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All about genetics

This is a short guide to the different ways that inherited disorders can be passed on through families.

Use it to

  • find out about the different types of inheritance
  • read about how it all fits together
  • look up words and terms in the glossary to find out what they mean

Types of inheritance

Here's Prof Shomi Bhattacharya of University College London explaining three different ways that retinitis pigmentosa can be passed on through families.

 

 

You can find out more about the different types of inheritance by clicking the headings below.

  • Autosomal recessive inheritance

    Autosomal recessive conditions can happen when both parents are carriers of faults in a particular gene. They each have 1 heathy copy and 1 faulty copy of the relevant gene.

    Family tree of 2 parents and 4 children showing autosomal recessive inheritance, where both parents carry a fault on the same gene.

    The parents won’t usually have symptoms because 1 healthy copy is enough for the cells that use the gene to do their job. But there is a chance that the parents will pass the condition on to their children.

    Each child whose parents are both carriers has

    • 1 in 4 chance of inheriting 2 healthy copies and being totally unaffected
    • 2 in 4 chance of being a carrier, due to getting 1 healthy copy and 1 faulty copy
    • 1 in 4 chance of inheriting 2 faulty copies and being affected
    Autosomal recessive conditions can also happen if one parent is affected by the recessive eye condition and the other parent is a carrier. The affected parent has 2 faulty copies of the relevant gene while the carrier parent has a faulty copy and a healthy one.
    Family tree of 2 parents, 4 kids, showing autosomal recessive inheritance when parent 1 has 2 faulty copies of a gene and parent 2 is a carrier, with 1 faulty copy.

    Each child with 1 parent who is affected and 1 who is a carrier has

    • 2 in 4 chance of being a carrier due to inheriting 1 healthy copy and 1 faulty copy
    • 2 in 4 chance of being affected, due to having 2 faulty copies

    Autosomal recessive conditions include

  • Autosomal dominant inheritance
    Family tree of 2 parents and 4 kids. One parent has a faulty copy and so do 2 of the kids.

    Autosomal dominant conditions happen when just 1 faulty copy of a particular gene is passed from parent to child. Even though the parent and child each have 1 healthy copy, the faulty copy ‘dominates’. This means that cells using the gene can’t work as well as they need to, and may lead to symptoms.

    Each child with 1 affected parent and 1 unaffected parent has
    • 2 in 4 chance of inheriting 2 healthy copies and being totally unaffected.
    • 2 in 4 chance of inheriting 1 faulty copy and being affected

    Autosomal dominant conditions include

  • X-linked recessive inheritance
    X-linked recessive conditions are passed on via the X chromosome.
    Family tree showing a mother who can pass on either x-chromosome to sons and daughters, a carrier daughter and affected son.
    If a female has a faulty version of a gene on one of her X chromosomes and a healthy version on the other X chromosome, she will be a carrier. She may also experience some symptoms herself, depending on the particular fault.

    If she has children with an unaffected male:

    Each son will have

    • 1 in 2 chance of inheriting the healthy X chromosome from the mother and being totally unaffected
    • 1 in 2 chance of inheriting the faulty X chromosome from the mother and being affected, as males only have 1 copy of the X chromosome

    Each daughter will have

    • 1 in 2 chance of inheriting the mother’s healthy X chromosome and being completely unaffected
    • 1 in 2 chance of inheriting the mother’s faulty X chromosome and being a carrier

    X-linked recessive conditions include

  • Maternal/mitochondrial inheritance

    Mitochondrial inheritance is also known as maternal inheritance because almost all of the mitochondria inside someone’s cells come from their mother (they were in the egg that got fertilised). If mitochondria in the fertilised egg contain genetic faults, these will be passed on.

    Family tree showing the connections from the mother to 4 children. The children are increasingly severely affected from left to right in the picture due to starting off as an embryo with less --> more faulty mitochondria.

    Whether and how badly any genetic faults in the mitochondria affect health partly depends on how many are working well. Someone who only inherits a few faulty mitochondria may not have any symptoms at all, whereas someone who inherits mostly faulty mitochondria will be severely affected.

    Mitochondrial conditions that affect vision include

How it all fits together

 

Genes, proteins and family traits

Family traits are passed on from parents to children via genes. Our genes are like chapters in an instruction manual. They tell the body how to build the machinery it needs to keep working from day to day.

For example, cells in the body need many different types of protein to do a wide range of jobs. Some proteins respond to light, some help cells to communicate with each other, some help fight off infection and so on.

Proteins are large molecules made of smaller molecules called amino acids. The amino acids are building blocks that need to be strung together in a specific order to make a particular protein.

Genes are a step by step guide to tell the protein-making machinery inside a cell which amino acid should be added next. They also include other instructions such as when to stop building because the protein is complete.

What are genes made of?

Each gene is made of a specific sequence of small molecules called nucleotides. The nucleotides are like letters in an alphabet.

There are only 4 letters: C, G, A and T. They come in groups of 3-letter words called codons.

Each codon has a specific meaning. For example, the sequence GGC is a codon that means: make a molecule of the amino acid called glycine.

Where does DNA come in?

DNA stands for ‘deoxyribonucleic acid’. It’s a very long molecule made up of hundreds or thousands of genes strung together.

There are about 2 metres of DNA inside each human cell, but it all has to fit inside the cell nucleus (the control centre) which is tiny. If you scaled it up, it would be like trying to pack 24 miles of thread into a tennis ball.

So DNA is wound around specialised proteins that help it fold up tightly into a package. There are 2 strands of DNA in each package, twisted together to form a ‘double helix’. (A helix is another name for a spiral or corkscrew shape.)

Chromosomes and sex

A package of DNA is called a chromosome. Chromosomes are also linked to proteins that help DNA to be copied, read and repaired when needed.

Our chromosomes come as a set, like a set of encyclopaedias. Each one covers different information, so you need them all to get the full picture.

Each human cell contains 46 chromosomes in total. There are 22 matching pairs (one copy from each parent), plus 2 sex chromosomes.

The sex chromosomes are named X and Y. Most females have a pair of X chromosomes (XX) and most males have an X and a Y (XY).

Most females inherit one X chromosome from their biological mother and one from their biological father. Males usually inherit one or other of the mother’s X chromosomes plus the Y chromosome from the father.

Faulty genes

Each gene makes a different protein, and genes often have slightly different versions of their DNA sequence that make slightly different versions of the protein. Some variations cause harm and are called a ‘genetic fault’ or a ‘mutation’, while others may have little or no noticeable effect.

Some genetic variations are inherited; they already exist in the family and can be passed on to biological children and grandchildren.

Genetic variations can also appear for the first time in a person with no family history of a condition linked to the gene. This can happen if there are errors when DNA is being copied as an embryo develops.

Not all genetic faults mean that a person will definitely develop a condition. It might just increase the risk that the person will be affected, especially under certain conditions, for example if they smoke or are exposed to a particular virus. And sometimes it takes variations in several genes together to change the risk of being affected.

Glossary

Allosome

  • Either of the sex chromosomes (X and Y).

Autosome

  • Any chromosome that isn't a sex chromosome.

Carrier

  • A person with just one copy of a genetic variation that doesn't usually give them symptoms but can be passed to their children.

DNA

  • The molecules that contain our genes.

Dominant inheritance

  • When a genetic variation only needs to appear in one copy of a gene to have an effect.

Genes

  • Sections of DNA that tell protein-making machinery inside cells how to build particular proteins.

Mitochondria

  • Power station compartments inside cells that make the energy cells use to work.

Molecule

  • A group of atoms, held together by chemical bonds, with no positive or negative electrical charge.

Nucleotide

  • Small molecules that are used like letters of an alphabet to write genes. DNA uses 4 types of nucleotide: adenine (A); cytosine (C); guanine (G) and thymine (T).

Recessive inheritance

  • When a genetic variation needs to be in both copies of a gene to have an effect.