means that these ends simply re-attach the bacterial plasmid.

  • The human DNA is inserted into the bacterial DNA using a different enzyme.

Now, the bacterial plasmid (containing the insulin gene) is placed into a bacterial cell. Moreover, this bacterial cell reproduces rapidly in a fermenter.

After this, the insulin is extracted, purified and packaged. This process is known as “downstream“.

With this, it is time to talk about the advantages and disadvantages of genetic engineering.

Genetic Engineering: Advantages and disadvantages

Here are some advantages:

  • Traits can be developed.

Through genetic engineering, we can grow crops with our desired characteristics. Moreover, animals can be modified to provide better meat and more milk.

This is because genetic engineering is gene-specific.

  • New products can be made by combining different profiles together.

These new products can provide more yield. For example, more fruits can be grown from a tree. This will bring greater profits to the farmers because of more sales.

As a result, the problem of the increasing population can also be countered.

  • Disease resistance in crops.

Due to genetic modifications, we have plant varieties that are more capable of dealing with insects. This prevents the loss of yield due to problems such as pests, insects and lack of water.

Here is an example.

Today, we have drought resistance types as well. This simply means that the genetic modification in plants has allowed them to grow even with less water.

Now, here are some disadvantages of genetic engineering.

  • Decrease in nutritional value.

When a plant grows and matures quickly, its nutritional value may reduce. You can take poultry as an example.

In fact, some chicken products have greater than 200% additional fat content as compared to (chicken) products a generation ago.

  • The technology can be abused.

Although genetic engineering is being used to treat disorders and diseases, yet it can also be abused. In humans, it can be used to develop ethically questionable traits.

Wrapping Up:

With this, our topic about inheritance and genetic engineering has come to an end. If you have any questions feel free to get in touch.

To recap, the topics covered are the test cross, genes, alleles, chromosomes and natural selection. Which part of this topic do you find challenging? Do let me know.

Thank you for reading and staying with me till the end. Stay tuned for more. And remember, practise past papers for this topic as well.

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