Chapter 5 of 8

All 8 chapters
  1. 1DNA, genes and chromosomes, readunderstood
  2. 2The structure of DNA, readunderstood
  3. 3Meiosis and sex cells, readunderstood
  4. 4Boy or girl: X and Y, readunderstood
  5. 5Dominant and recessive, readunderstood
  6. 6Family trees and genetic disorders, readunderstood
  7. 7Variation, readunderstood
  8. 8Mutation and genetic engineering, readunderstood

S4.2.5 Chapter 5

Dominant and recessive

A pea plant with round seeds is crossed with one with wrinkled seeds. All the offspring have round seeds. Where did "wrinkled" go?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Use the words allele, dominant, recessive, genotype and phenotype correctly
  • Predict offspring ratios with a Punnett square

You have two copies of most genes. A man has one X and one Y, so he has only one copy of the genes on each of them. One copy is on the chromosome you got from your mother, and one is on the chromosome you got from your father. The two copies are not always the same.

Different versions of one gene are called alleles. In the 1800s a monk named Gregor Mendel grew thousands of pea plants and counted what they made. The seed shape of a pea plant has two alleles. One makes round seeds and one makes wrinkled seeds.

We write alleles as letters. R is the allele for round seeds and r is the allele for wrinkled seeds. A pea plant has two alleles, so its pair of letters is RR, Rr or rr.

Mendel found that a seed with Rr is round. One copy of R is enough. We say that R is dominant and r is recessive. A dominant allele shows its effect when there is one copy or two. A recessive allele shows only when there are two copies and no dominant allele.

The pair of alleles a plant has is its genotype, such as Rr. What you can see is its phenotype, such as round seeds.

Seed shape belongs to the seed itself. The seeds on a plant are its offspring, so each seed has its own genotype. A round plant means a plant grown from a round seed.

A table with the columns Genotype and Phenotype. RR is round seeds. Rr is round seeds. rr is wrinkled seeds.GenotypePhenotypeRRround seedsRrround seedsrrwrinkled seeds
Two genotypes, RR and Rr, give the same phenotype. Only rr gives wrinkled seeds.

RR and rr are homozygous: the two alleles are the same. Rr is heterozygous: the two alleles are different. An Rr seed is round, but it carries r. The plant that grows from it can pass r on to its offspring.

Now we predict. First, cross a plant with two R alleles and a plant with two r alleles. Each parent can give only one kind of allele, so every offspring gets one R and one r.

A Punnett square of four boxes. The mother, with R and R, is down the side and the father, with r and r, is across the top. All four boxes read Rr. The words father and mother name the sides, and the alleles R, R, r and r are shown.fathermotherrrRRRrRrRrRr
Every offspring is Rr, so every seed is round. The wrinkled allele is hidden, but not lost.

Next, cross two of these Rr plants. Each parent gives R or r with an equal chance. The square shows the four ways the alleles can meet.

Draw it first

Cross two Rr plants

  1. A Punnett square. The mother, with R and r, is down the side and the father, with R and r, is across the top. In the first picture the four boxes are empty. In the second picture they read RR, Rr, Rr and rr. In the third picture the box rr is hatched and the key says wrinkled seeds.fathermotherRrRr

    1 Write the mother's alleles, R and r, down the side. Write the father's alleles, R and r, across the top.

  2. A Punnett square. The mother, with R and r, is down the side and the father, with R and r, is across the top. In the first picture the four boxes are empty. In the second picture they read RR, Rr, Rr and rr. In the third picture the box rr is hatched and the key says wrinkled seeds.fathermotherRrRrRRRrRrrr

    2 Fill each box with the two alleles that meet there. The boxes read RR, Rr, Rr and rr.

  3. A Punnett square. The mother, with R and r, is down the side and the father, with R and r, is across the top. In the first picture the four boxes are empty. In the second picture they read RR, Rr, Rr and rr. In the third picture the box rr is hatched and the key says wrinkled seeds.fathermotherRrRrRRRrRrrrwrinkled seeds

    3 Hatch the box that shows the recessive phenotype. Only rr has wrinkled seeds.

Worked example

Two Rr pea plants are crossed. What is the ratio of round seeds to wrinkled seeds among their offspring?

  1. The square has 4 boxes. 1 reads RR, 2 read Rr and 1 reads rr.
  2. Round seeds are RR and Rr, so 3 boxes are round.
  3. Wrinkled seeds are rr, so 1 box is wrinkled.
  4. The ratio of round to wrinkled is 3 : 1.

Answer: 3 : 1

The ratio of genotypes is 1 RR : 2 Rr : 1 rr. The ratio of phenotypes is 3 round : 1 wrinkled. Three quarters of the offspring are round and one quarter are wrinkled. A ratio is a chance for each seed. It is not a promise for one pod.

Worked example

Mendel counted the seeds of the second generation. He found 5 474 round seeds and 1 850 wrinkled seeds. How many round seeds were there for every wrinkled seed? Give the answer to one decimal place.

  1. Divide the round seeds by the wrinkled seeds.
  2. 5 474 ÷ 1 850 = 2.96 to two decimal places.
  3. To one decimal place this is 3.0, so there were about 3 round seeds for each wrinkled seed.

Answer: 3.0

A bar chart of seeds counted by Mendel. The axis is labelled number of seeds, with a scale from 0 to 6000 in steps of 2000. The bar for round reaches 5474. The bar for wrinkled reaches 1850.0200040006000roundwrinklednumber of seeds
Mendel counted every seed of the second generation. The result is close to 3 : 1, but it is not exactly 3 : 1. Chance seldom gives an exact ratio.

How can you tell if a round plant is RR or Rr? You cannot tell by looking. Cross it with a wrinkled plant, which is always rr. If the round plant is RR, every seed is round. If it is Rr, about half the seeds are wrinkled.

A Punnett square. The mother, with R and r, is down the side and the father, with r and r, is across the top. Two boxes read Rr and two boxes read rr. The two rr boxes are hatched and the key says wrinkled seeds.fathermotherrrRrRrRrrrrrwrinkled seeds
Rr crossed with rr. Half of the boxes are Rr and round. Half are rr and wrinkled. The ratio is 1 : 1.

People have genes in pairs too, and some features follow this pattern. In some families, extra fingers or toes are caused by a dominant allele. This feature is rare, so dominant does not mean common.

Be careful with other examples. Eye colour, tongue rolling and earlobe shape are often taught as single gene features. They are not. Several genes and other things affect them. This is a well known myth.

Try it

Fold a Punnett square

You need: an old sheet of paper, a pencil.

  1. Fold the paper in half, then in half again the other way. Open it. You have four boxes. Draw over the fold lines.
  2. Write R and r across the top for the father. Write R and r down the side for the mother.
  3. In each box write the letter from its column and the letter from its row. Write the capital letter first.
  4. Count the boxes of each genotype. Colour in the rr box.

What you will see: The boxes read RR, Rr, Rr and rr. One box in four is rr. The coloured box is the plant with wrinkled seeds.

If you cannot try it, this is what happens.
A table with the columns Genotype and Boxes. RR is 1 box. Rr is 2 boxes. rr is 1 box.GenotypeBoxesRR1Rr2rr1

A wrong idea many people have

A dominant allele is the one that most people have, or the stronger one.

Why it feels right: In everyday speech, "dominant" means winning or strong.

The test that breaks it: In some families the allele for extra fingers is dominant, yet very few people have it. A dominant allele only needs one copy to show.

What is true: Dominant means that one copy is enough to show. It says nothing about how common, strong or healthy the allele is. A recessive allele is not weaker. It is hidden when a dominant allele is present.

Key words

allele
one version of a gene.
dominant
an allele that shows its effect when there is one copy or two.
recessive
an allele that shows only when there are two copies and no dominant allele.
genotype
the pair of alleles an organism has, such as Rr.
phenotype
what you can see or measure, such as round seeds.
homozygous
having two of the same allele, such as RR or rr.
heterozygous
having two different alleles, such as Rr.

Back to your guess

The answer is A: It is still there, hidden, and it can come back in a later generation.

The wrinkled allele is still there. It is hidden by the round allele. When two of these offspring are crossed, about 1 in 4 of their seeds are wrinkled again.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A pea seed has the genotype Rr. R is dominant. What shape is the seed?

    Tap your answer.

    1. Not this one. A wrinkled seed needs two copies of r. This seed has one R, and R is dominant, so the seed is round. Try another.

    2. Not this one. Alleles do not blend. One R is enough, so the whole seed is round. Try another.

    3. Yes, that is right. R is dominant, so one copy is enough, and the seed is round.

    See the answer

    Round

    R is dominant, so one copy is enough, and the seed is round.

    • If you chose A: A wrinkled seed needs two copies of r. This seed has one R, and R is dominant, so the seed is round.
    • If you chose B: Alleles do not blend. One R is enough, so the whole seed is round.

  2. Which of these plants is heterozygous?

    Tap your answer.

    1. Yes, that is right. Hetero means different. Rr has two different alleles. RR and rr each have two the same.

    2. Not this one. RR has two alleles that are the same. That is homozygous. Try another.

    3. Not this one. rr has two alleles that are the same. That is homozygous too. Try another.

    See the answer

    Rr

    Hetero means different. Rr has two different alleles. RR and rr each have two the same.

    • If you chose B: RR has two alleles that are the same. That is homozygous.
    • If you chose C: rr has two alleles that are the same. That is homozygous too.

  3. Two Rr plants are crossed. What chance does each seed have of being wrinkled?

    Tap your answer.

    1. Yes, that is right. Only one box of four reads rr, so the chance of a wrinkled seed is 1 in 4.

    2. Not this one. That is the chance when Rr is crossed with rr. Here only one box in four is rr. Try another.

    3. Not this one. Three in four is the chance of a round seed. Wrinkled is the box with rr, one in four. Try another.

    4. Not this one. Both parents carry r. Each can pass it on, and a seed that gets r from both is rr. Try another.

    See the answer

    1 in 4

    Only one box of four reads rr, so the chance of a wrinkled seed is 1 in 4.

    • If you chose B: That is the chance when Rr is crossed with rr. Here only one box in four is rr.
    • If you chose C: Three in four is the chance of a round seed. Wrinkled is the box with rr, one in four.
    • If you chose D: Both parents carry r. Each can pass it on, and a seed that gets r from both is rr.

  4. What does "recessive" mean?

    Tap your answer.

    1. Not this one. Recessive is about how an allele shows, not how many people have it. Some recessive alleles are common. Try another.

    2. Not this one. It does not disappear. It is still in the cell, and it can be passed on to the next generation. Try another.

    3. Yes, that is right. A recessive allele has an effect only when the genotype has two copies of it.

    4. Not this one. Recessive does not mean weak or harmful. A recessive allele can code for a perfectly good protein. Try another.

    See the answer

    An allele that shows only when there is no dominant allele with it

    A recessive allele has an effect only when the genotype has two copies of it.

    • If you chose A: Recessive is about how an allele shows, not how many people have it. Some recessive alleles are common.
    • If you chose B: It does not disappear. It is still in the cell, and it can be passed on to the next generation.
    • If you chose D: Recessive does not mean weak or harmful. A recessive allele can code for a perfectly good protein.

  5. A round seeded plant is crossed with a wrinkled plant. About half of the seeds are wrinkled. What is the genotype of the round plant?

    Tap your answer.

    1. Not this one. An RR plant gives an R to every seed. Every seed would be round. Try another.

    2. Not this one. An rr plant has wrinkled seeds. This plant has round seeds, so it has at least one R. Try another.

    3. Yes, that is right. About half wrinkled means about half the seeds got r from the round plant. So the plant has one r, and it is Rr.

    See the answer

    Rr

    About half wrinkled means about half the seeds got r from the round plant. So the plant has one r, and it is Rr.

    • If you chose A: An RR plant gives an R to every seed. Every seed would be round.
    • If you chose B: An rr plant has wrinkled seeds. This plant has round seeds, so it has at least one R.

  6. Which of these is a phenotype?

    Tap your answer.

    1. Not this one. The letters give the genotype, the alleles in the cell. A phenotype is what you can see. Try another.

    2. Yes, that is right. A phenotype is what you can see or measure. Round seeds are seen. The alleles are not.

    3. Not this one. That is the genotype. A phenotype is the feature that results from it. Try another.

    See the answer

    Round seeds

    A phenotype is what you can see or measure. Round seeds are seen. The alleles are not.

    • If you chose A: The letters give the genotype, the alleles in the cell. A phenotype is what you can see.
    • If you chose C: That is the genotype. A phenotype is the feature that results from it.

  7. Which statement about eye colour is true?

    Tap your answer.

    1. Not this one. This is a myth from old school books. Eye colour is affected by several genes. Try another.

    2. Yes, that is right. Many features, such as eye colour, depend on several genes together. A one gene pattern, like Mendel's peas, applies only to a feature that one gene decides.

    3. Not this one. Both parents give genes for eye colour. The genes are on chromosomes that come from each parent. Try another.

    See the answer

    Several genes help decide eye colour, so it does not follow a plain dominant and recessive pattern.

    Many features, such as eye colour, depend on several genes together. A one gene pattern, like Mendel's peas, applies only to a feature that one gene decides.

    • If you chose A: This is a myth from old school books. Eye colour is affected by several genes.
    • If you chose C: Both parents give genes for eye colour. The genes are on chromosomes that come from each parent.

  8. From an earlier chapter

    Each baby has a 1 in 2 chance of being a girl. How many of the four boxes in the Punnett square for an XX mother and XY father read XX?

    Tap your answer.

    1. Not this one. Both eggs carry an X, so the two boxes with an X sperm both read XX. Go to S4.2.4. Try another.

    2. Yes, that is right. Two boxes read XX and two read XY. So the chance of a girl is 2 out of 4, which is 1 in 2.

    3. Not this one. The Y sperm gives XY, so only the X sperm boxes read XX. Go to S4.2.4. Try another.

    See the answer

    2 boxes

    Two boxes read XX and two read XY. So the chance of a girl is 2 out of 4, which is 1 in 2.

    • If you chose A: Both eggs carry an X, so the two boxes with an X sperm both read XX. Go to S4.2.4.
    • If you chose C: The Y sperm gives XY, so only the X sperm boxes read XX. Go to S4.2.4.

  9. Look at the empty square for two Rr plants. When it is filled in, how many boxes read Rr?

    An empty Punnett square of four boxes. The mother, with R and r, is down the side and the father, with R and r, is across the top.fathermotherRrRr
    See the answer

    2 boxes

    The mother's R meets the father's r, and the mother's r meets the father's R. Two boxes read Rr.

  10. Two Rr plants are crossed and give 120 seeds. About how many are wrinkled?

    See the answer

    About 30 seeds

    One box in four is rr, so one seed in four is wrinkled. 120 ÷ 4 = 30.

  11. An Rr plant is crossed with an rr plant and gives 80 seeds. About how many are round?

    See the answer

    About 40 seeds

    Two boxes in four read Rr, which is round. That is half. 80 ÷ 2 = 40.

Keep it

Copy this into your exercise book.

Alleles are versions of a gene. A dominant allele shows with one copy. A recessive allele needs two. Genotype is the alleles, phenotype is what you see. