Human Inheritance 101



Genetics – the science of Inheritance – can get very complicated.  If you are looking for information about the details of inheritance, you should speak to a qualified medical doctor or genetic counselor.

This page is a short introduction to the basic ideas.


The Cell

Simplified diagram of a cell

Your body is made up of a vast number of little compartments called CELLS.

The diagram shows the parts of a cell important to inheritance.

The cell is surrounded  by a thin wall called the CELL MEMBRANE.

The CELL NUCLEUS is where most of the cells’ DNA is stored.

The MITOCHONDRIA (pronounced mite-oh-kon-dree-ah) are tiny parts of the cell responsible for releasing most of the cell’s energy from food, creating fuel called ATP that can be “burned” by the cell.  If there is only one, we call it a MITOCHONDRION (mite-oh-kon-dree-on).

The number of cells varies a lot, but one scientific estimate puts it at around 37,000,000,000 (23829164)

Every one of these cells has developed from one original – the fertilized egg.

In the adult body, most cells are specialized and grouped together to make TISSUES.  For example there are Liver cells, Kidney Cells, Bone Cells and so on.

There are two things influencing how the egg develops into a body, and how the cells of the body behave in the future.



The parents pass a lot of information down to their child inside the fertilized egg.

This is the information telling the egg how to grow and develop into an adult human being, and not, say, a chicken!

The information tells the egg how to produce each organ and body part, blood group, color of eyes, color of hair, color of skin, body shape and so on.

Scientists call a single unit of inheritance a GENE.

ABO Blood Group Gene

ABO Blood Group is an example of something controlled by one GENE.

This ABO gene has three possible versions.   Sometimes scientists use the word ALLELE to mean a version of a gene.

The three possible  versions are called A, B and O.

You inherit one copy of each gene from your Father, and one copy from your Mother.

This table shows the nine possible combinations of A, B and O you might inherit.

ABO Blood Group Inheritance

From Mother

If you have inherited genes AA or AO or OA you will be Blood Group A.

If you have inherited genes BB or BO or OB you will be Blood Group B.

If you have inherited genes AB or BA you will be Blood Group AB.

If you have inherited genes OO you will be Blood Group O.


Genes in the Cell Nucleus

Almost all human genes are held inside the CELL NUCLEUS.

Genes are bunched together into structures called CHROMOSOMES.

In Humans, there are 46 Chromosomes, grouped together in  23 Chromosome pairs.

One Chromosome from each pair was inherited from your Father, and the other one from your Mother.

Scientists can extract the chromosomes from a cell, stain them and look at them under a microscope. They can put together the pairs using the size and banding patterns to match the pairs together.

For convenience, scientists have numbered the chromosome pairs 1 to 22, from the largest to the smallest.

22 of the 23 pairs are pretty much the same for men and women.

Pair number 23 is special. This pair is known as the Sex Chromosomes.  Women have two X-shaped chromosomes, and Men have one X and one Y-shaped chromosome.

This image is a photograph of chromosomes from the nucleus of a Human Male – notice the very small Y chromosome.

Picture of Human Nuclear Chromosomes

(Source: UK Open University Open Learn Works )

Genes in Eggs and Sperm

When an Egg Cell is formed by a Woman, or a Sperm Cell is formed by a Man, only one of each pair of chromosomes is copied.

That means each Sperm gets one each of Chromosomes 1 thru 22, and either one X chromosome or one Y Chromosome.

Each Egg cell gets one each of Chromosome 1 thru 22 plus one X Chromosome.

That means the fertilized egg, fromed when one Sperm fertilizes one Egg, ends up with 23 pairs of Chromosomes.  If the Sperm had an X Chromosme, the child will be Female, and if the Sperm had a Y chromosome, the child will be Male.

For Chromosome numbers 1 thru 22, the same Genes appear one each chromosome. You inherited one of each pair from your Father, and one from your Mother.

Take a look at the page on Eggs, Sperm and Mitochondria.

Here are some examples using the ABO genes described above.

Father AA Mother OO

All the Father’s sperm have chromosomes with the A version of the ABO gene.

