GCSE · Biology · Edexcel · Spec 1BI0

Alleles and inherited characteristics

You carry two copies of every gene, one from each parent, and the two copies don't have to match. That one fact is why individuals differ.

From a pair of chromosomes to a visible feature

Stage 1 of 5: Chromosome pair. paused

Chromosome pair · 1/5Five stages, one chain: pair of chromosomes, then alleles, then proteins, then phenotype, then dominance.

You have a pair of chromosomes. Each gene has two copies in that pair, and you got one copy from each parent. So you inherit two alleles for every gene.

Drag the slider or press Play. Follow one gene from the chromosome pair all the way to the characteristic it affects.

Check your gut

What do you get from each parent?

Take one gene. Your biological parents each pass something on to you for it.

Which of these is closest to what you think?

I inherit two genes for a characteristic, one from each parent.One gene decides each characteristic, so there is nothing else involved.For each gene I get one allele from each parent. Together they make my genotype.

How sure are you?

I'm sureI think soI'm guessing

I inherit two genes for a characteristic, one from each parent.

This is a common misconception

Close, and an easy slip. What you inherit is two copies of the same gene, one from each parent. The copies can be different versions, and versions of a gene are called alleles. So it is two alleles of one gene, not two genes.

One gene decides each characteristic, so there is nothing else involved.

This is a common misconception

That's a tidy idea, but it's too simple. Most characteristics are influenced by multiple genes. Some characteristics are determined by just one gene, so the idea fits a few cases and not most.

For each gene I get one allele from each parent. Together they make my genotype.

This matches the evidence

Yes. Two alleles of each gene, one from each parent, and the combination of alleles is your genotype. Both alleles contribute to it.

Two alleles of each gene, one from each parent. And most characteristics involve many genes.

Predict, then check

A capital letter stands for the dominant allele and a lowercase letter for the recessive allele.

A gene has a dominant allele B and a recessive allele b. A person's genotype is Bb. Which version of the characteristic do they show?

Is it all in the genes?

Set by genotype onlyvsGenotype and environment

Phenotypes come in two kinds. Compare them.

Focus

What sets the phenotype

Set by genotype only

The genotype alone

Genotype and environment

The genotype and the environment

The insight

Some phenotypes are determined only by genotype, while others are also affected by the environment.

Same alleles, same version?

Set by genotype only

Yes. The alleles decide the version.

Genotype and environment

Not necessarily. The environment is also at work.

Role of the genes

Set by genotype only

The genetic code is the whole story

Genotype and environment

The genetic code is part of the story

Say it in your own words

Write the chain, then mark it

Explain why different alleles of the same gene can lead to different phenotypes. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Alleles are different versions of the same gene. (1 mark)
Different alleles code for different versions of the same protein. (1 mark)
The different protein structures lead to different characteristics (phenotypes). (1 mark)

Model answer

Alleles are different versions of the same gene. Different alleles code for different versions of the same protein. The different protein structures lead to different characteristics, so the phenotypes are different.

Alleles, then proteins, then phenotype. Write the chain in that order and each link earns its own mark.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Two copies of every gene. They don't have to match.

What you need to know

You inherit two copies of each gene, one from each parent, in a pair of chromosomes.
Versions of a gene are called alleles, so you inherit two alleles for each gene.
The combination of alleles you have is your genotype. The features that result, at least partly, from your genes are your phenotype.
Have a goA classmate announces, 'My genotype is the stuff people can see about me.' Confidently wrong. What would you tell them?
That's phenotype. Genotype is the combination of alleles you carry.
Genotype is the alleles themselves. Phenotype is the features that result, at least partly, from your genes.
Different alleles code for different versions of the same protein, and the different protein structures lead to different characteristics.
Have a goTwo people carry different alleles of the same gene. Will the protein that gene codes for be exactly identical in both?
No. Different alleles code for different versions of the same protein.
They are versions of the same protein with different structures, which is what leads to different characteristics.
In some cases one allele is dominant, written as a capital letter. The recessive allele gets the lowercase letter.
A dominant characteristic needs one or both dominant alleles in the genotype. A recessive one needs two recessive alleles.
When one allele is dominant it will always be expressed, but the recessive allele is still present.
Two of the same allele means homozygous. Two different alleles means heterozygous.
Have a goYour friend says, 'I must be heterozygous for this gene, because I got one allele from each parent.' What's wrong?
Everyone gets one allele from each parent. Heterozygous means the two alleles are different.
Homozygous and heterozygous describe whether the two alleles match, not where they came from.
Most characteristics are influenced by multiple genes, but some are determined by just one gene.
Some phenotypes are determined only by genotype, while others are also affected by the environment.

The big picture

You inherit two alleles of each gene, one from each parent. Different alleles code for different versions of the same protein, and the different protein structures lead to different characteristics. In some cases one allele is dominant, so it is expressed while the recessive allele is still there. Most characteristics involve many genes, and some are also affected by the environment.

Key points

1You inherit two alleles of each gene, one from each parent. It is alleles, never two genes.

2Different alleles code for different versions of the same protein. The different protein structures lead to different phenotypes.

3In some cases one allele is dominant and is always expressed. The recessive allele is still present, and shows only with two recessive alleles.

4Most characteristics are influenced by multiple genes, and some phenotypes are also affected by the environment.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

For one gene, H is the dominant allele and h is the recessive allele. An individual has the genotype hh. Describe the genotype, say which characteristic is expressed, and explain why.

⚠ Watch out

Saying alleles code for 'the same protein'. Different alleles code for different versions of the same protein, and it is the different protein structures that lead to different characteristics.

🧠

Memory hook

Two alleles in, one from each parent. Dominant gets expressed. Recessive may hide, but hiding is not leaving.

✓

Check yourself

Without looking back: what is the difference between a genotype and a phenotype, and which of the two can the environment also affect?

Flashcards

(13)

How many alleles of each gene do you inherit, and from where?

Two: one from each parent. Each gene has two copies in a pair of chromosomes.

What is an allele?

A version of a gene, a genetic variant of the same gene.

Two alleles of a gene differ. What does that mean for the characteristic?

Each allele is associated with a different version of the same characteristic.

What is a genotype?

The combination of alleles an organism has.

What is a phenotype?

The features that result, at least partly, from the genetic code of an organism's genes.

What do different alleles of a gene code for?

Different versions of the same protein. The different protein structures lead to different phenotypes.

Which letter case marks a dominant allele, and which a recessive allele?

Capital letter: dominant allele. Lowercase letter: recessive allele.

What must the genotype contain for a dominant characteristic to be expressed?

One or both dominant alleles.

What must the genotype contain for a recessive characteristic to be expressed?

Two recessive alleles.

Does a dominant allele remove the recessive allele?

No. The dominant allele is expressed, but the recessive allele is still present.

Homozygous or heterozygous?

Homozygous: two of the same allele. Heterozygous: two different alleles.

How many genes influence most characteristics?

Multiple genes. Some characteristics are determined by just one gene.

Are all phenotypes set by genotype alone?

No. Some are determined only by genotype, while others are also affected by the environment.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

From a pair of chromosomes to a visible feature

What do you get from each parent?

Write the chain, then mark it

More Edexcel GCSE Biology topics