GCSE · Biology · Edexcel · Spec 1BI0

Key genetics terminology

A pea plant can have purple flowers and still be hiding the code for white ones. How can you tell? First, a quick sort.

Try it first

Sort the genotypes

A genotype is a pair of alleles. A capital letter is a dominant allele and a lowercase letter is the recessive one. In pea plants P gives purple flowers and p gives white; B and b follow the same pattern. Tick every set each genotype belongs to, then check.

A Shows the dominant characteristic
B Homozygous (two of the same allele)
C Has a recessive allele in its genotype

Zoom in

From the nucleus down to one pair of alleles

View: Alleles. Showing 4 layers: Nucleus, A pair of chromosomes, A gene, Alleles

View

Layers

Explore

zoom in with the View buttons, then tap a part to explore it

At that location each chromosome carries one allele: two versions of the same gene.

Use the View buttons to zoom in step by step, then tap any part.

Predict, then check

Two gametes fuse at fertilisation. Think about what the zygote needs.

A human body cell has 46 chromosomes in its nucleus. How many chromosomes does one human sperm cell carry?

The most-swapped pair

GenotypevsPhenotype

Genotype and phenotype are easy to mix up. Compare them side by side.

Focus

What it is

Genotype

The combination of alleles an organism has for a characteristic

Phenotype

The physical characteristics of the organism: features that result, at least partly, from its genes

The insight

Genotype is the code you carry. Phenotype is what that code (at least partly) produces.

How you write it

Genotype

Letters, such as Pp

Phenotype

A description, such as purple flower

Pea plant flowers

Genotype

PP, Pp or pp

Phenotype

Purple (PP), purple (Pp) or white (pp)

Does the environment play a part?

Genotype

It is simply the combination of alleles

Phenotype

Some phenotypes are determined only by genotype; others are also affected by the environment and lifestyle

Which idea is yours?

What happens to p?

A pea plant has the genotype Pp and purple flowers.

Which of these is closest to what you think is going on?

A gene and an allele are the same thing, so P and p are just two names for one thing.p is still in the genotype. P is the allele that is expressed, so the flowers are purple.P is the stronger allele, so it removes p and only P is left.

How sure are you?

I'm sureI think soI'm guessing

A gene and an allele are the same thing, so P and p are just two names for one thing.

This is a common misconception

Not quite. A gene is a short section of DNA that codes for a particular protein. Alleles are different versions of the same gene, so P and p are two alleles of one gene, one from each parent.

p is still in the genotype. P is the allele that is expressed, so the flowers are purple.

This matches the evidence

Yes. Both alleles are in the genotype. One dominant allele is enough for the dominant characteristic to be expressed, and the recessive allele is still there.

P is the stronger allele, so it removes p and only P is left.

This is a common misconception

It is an easy idea to fall for, but dominant is not "stronger". Both alleles contribute to the genotype, and a recessive allele is still present when a dominant allele is expressed.

Dominant is about which characteristic shows. In Pp the dominant allele is expressed and the recessive allele is still in the genotype.

Now you write

Describe a Pp plant

In pea plants, the allele for purple flowers is P and the allele for white flowers is p. A pea plant has the genotype Pp. Describe this plant using the terms allele, genotype, phenotype, heterozygous, dominant and recessive. [5 marks]

Your answer

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

The mark scheme

Says Pp is the plant's genotype (the combination of alleles it has) (1 mark)
Says the plant is heterozygous because P and p are two different alleles (1 mark)
Says P is the dominant allele (capital letter) and p is the recessive allele (lowercase letter) (1 mark)
Says the phenotype is purple flowers, because one dominant allele is enough for the dominant characteristic to be expressed (1 mark)
Says the recessive allele p is still present in the genotype even though it is not shown (1 mark)

Model answer

The plant's genotype is Pp, the combination of alleles it has. P and p are two different alleles of the same gene, so the plant is heterozygous. P is the dominant allele (capital letter) and p is the recessive allele (lowercase). The phenotype is purple flowers, because one dominant allele is enough for the dominant characteristic to be expressed. The recessive allele p is still present in the genotype, even though it is not shown.

Name the genotype, name the phenotype, and say what the recessive allele is doing: still there, not shown.

Exam line: Use each key term accurately on the plant you are given, in your own sentences.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Chromosome, gene, allele, genotype, phenotype: what each word names and how they fit together.

