Single gene disorders - Autosomal recessive
There are several different patterns of inheritance of single gene disorders that we can study including:
- autosomal recessive
- autosomal dominant
- incomplete dominance
- sex-linked recessive
X and Y chromosomes = sex chromosomes
All other chromosomes = Chromosomes that control the characteristics of an organism but do not determine its sex.
Autosomal recessive inheritance
Autosomal recessive traits are expressed rarely, sufferers require two Alternative form of a gene that is expressed only if a dominant allele of that gene is not present. An organism must have two copies of a recessive allele for that allele to be expressed. to be affected (homozygous recessive).
Recessive traits may skip generations and will affect both genders equally. An example of an autosomal recessive condition is cystic fibrosis.
A disorder that mainly affects the lungs, pancreas, liver, and intestine. The main symptom is difficulty breathing. is an inherited disorder of cell membranes that mainly affects the lungs and digestive system. They can become clogged with lots of thick, sticky mucus as too much is produced.
Over many years, the lungs become increasingly damaged and may eventually stop working properly. A number of treatments are available to help reduce the problems caused by the condition, but unfortunately average life expectancy is reduced for people who have it.
It is caused by a faulty Alternative form of a gene that is expressed only if a dominant allele of that gene is not present. An organism must have two copies of a recessive allele for that allele to be expressed. on chromosome 7. To be born with cystic fibrosis, a child has to inherit two copies of this faulty gene - one from each of their parents. Their parents will not usually have the condition themselves, because they will only carry one faulty gene and one that works normally.
In the diagram below cystic fibrosis involves:
- the recessive allele (lower case), which can be shown as f
- the dominant allele (capital letter), which can be shown as F
An individual who is This describes a genotype in which the two alleles for the characteristic are identical. (ff) with the recessive allele will develop cystic fibrosis. Someone who is This describes a genotype in which the two alleles for a particular characteristic are different. (Ff) will be a carrier of the recessive allele, but will not develop cystic fibrosis and have no symptoms.
Someone who is homozygous with the An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself. allele (FF) will not develop cystic fibrosis, as you need two faulty alleles (ff) for the condition. In this combination, no faulty alleles are present.
Example 1
In example 1, both parents are heterozygous, Ff. The chance of them producing a child with cystic fibrosis is 1 in 4, or 25%. The parents are Someone who does not suffer from a condition but carries the allele and can pass it to his or her offspring. of the disorder, and it is possible for them to produce a child with cystic fibrosis, without having it themselves. Carriers have no symptoms and are usually unaware they are carrying the recessive allele.
Example 2
In example 2, only one parent (the father) has a copy of the recessive allele (Ff). There is no chance of them producing a child with cystic fibrosis. However, half the possible offspring will be homozygous, FF, and be unaffected, and half will be heterozygous, Ff and carry the recessive allele. The ratio of FF to Ff is 1:1 or 50%.
This genetic diagram shows how cystic fibrosis is inherited.