All the Mother’s eggs  have chromosomes with the O version of the ABO gene.

The child MUST end up with genes AO, and so will be Blood Group A.

Father AO Mother OO

Half the Father’s Sperm will charry the A version of the gene, and half will carry the O version of the gene.

All the Mother’s eggs  have chromosomes with the O version of the ABO gene.

A child will have a 50% chance of an A from the Father, and 50%  chance of an O  from the Father, but will always get O from the Mother.

Each child will have a 50/50 chance of being AO or OO.

An AO child will show Blood Group A, a OO child will show Blood Group O.

Father AB Mother AB

Each of the Father’s Sperm will carry either version A or B.

Each of the Mother’s Eggs will carry either version A or B.

Each child will have a 25% chance of being AA, 25% chance of being AB, 25% chance of being BA and 25% chance of being BB.

Since both AB and BA show as Blood Group AB, each child has a 1 in 4 chance of being Blood Group A, a 1 in 4 chance of being Blood Group B, and a 1 in 2 chance of being Blood Group AB.


This is the short, handy name for the chemical that actually stores the information of genes.

Why is this chemical so special?

  • It is built up from a small number of simple repeating groups.
  • It can form extremely long chains holding a lot of information
  • It can be reliably copied over and over again.

Ignoring the complicated chemical structure, you can think of each  piece of DNA as a very long string of letters.


This is a very simple alphabet with only four letters, A, C, G and T.

These represent four chmicals called BASES. The chemicals easily link together in pairs – every A to a T and every G to a C.

So two  strings or strands of DNA can pair together like so:






These two strands are linked together into a very stable very large molecule.

Position 1 in the molecule is the BASE PAIR A-T, position 2 is C-G, position 3 is A-T and so on.

This structure keeps DNA very stable when it is not being used, and it can be easily copied when needed. The two strands are seperated, and then a new set of bases attached following the same pair-matching rules.

Although there are only four different “letters” in this code, it can easily be used to hold a vast amount of information.

Diagram showing a body, a cell, a chromosome and a gene.



In the late 1980s, researchers discovered that the Mitochondria of every cell carry genes as well as the Nucleus.

There are a some big differences between inheritance through the Nucleus and inheritance through the Mitochondria.

Mitochondria are inherited through the egg cell, not the sperm, so mitochondrial DNA can only come from the Mother, not the Father.

One cell has only one Nucleus and so only has two copies of each Nuclear Gene.

There is only One Mitochondrial Chromosome, but one cell can have thousands of Mitochondria, and each one can hold many copies of the Mitochondrial Chromosome.

The Egg Cell can have thousands of mitochondria,  each with its own copies of the Mitochondrial genes.

If all the Mitochondria have the same version of a gene, the cell is called HOMOPLASMIC.

if some of the Mitochondria have one version of a gene, and some hae a different version, the cell is called HETEROPLASMIC.

The Mitochondrial Chromosome

Unlike the chromosomes in the Nucleus, the chromosome in Mitochondria is a circle.  In humans it has 16569 positions or  BASE PAIRS, numbered from 1 to 16569.

It has information for 37 genes.  All of these genes are involved in the mechanism to release energy from food.

13 of the genes tell the mitochondria how to make essential protein subunits (or peptides).  All of these protein subunits are vital parts of the ELECTRON TRANSPORT CHAIN, as set of complex proteins found inside mitochondria to release energy from food.

Most of the genes used to build mitochondria are on the chromosomes of the Nucleus. For example, there are more than 40 subunits in Complex I or NAD Dehydrogenase , but only subunits 1 to 6 have genes inside Mitochondria.

This means to build a fully working copy of Complex I, the cell has to use genes inside the Nucleus to make protein subunits and genes inside the Mitochondria to make protein subunits.

This is a simplified diagram of the Mitochondrial chromosome. Diagram of genes on the Mitochondrial Chromosome


All of the gene mutations linked to LHON are in this chromosome.

The three commonest are at positions (base pairs) 11778, 3460 and 14484.

Diagram of Mitochonrial Chromosome showing 3 commonest LHON mutations


This page was last updated September 2 2015


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