What you need to know

DNA is packaged into chromosomes, and a gene is a short section of DNA that codes for a particular protein.
Humans have 46 chromosomes in their cell nuclei, each storing thousands of genes. The number differs between organisms.
Have a goSam announces, very confidently, "Every human cell has 46 genes in its nucleus." What has Sam got mixed up?
46 is the number of chromosomes, not genes.
Each chromosome stores thousands of genes, so a nucleus holds far more than 46 genes.
A human gamete (egg or sperm) has 23 chromosomes, one of each pair. The zygote's nucleus gets 23 from each gamete.
You inherit two copies of each gene, one from each parent, at the same locations on a pair of chromosomes.
Alleles are different versions of the same gene, and they give different versions of the same characteristic.
Alleles of one gene share a letter, and you can have two copies of one allele or two different ones.
Here, an allele is dominant (capital letter) or recessive (lowercase letter).
Have a goA student labels two alleles of one gene K and k. Which of the two is the recessive allele?
k, the lowercase one.
Alleles of the same gene share a letter. The capital is the dominant allele and the lowercase version is the recessive one.
Genotype is your combination of alleles for a characteristic. Phenotype is the physical characteristics that result, at least partly, from them.
Two different alleles is heterozygous. Two of the same is homozygous: homozygous dominant (BB) or homozygous recessive (bb).
Have a goJess says, "Rr and rr are both homozygous, because they both have an r in them." Is she right?
No. Rr is heterozygous; only rr is homozygous (recessive).
Homozygous means two of the same allele. Having an r in the pair does not matter: Rr has two different alleles.
A dominant characteristic shows with one or two dominant alleles; a recessive one needs two recessive alleles.

The big picture

DNA is packaged into chromosomes, and a gene is a short section of DNA that codes for a particular protein. Different versions of a gene are alleles, and the pair of alleles an organism has is its genotype. What you can actually see is its phenotype, and a recessive allele can be present without being shown.

Key points

1A chromosome is a big package of DNA; a gene is a section of it; alleles are different versions of one gene.

2Genotype is the alleles (the letters); phenotype is the physical characteristics that result.

3A dominant characteristic shows with one or two dominant alleles, so a heterozygote can show it while a recessive allele is still present.

4Homozygous means two of the same allele; heterozygous means two different alleles.

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Worked example

Problem

A gene has a dominant allele F and a recessive allele f. An organism has the genotype ff. Describe it using the key terms, and say which characteristic it shows.

⚠ Watch out

Writing "the phenotype is Pp". Pp is the genotype: the alleles. The phenotype is what you can see, such as purple flowers.

🧠

Memory hook

Genotype = the Genes you carry (the letters). Phenotype = what you Physically see.

✓

Check yourself

From memory: which sits inside which — gene, chromosome, nucleus — and where do alleles fit in? (Answer: genes sit inside chromosomes, chromosomes sit inside the nucleus, and alleles are different versions of one gene.)

Flashcards

(15)

Chromosome

A big package of DNA in the nucleus. Humans have 46, and each one stores thousands of genes.

Gene

A short section of DNA that codes for a particular protein.

Allele

A genetic variant of the same gene. Different alleles give different versions of the same characteristic.

Gamete

An egg cell or sperm cell. It carries half the chromosomes, one of each pair: 23 in a human.

What does the nucleus of a zygote contain?

The 23 chromosomes from each gamete.

Genotype

The combination of alleles an organism has for a characteristic. An individual has a genotype for each gene.

Phenotype

The physical characteristics of an organism, resulting at least partly from the genetic code of its genes.

Dominant allele

Shown with a capital letter. One or both dominant alleles in the genotype are enough for the dominant characteristic to be expressed.

Recessive allele

Written as the lowercase version of the gene's letter. To show its characteristic, an organism needs two copies of it.

Homozygous

Two of the same allele: homozygous dominant (for example BB) or homozygous recessive (for example bb).

Heterozygous

Two different alleles in the genotype, for example Pp.

In pea plants (purple P, white p), which genotypes give purple flowers and which give white?

PP and Pp give purple flowers. Only pp gives white.

A dominant allele is paired with a recessive one. What has happened to the recessive allele?

It is still present in the genotype. Dominant does not mean "stronger"; both alleles contribute to the genotype.

Is a scar part of someone's phenotype?

No. Damage or scars result from lifestyle or the environment, so they are not phenotypic.

True or false: every phenotype is decided by genotype alone.

False. Some are determined only by genotype; others are also affected by the environment and lifestyle.

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.

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Sort the genotypes

What happens to p?

Describe a Pp plant

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