Title: Inheritance Patterns - Dr Neeta Lahkani
Description: The podcast discusses the basic concepts of inheritance patterns (autosomal dominant and autosomal recessive).
Transcript:
During this particular podcast I am going to be talking about inheritance patterns.
The way traits or characteristics are passed down from generation to generation has been observed for a long time, however in the nineteenth century a Monk named GregorMendal did several scientific studies that showed us the exact working of how one trait is passed on from parent to child. These were then known as themendelian inheritance patterns.
In order to explain the way this inheritance works, firstly we need to talk about how things are organised in our bodies
Every normal person has two copies of their genetic information (which we call genes). It’s like having two copies of the same book.
One of these copies is acquired from mum and the other from dad.
Many genes are stored together on chromosomes like lots of books on a bookshelf.
In this case all the genes i.e books from dad are stored on one chromosome and all those from mum on another copy of the exact chromosome like two identical bookshelves with the same books on them, one from mum, one from dad. So each cell contains two copies of each chromosome, and two copies of each gene, one on one chromosome and one on the other.
The exceptions to this rule are the sex chromosomes that determine whether we are boys or girls. The paired chromosomes that are not involved in sex determination are called autosomes. Human beings have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (X and Y).
The different parts of a gene are known as alleles, like chapters in the books. Therefore we have two alleles for each characteristic - one inherited from the dad, one inherited from the mom.
Often when explaining inheritance it is easier to think of alleles as genes. i.e. as the different chapters of the book as a whole book.
A mutation in an allele can be thought of as a misprint in one word in the book, meaning the whole sentence doesn’t make sense.
So in summary we now know that we have 22 pairs of chromosomes (bookshelves) one of each pair from mum and one of each pair from dad.
On those chromosomes there are numerous genes (books on the bookshelves)
Each gene has different alleles, which determine the genes characteristics, like chapters in the book, which make the entire story.
Individuals that have two exact copies of the same allele, i.e. Two exact copies of the same book – one from mum and one from dad) are called homozygous
Individuals have different alleles, in instances where a mutation in one has occurred they are known as heterozygous i.e. mum gives a book with a slightly different story in it to the one dad gives.
In this case the dilemma is which book is used to make the final product. The inheritance patterns observed will depend on whether the allele is dominant or recessive.
Autosomaldominant
We already know from what we have discussed that we are talking about one of our 22 pairs of autosomes.
The dominant part of the pattern refers to which of the two different books to going to be used to make the final product. In this inheritance pattern the mutated allele or misprinted book is the dominant one and if inherited will cause the disease to manifest
For example
Mum has two genes (one from her mum and one from her dad) she is homozygous (same)
Dad has two genes (one from his mum and one from his dad) he is heterozygous (different because one book reads wrong, it has a misprint i.e. mutation)
Mum gives one of her two genes to each of her children (which gene this is random) and dad gives one of his genes to each of his children (which is also random)
In this case if a child inherits a mutated gene from dad and a normal gene from mum the child is a heterozygote. However will manifest the disease as dad’s faulty gene is dominant over mum’s normal gene.
The manifestation of disease occurs around 50% of the time in a situation where one parent has the fault gene
Examples of autosomal dominant diseases are FAP and BRCA inherited cancers
With these examples I will briefly mention penetrance
Penetrance means the likelihood of developing the condition if you definitely carry the gene. For example in FAP penetrance is almost 100% meaning with a mutated FAP gene will go on to develop cancer.
In BRCA inherited cancers the penetrance is not 100% meaning inheriting the gene does not mean necessary lead to development of cancer.
Autosomal recessive
In this inheritance pattern the characteristics are only seen if the person carriers two of the mutated genes (i.e. both of their books have misprints) The mutated gene is recessive to the normal gene.
So if mum only has one faulty gene and dad only has one faulty gene then neither of them will have the disease, however if mum and dad both give their faulty gene to the same child then the child will develop the condition. As there are no normal genes to dominant over the faulty ones. In this situation there is a 25% chance a child will develop the condition, a 25% that the child will not inherit either gene and a 50% that they will inherit either one from mum or one from dad. In this case they are called carriers.
So in summary we have discussed gene, chromosomes, autosomes, homogygous and heterozygous. In addition to penetrance autosomal dominant and autosomal recessive inheritance.