Inheritance and Genetic Disorders

In this guide, we will cover the key topics related to inheritance and genetic disorders. This includes understanding genetic diagrams, how sex is inherited, and the inheritance patterns of genetic disorders. These concepts are vital to understanding how traits and conditions are passed from one generation to the next.

Genetic Diagrams

Genetic diagrams are tools used to predict the possible inheritance of traits from parents to offspring. The most commonly used genetic diagrams are Punnett squares, which show the combinations of alleles that can occur when gametes (egg and sperm cells) combine during fertilisation.

Alleles

Alleles are different versions of a gene. Each individual has two alleles for each gene, one inherited from each parent. These alleles can be: 

• Dominant: An allele that will express its trait even if only one copy is present. For example, the allele for brown eyes (B) is dominant over the allele for blue eyes (b).
• Recessive: An allele that will only express its trait if two copies are present, one from each parent. For example, the allele for blue eyes (b) is recessive.

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Punnett Square Example

Let’s consider the inheritance of a single trait where one parent is heterozygous for brown eyes (Bb), and the other parent has blue eyes (bb). The Punnett square for this cross is as follows:

• Genotype: 50% Bb (brown eyes), 50% bb (blue eyes).
• Phenotype: 50% with brown eyes, 50% with blue eyes.

The Punnett square shows the probability of offspring having either brown or blue eyes, based on the parents’ alleles.

Inheritance of Sex

In humans, biological sex is determined by the presence of specific sex chromosomes. There are two types of sex chromosomes: X and Y. The combination of these chromosomes determines whether an individual is male or female.

Sex Chromosomes

• Females have two X chromosomes (XX).
• Males have one X and one Y chromosome (XY).

During reproduction, males produce two types of sperm: one with an X chromosome and one with a Y chromosome. Females only produce eggs with an X chromosome. The combination of these chromosomes at fertilisation determines the sex of the offspring:

• If the sperm carrying an X chromosome fertilises the egg, the offspring will be female (XX).
• If the sperm carrying a Y chromosome fertilises the egg, the offspring will be male (XY).

This means that the father’s sperm determines the sex of the baby.

Sex-Linked Inheritance

Some genetic disorders are linked to the sex chromosomes, particularly the X chromosome. These are called sex-linked disorders.

• X-Linked Recessive Traits: Because males only have one X chromosome, they are more likely to be affected by X-linked recessive disorders, such as colour blindness or haemophilia. In females, two copies of the mutated allele are needed for the disorder to be expressed, as they have two X chromosomes.

Example of X-linked inheritance:

• A female carrier for colour blindness (X^CX^c) crosses with a male who has normal vision (X^CY). The Punnett square for this cross looks like this:

• The female offspring (X^CX^C) will have normal vision, while the male offspring (X^CY) will also have normal vision. However, if the male inherits the X^c allele, he will be colour blind, as he has no second X chromosome to compensate for the defective gene.

Inherited Diseases

Inherited diseases are conditions caused by genetic mutations that are passed down from parents to offspring. These diseases can be inherited in different ways depending on whether the mutated gene is dominant or recessive, and whether it is located on an autosome (non-sex chromosome) or a sex chromosome.

Autosomal Dominant Inheritance

In autosomal dominant inheritance, only one copy of the mutated gene is needed to express the disease. Affected individuals have a 50% chance of passing the gene on to their offspring.

Example: Huntington’s disease

• Huntington’s disease is caused by a dominant allele (H). If one parent has the allele (Hh), the offspring will have a 50% chance of inheriting the disease (Hh) and developing the condition in adulthood.

Autosomal Recessive Inheritance

In autosomal recessive inheritance, two copies of the mutated gene (one from each parent) are required for the disease to be expressed. If an individual only inherits one copy of the mutated gene (a carrier), they will not show symptoms but can pass the gene on to their children.

Example: Cystic fibrosis

• Cystic fibrosis is caused by a recessive allele (f). If both parents are carriers (Ff), there is a 25% chance of the child inheriting two copies of the mutated gene (ff) and developing the disease.

• In this case, 25% of the offspring will have cystic fibrosis (ff), 50% will be carriers (Ff), and 25% will be unaffected (FF).

Sex-Linked Inherited Diseases

Some genetic disorders are linked to genes located on the X chromosome. These conditions are more commonly seen in males, as they only have one X chromosome.

Example: Haemophilia

• Haemophilia is a blood clotting disorder caused by a mutation in a gene on the X chromosome. If a female inherits one copy of the mutated gene, she will be a carrier, but the disorder will only be expressed if she inherits two copies of the mutated gene (one from each parent). Males, however, will be affected if they inherit the mutated gene from their mother because they only have one X chromosome.

Summary

Inheritance is the process by which traits and genetic conditions are passed down from one generation to the next. Genetic diagrams, such as Punnett squares, help predict the inheritance of traits. The inheritance of sex is determined by the X and Y chromosomes, with males passing on either an X or a Y chromosome to their offspring. Inherited diseases can follow different inheritance patterns, including autosomal dominant, autosomal recessive, and sex-linked inheritance. Understanding these principles is crucial for understanding how genetic disorders are passed on in families and how traits are inherited through generations.