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Updated Mar 18, 2026

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5 pages

Understanding Alleles and Genetic Inheritance

Ever wondered why you have your mum's eyes or your... Show more

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# Alleles and Inheritance

What are alleles and inheritance?

This is about how we get our traits, like eye colour or hair colour, from our

Alleles and Inheritance Basics

Think of genes as instruction manuals for specific traits like eye colour or height. Each gene sits on your DNA and tells your body how to build certain characteristics.

Alleles are simply different versions of the same gene. You inherit one allele from your mum and one from your dad for every single gene. So if there's a gene for eye colour, you might get a brown-eye allele from one parent and a blue-eye allele from the other.

Here's where it gets interesting: dominant alleles (written as capital letters like B) always win when paired with recessive alleles (written as lowercase letters like b). The dominant allele masks the recessive one completely.

Remember: Your genotype is the actual alleles you have (like Bb), whilst your phenotype is what people can see (like brown eyes).

Homozygous means having two identical alleles (BB or bb), whilst heterozygous means having two different alleles (Bb). These terms pop up constantly in genetics questions, so nail them down now!

How Alleles Actually Work

Let's use eye colour to make this crystal clear. The brown eye allele (B) dominates the blue eye allele (b) every time they're together.

If your genotype is BB, you'll have brown eyes. If it's bb, you'll have blue eyes. But here's the crucial bit - if you're Bb (heterozygous), you'll still have brown eyes because that dominant B allele takes charge.

The only way to express a recessive trait like blue eyes is to have two recessive alleles (bb). One dominant allele is enough to completely hide the recessive trait.

Key Point: In heterozygous individuals, only the dominant trait shows up. You need two recessive alleles to see the recessive trait.

This explains why two brown-eyed parents can sometimes have a blue-eyed child. Both parents could be Bb, and there's a chance their child inherits the b allele from each parent, resulting in bb genotype and blue eyes.

Punnett Squares Made Simple

Punnett squares are your best mate for predicting genetic outcomes. They show every possible combination when two organisms reproduce.

Let's cross a homozygous brown-eyed parent (BB) with a homozygous blue-eyed parent (bb). Each parent can only contribute one type of gamete (sex cell) - the BB parent gives B, the bb parent gives b.

Draw a grid: put one parent's possible gametes across the top, the other's down the side. Fill in each box by combining the alleles. In this case, every single offspring gets Bb.

Result Check: 100% of offspring will be Bb (genotype) with brown eyes (phenotype).

The beauty of Punnett squares is they give you exact probabilities. You can predict not just what traits might appear, but the chances of each outcome occurring.

The Classic 3:1 Ratio

Now let's cross two heterozygous parents (Bb x Bb) - this is the money shot of genetics! Each parent can contribute either a B or b allele.

Your Punnett square will show: BB, Bb, Bb, bb. That's a genotypic ratio of 1:2:1 (25% BB, 50% Bb, 25% bb).

But the phenotypic ratio is 3:1 because both BB and Bb produce brown eyes. Only the bb genotype gives blue eyes. So you get 75% brown eyes to 25% blue eyes.

Exam Tip: The 3:1 phenotypic ratio from a heterozygous cross is genetics gold - learn it inside out!

Remember, these ratios show probability for each individual offspring. If parents have four kids, it doesn't guarantee exactly three will have brown eyes - each child independently has a 75% chance.

Common Mistakes to Avoid

Don't fall into these traps! Dominant doesn't mean better or more common - it just means the allele that gets expressed in heterozygous pairs. Some dominant alleles cause genetic disorders.

Always use the same letter for related alleles - B for dominant, b for recessive. Never mix different letters like B and G for the same gene.

Watch out for genotype versus phenotype questions. If asked for genotype, give the letters (Bb). If asked for phenotype, describe what you'd see (brown eyes).

Probability Alert: Punnett squares show chances, not guarantees. Each offspring is an independent event with the same odds.

The beauty of inheritance is its predictability. Once you master these patterns, you can work out the genetics behind any trait - from pet breeding to human characteristics. These fundamentals unlock the entire world of genetics!

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Biology

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Updated Mar 18, 2026

•

5 pages

Understanding Alleles and Genetic Inheritance

Ever wondered why you have your mum's eyes or your dad's nose? It all comes down to alleles- different versions of genes that determine your traits. Understanding how these genetic instructions pass from parents to children is key to... Show more

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Alleles and Inheritance Basics

Think of genes as instruction manuals for specific traits like eye colour or height. Each gene sits on your DNA and tells your body how to build certain characteristics.

Remember: Your genotype is the actual alleles you have (like Bb), whilst your phenotype is what people can see (like brown eyes).

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

How Alleles Actually Work

Let's use eye colour to make this crystal clear. The brown eye allele (B) dominates the blue eye allele (b) every time they're together.

The only way to express a recessive trait like blue eyes is to have two recessive alleles (bb). One dominant allele is enough to completely hide the recessive trait.

Key Point: In heterozygous individuals, only the dominant trait shows up. You need two recessive alleles to see the recessive trait.

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Access to all documents

Improve your grades

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Punnett Squares Made Simple

Punnett squares are your best mate for predicting genetic outcomes. They show every possible combination when two organisms reproduce.

Let's cross a homozygous brown-eyed parent (BB) with a homozygous blue-eyed parent (bb). Each parent can only contribute one type of gamete (sex cell) - the BB parent gives B, the bb parent gives b.

Draw a grid: put one parent's possible gametes across the top, the other's down the side. Fill in each box by combining the alleles. In this case, every single offspring gets Bb.

Result Check: 100% of offspring will be Bb (genotype) with brown eyes (phenotype).

The beauty of Punnett squares is they give you exact probabilities. You can predict not just what traits might appear, but the chances of each outcome occurring.

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

The Classic 3:1 Ratio

Now let's cross two heterozygous parents (Bb x Bb) - this is the money shot of genetics! Each parent can contribute either a B or b allele.

Your Punnett square will show: BB, Bb, Bb, bb. That's a genotypic ratio of 1:2:1 (25% BB, 50% Bb, 25% bb).

But the phenotypic ratio is 3:1 because both BB and Bb produce brown eyes. Only the bb genotype gives blue eyes. So you get 75% brown eyes to 25% blue eyes.

Exam Tip: The 3:1 phenotypic ratio from a heterozygous cross is genetics gold - learn it inside out!

Remember, these ratios show probability for each individual offspring. If parents have four kids, it doesn't guarantee exactly three will have brown eyes - each child independently has a 75% chance.

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

Common Mistakes to Avoid

Don't fall into these traps! Dominant doesn't mean better or more common - it just means the allele that gets expressed in heterozygous pairs. Some dominant alleles cause genetic disorders.

Always use the same letter for related alleles - B for dominant, b for recessive. Never mix different letters like B and G for the same gene.

Watch out for genotype versus phenotype questions. If asked for genotype, give the letters (Bb). If asked for phenotype, describe what you'd see (brown eyes).

Probability Alert: Punnett squares show chances, not guarantees. Each offspring is an independent event with the same odds.