Inherited Disorders

An inherited disorder is a condition that arises when an individual inherits certain faulty genes or alleles from their parents. So the offspring inherits certain alleles.

Some examples of inherited disorders are:

  • Sickle cell anaemia
  • Cystic fibrosis
  • Polydactyly
  • Tay-Sachs disease

Cystic Fibrosis

Cystic fibrosis is a genetic disorder affecting cell membranes, particularly in the lungs and digestive system. It causes the body to produce large amounts of thick, sticky mucus. Over time, the buildup of this mucus can damage the organs, leading to respiratory and digestive problems.

Cystic fibrosis is controlled by a single gene, with two alleles:

  • The allele for normal cell function is dominant (F)
  • The allele for a defective cell membrane is recessive (f)

So, heterozygous people (who only carry one copy of the recessive allele), will not have cystic fibrosis. However, they are still carriers. For a person to have the disorder, they need to carry two copies of the recessive allele.

Example

Let’s look at how cystic fibrosis is inherited using a Punnett square:

In this example, the father has one allele for cystic fibrosis and one normal allele. Therefore, he is heterozygous, and he also carries the recessive allele. The mother is also heterozygous, carrying the recessive allele.

Remember that when constructing a Punnett square, we write down the genotypes of the gametes, which are haploid. So, with this example, half of the male gametes will have the cystic fibrosis allele and the other half will have the normal allele. This is the same with the female gametes.

During fertilisation, the gametes combine:

  • 25% of the offspring are homozygous for the dominant allele. So, they do not have cystic fibrosis and they are not a carrier of the recessive allele.
  • 50% of the offspring are heterozygous. So, they do not have cystic fibrosis, but they are a carrier of the recessive allele.
  • 25% of the offspring are homozygous for the recessive allele. So, they have cystic fibrosis, and they also carry the recessive allele.

These numbers are simply averages. So, on average, 25% of the offspring will have cystic fibrosis.

The genetic diagram below shows how cystic fibrosis is inherited.

Polydactyly

Polydactyly is a genetic disorder that causes a person to be born with extra fingers or toes.

It is caused by the dominant allele (D). So, if a person is homozygous for the dominant allele (DD) or heterozygous (Dd), then they will have polydactyly. Even if only one parent is a carrier of polydactyly, the offspring can still inherit the disorder.

Example

Let’s look at how polydactyly is inherited using a Punnett square:

In this example, the mother is homozygous for the recessive allele, so she does not have the allele for polydactyly. However, the father is heterozygous, so he carries the dominant allele.

During fertilisation, the gametes combine:

  • 50% of the offspring are heterozygous. So, they have polydactyly, and they are also a carrier of the dominant allele.
  • 50% of the offspring are homozygous for the recessive allele. So, they do not have polydactyly and they do not carry the dominant allele.

So, on average, 50% of the offspring will have polydactyly.

The genetic diagram below shows how polydactyly is inherited.

Embryo Screening

There are many inherited disorders in humans, and one potential solution is embryo screening. This process involves testing embryos for the presence of alleles linked to inherited disorders. Embryos without these defective alleles are selected for implantation, which increases the chances of a healthy pregnancy.

Benefits of embryo screening

  • Treatments for disorders can be expensive. So, by reducing the number of people with inherited disorders, more money can be invested in improving other health services.
  • Many inherited disorders can cause a lot of harm and trauma to people. Embryo screening can prevent children from being born with inherited disorders.

Issues with embryo screening

  • Embryo screening is expensive. So, it is not available to all patients and many people think this money should be spent on other health services.
  • A large number of embryos are created. However, only a few are implanted. So, destroying healthy embryos raises ethical concerns.
  • In the future, scientists may be able to screen embryos to produce offspring with more desirable features. This will raise many ethical concerns and may also reduce variation.

Gene Therapy

Gene therapy involves introducing functional alleles into the cells of an individual with defective genes. As a relatively new treatment, gene therapy isn’t always successful. However, scientists are trying to develop and improve its effectiveness.

Similar to embryo screening, gene therapy also raises many ethical concerns.

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