[inheritance] inherited disorders

[ BBC Bitesize 'Inheritance, variation and evolution ⇢ Genetic inheritance' page 1-9 ]

Alleles

Different forms of the same gene. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Genotype

The alleles that an organism has for a particular characteristic’s gene, usually written as letters. eg. Gg Hh

Phenotype

The visible characteristics of an organism which occur as a result of its genes.

Dominant Allele

An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself. They are represented by a capital letter, for example, A.

Recessive Allele

Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene. They are represented by a lowercase letter, for example, a.

Homozygous

This describes a genotype in which the two alleles for the characteristic are identical. eg. AA

Heterozygous

This describes a genotype in which the two alleles for a particular characteristic are different. eg. Aa

Most characteristics are a result of multiple genes interacting, rather than…

… a single gene.

Punnett Square

A table in which all of the possible outcomes for a genetic cross between two individuals, with known genotypes, are given.

Chromosomes in a Male

They have two different sex chromosomes, X Y

Chromosomes in a Female

They have two X chromosomes, XX

Human Karyotypes

They show all the chromosomes aligned in pairs. The blue box shows the male chromosomes and the red box shows the female chromosomes.

Genetic Diagram

Like a Punnett square, they show how chromosomes may combine in zygotes. Usually, the diagram states the ratio of female to male offspring is 1:1 - on average, half of the offspring will be girls and half will be boys.

Carriers

People who have no symptoms and are usually unaware they are carrying an allele which causes an inherited disorder to be passed down to their offspring.

Cystic Fibrosis

An inherited disorder affecting cell membranes, primarily lungs and digestive system.

Symptoms of Cystic Fibrosis:

• build up of mucus in lungs and digestive system

• difficulty breathing

• poor growth

• gastrointestinal issues

Cause of Cystic Fibrosis

Caused by faulty recessive allele on chromosome 7. Child must inherit two copies of faulty gene (one from each parent) to have the disorder. Homozygous (ff) individuals develop cystic fibrosis.

Therapies Used to Treat Cystic Fibrosis:

• no cure

• treatments to reduce symptoms → physiotherapy

Polydactyly

An inherited condition in which a person has extra fingers or toes.

Cause of Polydactyly

It is caused by a dominant allele of a gene. This means it can be passed on by just one allele from one parent if they have the disorder. Someone who is homozygous (PP) or heterozygous (Pp) for the dominant allele will develop polydactyly.

Genetic Testing:

Involves analysis of a person's DNA to see if they carry alleles that cause genetic disorders. It can be done at any stage in a person's life.

Role of Placenta:

• oxygen, water and other nutrients are transported to the foetus’ body through the mother’s blood, which travels from the placenta to the umbilical cord

• facilitates the movements of nutrients from the mother’s blood to the baby’s body

• waste products are removed from the baby’s body by the mother’s body taking it in and excreting it themself

Embryo Screening

A procedure where the embryo can be examined to see if any genetic disorders are produced in embryo’s chromosomes.

Embryo Screening Process:

• samples can be taken from the placenta or fluid surrounding the foetus

• DNA of the foetus can be tested to determine which, if any, inherited disorders the child may/would develop

• parents and medical staff can use this information to be better prepared

Advantages of Embryo Screening:

• if couple knows about an inherited disorder, they can make plans early on to deal with it eg. termination

• treatment can be started early

• parents can outweigh benefits and disadvantages

Diadvantages of Embryo Screening:

• false negative and positives are possible

• increases the rick of a miscarriage

• screening is expensive

• ethical/moral issues

Antenatal Testing

Testing used to analyse an individual's DNA or chromosomes before they are born. This testing is offered to couples who may have an increased risk of producing a baby with an inherited disorder, but it can't detect all the risks of inherited disorders.

Neonatal Testing

Also known as the new born blood spot test, involves analysing a sample of blood that is taken from pricking a baby's heel. It detects genetic disorders in order to treat them early.

Pre-implantation Genetic Diagnosis (PGD)

Used on embryos before implantation, usually in the IVF process. Once the embryos have reached the eight-cell stage, one cell is removed. The cells are tested for the disorder causing alleles. Embryos that don't contain the disorder allele are implanted into the uterus.

Gene Therapy

Involves inserting copies of a normal allele into the chromosomes of an individual who carries a faulty allele. It is not always successful, and research is continuing to try and develop this possible treatment further.

Steps of Gene Therapy:

1. identify the gene involved in the genetic disorder

2. restriction enzymes cut out the normal allele

3. many copies of the allele are made

4. copies of the normal working allele are put into the cells of a person who has the genetic disorder due to a mutated or faulty copy of an allele

Problems with Gene Therapy:

• the alleles may not go into every target cell, which are cells that need the new non-faulty cell

• the alleles may be inserted into the chromosomes in random places, rather than in the required position, so they do not work properly

• some treated cells may be replaced naturally by the patient's own untreated cells, as cells are frequently replaced through the process of mitosis during growth and repair

Different methods are used to get the alleles into the patient's cells, including:

• using nose sprays, which allow a patient to introduce the working allele up their nose and it will be taken into their body and incorporated

• using cold viruses that are modified to carry the allele - the viruses go into the cells and infect them

• the direct injection of DNA

Family Tree / Pedigree Analysis Diagram Notation:

• males → squares

• females → circles

Gregor Mendel (1822-1884)

Scientist who studied pea plants' inheritance in mid-19th century.

Mendel’s Observations:

• red-flowered plants bred with white-flowered: all offspring had red flowers (dominant allele)

• crossbreeding resulted in most offspring having red flowers, some white (recessive allele)

• Mendel observed inheritance determined by unchanged 'units'

Why was Mendel’s Work not Accepted?

• Mendel's work predated DNA discovery, expanded genetic inheritance knowledge.

• Mendel's work initially not accepted due to communication, publication, and explanation issues

After Mendel:

• late 19th century: Idea of genes on chromosomes emerged with better microscopes

• early 20th century: Observation of chromosome and Mendel's 'units' similarity

• genes, previously 'units', now recognized on chromosomes

• mid-20th century: Watson and Crick determined DNA structure with Franklin and Wilkins' data