2.2. Explain dominant and recessive factors

This guide will help you answer 2.2. Explain dominant and recessive factors.

To understand dominant and recessive factors, we need to explore how genetic traits are inherited. These factors relate to the way traits or characteristics, such as hair colour, eye colour, or genetic disorders, pass from parents to their children. These are determined by genes, which are segments of DNA located on chromosomes. Each individual has two copies of every gene – one inherited from their mother and one from their father.

This guide covers the concepts of dominant and recessive traits.

What Are Genes?

Genes act as instructions for the body, dictating specific characteristics. For example, they determine whether someone has brown eyes or blue eyes. Human cells contain 23 pairs of chromosomes, and these chromosomes hold thousands of genes.

Each gene has different versions, called alleles. For example, the gene for eye colour might have one allele for brown eyes and another allele for blue eyes. Which gene version gets expressed depends on whether it is dominant or recessive.

Dominant and Recessive: What Do These Terms Mean?

• Dominant: A dominant allele will always show its effect, even if the person has only one copy of this allele. For example, if the allele for brown eyes is dominant, a person with one brown-eye allele and one blue-eye allele will have brown eyes. This is because the dominant allele “overpowers” the recessive allele.
• Recessive: A recessive allele will only show its effect if the person has two copies of this allele – one from each parent. For example, the allele for blue eyes is recessive. This means that to have blue eyes, a person must inherit one blue-eye allele from their mother and another blue-eye allele from their father.

How Does Inheritance Work?

Every person inherits two alleles for each characteristic, one from their mother and one from their father. Whether a child expresses a dominant or recessive trait depends on the combination of alleles they inherit. Let’s use examples to clarify.

Dominant Trait Example: Brown Eyes

• The allele for brown eyes (B) is dominant.
• The allele for blue eyes (b) is recessive.

If a child inherits one brown-eye allele (B) and one blue-eye allele (b), the child will have brown eyes because the dominant allele overrides the recessive allele. Even if both alleles are dominant (BB), the child will still have brown eyes.

Recessive Trait Example: Blue Eyes

For a child to have blue eyes, they must inherit two recessive alleles (bb) – one from each parent. If even one dominant allele (B) is present, the child will have brown eyes, as the dominant allele takes precedence.

Key Points About Dominant and Recessive Traits

• A dominant trait is expressed if at least one dominant allele is present.
• A recessive trait is expressed only if two recessive alleles are present.
• Dominant traits overshadow recessive traits in an individual’s physical appearance (phenotype).

Examples of Dominant and Recessive Traits

Some common traits influenced by dominant and recessive alleles include:

Dominant Traits:

• Brown eyes
• Dark hair
• Freckles
• Dimples
• Widow’s peak (a V-shaped hairline)

Recessive Traits:

• Blue eyes
• Blonde hair
• Straight hairline
• No freckles
• Attached earlobes

Punnett Squares: A Tool to Explain Inheritance

Geneticists often use a chart called a Punnett square to predict inheritance patterns. This square calculates the probability of a child inheriting specific traits from their parents.

Let’s say both parents have one dominant allele (B) and one recessive allele (b) for eye colour. The possible combinations for their child are:

Parent 1	B	b
Parent 2 B	BB	Bb
Parent 2 b	Bb	bb

Based on this Punnett square, the child has:

• A 25% chance of inheriting two dominant alleles (BB) – brown eyes.
• A 50% chance of inheriting one dominant allele and one recessive allele (Bb) – brown eyes.
• A 25% chance of inheriting two recessive alleles (bb) – blue eyes.

This demonstrates that even though both parents have brown eyes, there’s still a possibility the child might have blue eyes.

Understanding Genetic Disorders

Dominant and recessive inheritance also plays a role in genetic disorders. Some conditions require only one copy of a faulty allele (dominant disorders), while others require two copies (recessive disorders).

Dominant Genetic Disorders

In dominant disorders, inheriting one faulty allele from either parent is enough to cause the condition. Examples include:

• Huntington’s disease
• Marfan syndrome

If a parent has a dominant genetic disorder, there is a 50% chance they will pass the faulty allele to their child.

Recessive Genetic Disorders

In recessive disorders, a person must inherit two faulty alleles – one from each parent – to develop the condition. Examples include:

• Cystic fibrosis
• Sickle cell anaemia

If both parents are carriers (meaning they each have one faulty allele and one healthy allele), their child has:

• A 25% chance of inheriting both faulty alleles and developing the condition.
• A 50% chance of inheriting one faulty allele, becoming a carrier like the parents.
• A 25% chance of inheriting two healthy alleles, meaning the child will not have the condition or be a carrier.

Carriers of recessive disorders often do not show symptoms themselves but can still pass the faulty allele to their children.

Codominance and Incomplete Dominance

Not all traits follow simple dominant/recessive patterns. Some traits involve codominance or incomplete dominance.

Codominance

In codominance, both alleles are equally strong, and both are expressed. An example is blood type. A person with one allele for blood type A and one allele for blood type B will have blood type AB.

Incomplete Dominance

In incomplete dominance, neither allele is fully dominant over the other. The result is a blend of the two traits. For example, if a red-flowered plant and a white-flowered plant are crossed, the offspring might have pink flowers.

Why Is This Important in Health and Social Care?

Understanding dominant and recessive traits is crucial when working with individuals and families in health and social care. This knowledge helps professionals:

• Support individuals with genetic conditions.
• Explain inheritance patterns to families.
• Provide information about genetic testing.
• Help families make informed decisions about healthcare and future planning.

For example, if a child is diagnosed with a recessive genetic disorder, health and social care workers can guide the parents in understanding how the condition was inherited and what this means for future pregnancies.

Final Thoughts

Dominant and recessive factors play a significant role in the inheritance of traits and genetic conditions. A dominant allele will show its effect even if only one copy is present, while a recessive allele requires two copies to be expressed. Tools like Punnett squares can help predict inheritance, and understanding these patterns is essential in supporting individuals and families affected by genetic conditions. This knowledge empowers health and social care workers to provide compassionate, informed care.