2.1. Describe the laws of inheritance

This guide will help you answer 2.1. Describe the laws of inheritance.

The laws of inheritance explain how traits and characteristics are passed down from one generation to the next through genetic material. These laws are based on the study of genetics, which focuses on how DNA from biological parents contributes to the traits of their offspring. Understanding inheritance helps explain why children may look like their parents, why certain genetic conditions run in families, and the probability of these traits being passed on.

Inheritance refers to the transmission of genes from parents to their biological children. Genes are segments of DNA that contain instructions for various biological structures and functions, such as eye colour, blood type, and susceptibility to certain diseases. These genes are organised into chromosomes, which are found in the nucleus of most cells in the body. Humans have 23 pairs of chromosomes – half inherited from the mother and half from the father.

Gregor Mendel, known as the “Father of Genetics,” first established the basic principles of inheritance through experiments with pea plants. He formulated key rules that explain how traits are inherited, now referred to as Mendel’s Laws of Inheritance.

Mendel’s Laws of Inheritance

Mendel proposed three key laws of inheritance based on his findings:

Law of Segregation

The law of segregation states that every individual has two versions of each gene (known as alleles), one inherited from each parent. During reproduction, these alleles separate into different gametes (sperm and egg cells), ensuring that offspring receive one allele from each parent.

For example:

• If a person inherits one allele for brown eyes (B) and one allele for blue eyes (b), they may pass on either the B or the b allele to their child.

This process of segregation occurs during meiosis, a type of cell division essential for sexual reproduction.

Law of Independent Assortment

This law explains how genes for different traits are passed on independently of one another. For example, a gene for hair colour is inherited independently of a gene for height.

This means that inheriting one particular trait (e.g., brown eyes) does not affect the inheritance of another unrelated trait (e.g., freckles). However, this law applies more to genes located on different chromosomes or those far apart on the same chromosome.

Law of Dominance

The law of dominance states that some alleles are dominant, while others are recessive. A dominant allele masks the effect of a recessive allele when both are present.

For example:

• If B represents the dominant allele for black hair and b represents the recessive allele for blonde hair, a person with the genotype Bb (one dominant allele and one recessive allele) will have black hair.

This explains why some traits are more commonly expressed than others and why recessive traits may appear to “skip” generations.

Genotype and Phenotype

To understand the laws of inheritance, it’s important to define two key terms:

1. Genotype: The genetic makeup of an individual. This includes all the alleles they inherit from their parents, even those not outwardly visible.
2. Phenotype: The observable characteristics of an individual, such as eye colour, height, or hair type.

The relationship between genotype and phenotype often depends on the dominance of alleles. For instance, for the gene responsible for eye colour:

• Genotype BB (homozygous dominant) and Bb (heterozygous) produce the phenotype of brown eyes.
• Genotype bb (homozygous recessive) produces the phenotype of blue eyes.

Modes of Inheritance

Traits and characteristics may follow different patterns of inheritance. Here are the main types:

Autosomal Dominant Inheritance

With autosomal dominant inheritance, a single copy of the dominant allele is enough to express the trait or condition. If one parent carries the dominant allele, each child has a 50% chance of inheriting it.

Examples include:

• Huntington’s disease
• Marfan syndrome

Autosomal Recessive Inheritance

In this pattern, both copies of the gene must be recessive for the trait or condition to appear. If both parents are carriers (having one recessive allele and one normal allele), there is:

• A 25% chance the child will inherit the condition.
• A 50% chance the child will be a carrier.
• A 25% chance the child will not inherit the allele at all.

Examples include:

• Cystic fibrosis
• Sickle cell anaemia

X-Linked Inheritance

Some genes are located on the sex chromosomes, specifically the X chromosome. Males, with one X and one Y chromosome, are more likely to express X-linked recessive conditions if they inherit the affected X chromosome.

Examples include:

• Haemophilia
• Duchenne muscular dystrophy

Females, possessing two X chromosomes, are usually carriers unless they inherit two copies of the affected gene.

Codominance

In cases of codominance, both alleles in a pair are equally expressed in the phenotype. An example is the AB blood type, where both the A and B alleles are expressed.

Incomplete Dominance

This occurs when neither allele is completely dominant, and the phenotype is a blend of the two. An example is the pink flowers that result from crossing white and red flowered plants.

Mutations and Inheritance

Sometimes, changes occur in a gene, known as mutations. A mutation can cause a gene to function improperly, potentially causing a genetic condition. Some mutations can be inherited, while others occur spontaneously.

For example:

• A mutation in the BRCA1 gene can increase the risk of breast or ovarian cancer. This mutation can be inherited from a parent.

Environmental and Polygenic Factors

Not all traits are due to inheritance alone. Many traits result from an interplay between genetics and environmental factors. For example, height is influenced by genes but can be affected by nutrition.

Additionally, some traits are polygenic, meaning they are controlled by multiple genes. Examples include skin colour and intelligence.

Ethical Considerations in Genetic Inheritance

There are ethical issues surrounding the study and application of genetic inheritance, particularly in areas like genetic testing and reproductive choices. For instance:

• Screening for inherited conditions, such as sickle cell anaemia, can raise questions about how this information is used.
• Parents may face difficult decisions if they know their child could inherit a genetic condition.

The Human Genome Project has advanced our understanding of genetics, but it has also highlighted concerns about misuse or discrimination based on genetic information.

Why Inheritance Studies Are Important

Studying the laws of inheritance has practical applications in many areas, including:

• Diagnosing and treating genetic conditions.
• Predicting the likelihood of passing on inherited conditions.
• Helping individuals understand family health history.

Advances in genetics, such as gene therapy and personalised medicine, have the potential to change how we treat inherited conditions.

Conclusion

The laws of inheritance are the foundation for understanding how genetic traits and conditions are passed from generation to generation. These principles help explain not only physical features but also the likelihood of passing on genetic conditions. By understanding these laws, professionals in health and social care can provide support to individuals and families in making informed decisions about their health.