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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Dominant and Recessive Alleles
Have you ever wondered why you have your mum's eye colour but your dad's hair? The answer lies in the fascinating world of genetics and inheritance patterns. In this guide, we'll explore how traits are passed from parents to offspring through dominant and recessive alleles.
Key Definitions:
We inherit genes from our parents - one copy from mum and one from dad. These genes come in different versions called alleles. When these alleles are different from each other, one might "overpower" the other - this is called dominance. The "stronger" version is the dominant allele, while the "hidden" version is recessive.
Scientists use letters to represent alleles. Capital letters (like 'T') represent dominant alleles, while lowercase letters (like 't') represent recessive alleles. For example, if 'T' represents the allele for tall plants and 't' represents short plants, a plant with 'Tt' will be tall because 'T' is dominant.
When we talk about the genetic makeup (genotype) of an organism, we need to understand two important terms:
When both alleles for a gene are the same, we call it homozygous. This can be either:
When the two alleles for a gene are different, we call it heterozygous (Tt). In this case, the dominant allele will determine the phenotype (physical trait), so a heterozygous organism will show the dominant trait.
Punnett squares are simple diagrams that help us predict the possible combinations of alleles in offspring. They're named after Reginald Punnett, who developed them in the early 1900s.
1. Write the possible alleles from one parent across the top
2. Write the possible alleles from the other parent down the left side
3. Fill in the boxes with the combined alleles
4. Each box represents a possible combination in the offspring
Let's look at a cross between two heterozygous tall pea plants (Tt × Tt):
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
From this Punnett square, we can see that:
So the phenotype ratio is 75% tall : 25% short (or 3:1).
Brown eyes are generally dominant over blue eyes. People with blue eyes usually have two recessive alleles (bb), while people with brown eyes have at least one dominant allele (BB or Bb).
Free earlobes (lobes that hang) are dominant over attached earlobes. If you have attached earlobes, you likely have two recessive alleles (ee).
The ability to roll your tongue into a tube shape is dominant. If you can't roll your tongue, you probably have two recessive alleles.
Many genetic disorders follow dominant or recessive inheritance patterns. Understanding these patterns helps in genetic counselling and predicting the likelihood of a child inheriting a condition.
Recessive disorders only appear when a person has two copies of the recessive allele (homozygous recessive). Examples include:
Dominant disorders appear when a person has just one copy of the dominant allele. Examples include:
Sickle cell anaemia is a recessive genetic disorder affecting haemoglobin in red blood cells. People with two copies of the recessive allele (ss) have the disease, while those with one normal and one sickle cell allele (Ss) are carriers. Interestingly, carriers have some protection against malaria, which explains why the sickle cell allele is more common in regions where malaria is prevalent. This is an example of how a seemingly harmful recessive allele can provide an advantage in certain environments.
Understanding how to calculate probabilities is essential for predicting inheritance patterns.
Let's consider two parents who are both carriers for cystic fibrosis (Ff). What is the probability that their child will have cystic fibrosis?
| F | f | |
| F | FF (Normal) |
Ff (Carrier) |
| f | Ff (Carrier) |
ff (Affected) |
From this Punnett square:
Let's apply what we've learned with a practice problem:
In rabbits, black fur (B) is dominant to brown fur (b). If a heterozygous black rabbit (Bb) is crossed with a brown rabbit (bb), what percentage of offspring will have brown fur?
Solution: Let's create a Punnett square:
| B | b | |
| b | Bb (Black) |
bb (Brown) |
From this, we can see that 50% of the offspring will have brown fur (bb) and 50% will have black fur (Bb).
Understanding dominant and recessive alleles is crucial for predicting inheritance patterns and forms the foundation for more complex genetic concepts you'll learn in future biology studies.
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