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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Alleles and Characteristics
Have you ever wondered why you have your mum's eyes but your dad's hair colour? Or why some traits seem to "skip" generations? The answers lie in understanding alleles and how they determine our characteristics!
Key Definitions:
Think of genes as "houses" on the DNA "street". Each house (gene) controls one characteristic, like eye colour. Alleles are different "designs" of the same house. For eye colour, you might have a blue eye allele or a brown eye allele, but they're both versions of the eye colour gene.
Dominant alleles are like bossy siblings - they always get their way! We write them with capital letters (e.g., 'B'). Recessive alleles only show up when there's no dominant allele to overshadow them. We write them with lowercase letters (e.g., 'b').
We inherit two copies of each gene - one from mum and one from dad. These two alleles interact to determine our characteristics.
For any gene with two possible alleles (e.g., B and b), there are three possible genotype combinations:
Two copies of the dominant allele. The dominant characteristic will be displayed.
One dominant and one recessive allele. The dominant characteristic will be displayed.
Two copies of the recessive allele. The recessive characteristic will be displayed.
Monohybrid inheritance refers to the inheritance of a single gene with two alleles. Let's explore this with a classic example: pea plant height.
Gregor Mendel, often called the "father of genetics," studied pea plants in the 1860s. He discovered that when he crossed tall plants (TT) with short plants (tt), all the offspring in the first generation were tall (Tt). But when these offspring bred with each other, about 1/4 of the next generation were short! This helped him discover dominant and recessive inheritance.
Geneticists use diagrams to predict the outcomes of genetic crosses. The most common is the Punnett square.
Let's look at a cross between two heterozygous tall pea plants (Tt × Tt):
| T | t | |
| T | TT (Tall) |
Tt (Tall) |
| t | Tt (Tall) |
tt (Short) |
From this Punnett square, we can see that:
So the phenotype ratio is 3:1 (3 tall : 1 short).
Brown eyes are generally dominant over blue eyes. If you have the genotype BB or Bb, you'll have brown eyes. Only people with bb have blue eyes. This is why two brown-eyed parents can sometimes have a blue-eyed child (if both parents are Bb)!
Free earlobes (lobes that hang down) are dominant over attached earlobes. If you have the genotype EE or Ee, you'll have free earlobes. Only people with ee have attached earlobes.
Some genetic disorders follow simple inheritance patterns, while others are more complex.
Cystic fibrosis is caused by a recessive allele (let's call it 'f'). People with the genotype FF or Ff don't have the condition, but those with ff do. If two carriers (Ff) have children, there's a 25% chance their child will have cystic fibrosis. This is why genetic counselling is important for families with a history of genetic disorders.
A carrier has one copy of a recessive allele for a genetic disorder but doesn't show symptoms. For example, someone with the genotype Ff for cystic fibrosis is a carrier. They won't have cystic fibrosis themselves, but they could pass the 'f' allele to their children.
Genetic testing can identify carriers, helping people make informed decisions about having children if there's a risk of genetic disorders.
While we've focused on simple dominant and recessive alleles, inheritance can be more complex:
Both alleles are expressed equally. For example, in certain flowers, a red allele (R) and a white allele (W) together (RW) produce a pink flower.
The heterozygous phenotype is a blend of both homozygous phenotypes. For example, in snapdragon flowers, red (RR) and white (rr) produce pink (Rr).
Some genes have more than two possible alleles in a population. Human blood type (A, B, AB, O) is determined by multiple alleles.
Understanding alleles and inheritance patterns helps us explain why we look the way we do, predict the characteristics of offspring and understand genetic disorders. It's the foundation of modern genetics and has applications in medicine, agriculture and many other fields!
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