Inherited disorders

​​In a nutshell

Some medical disorders can be inherited from our parents. We can use genetic diagrams and family pedigree trees to determine whether someone will have an inherited genetic disorder.

Inheriting genetic disorders

Genetic medical disorders can arise when there is a mutation in a gene. This makes the gene faulty and results in a phenotype (the physical characteristic) being disordered. Normally, harmful mutations occur in recessive alleles of the gene.

Note: Recessive alleles are not expressed in the phenotype unless a person does not have a copy of the dominant allele (which is always expressed in the phenotype) or has two copies of the recessive allele. 

To inherit a genetic disorder, a person must receive a faulty recessive allele from each parent. If a person carries only one faulty recessive allele, they may be a carrier of the disorder, meaning that they are not affected by the disorder (due to the presence of the dominant allele) but their children are at risk of inheriting a recessive allele from them. 

Examples

There are some exceptions where inherited disorders are caused by the dominant allele.

Example

Polydactyly is a condition where an extra finger or toe is present on the hand or foot. This condition is caused by a dominant allele. The genotype of most with this condition is likely to be heterozygous (where they have received one dominant allele from one parent and one recessive allele from the other parent), although those with both parents also affected by polydactyly will have a genotype of homozygous dominant (where they have received a dominant allele from both parents).

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Genetic testing

Nowadays, it is possible to test adults and unborn babies for genetic disorders. This can help inform a parent whether they are able to look after the child once they are born. However, there are some ethical debates on whether we should be allowed to genetically test children. 

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Genetic diagrams

We can predict the odds of someone having a genetic disorder using genetic diagrams. Genetic diagrams are really good for determining the genotype; we can then predict the phenotype from this. 

Example

Cystic fibrosis is a genetic disorder caused by a recessive allele. It results in a person having very sticky mucus in their airways which makes it hard for them to breathe. 

A couple are thinking of having a child. Both the mother and the father do not have cystic fibrosis, but are carriers for the recessive allele that causes the disease. We can predict the chance that their child may have cystic fibrosis. 

The child has a $$25\%$$​ chance of having the disorder. 

Let's consider another scenario where the mother has cystic fibrosis but the father does not - although he is a carrier of the recessive allele. 

The child has a $$50\%$$ chance of having the disorder.

Family pedigrees

We can also determine whether a someone has inherited a disorder without knowing the exact alleles they carry. We can do this using family pedigree trees. These show multiple generations of a family and who was affected by a disease. Therefore, family pedigree trees are really good at determining the phenotype; we can then predict some people's genotypes using this. 

Example

The following family pedigree tree shows the generations that have been affected by cystic fibrosis in Sam's family.

We can use this diagram to predict that both Sam's parent's are carriers. Sam's father inherited a recessive allele from his mother who had cystic fibrosis, and a dominant allele from his father who may or may not have been a carrier. Sam's mother received a dominant and recessive allele from her parents - of whom at least one had to be a carrier of the recessive allele as they were both unaffected as well.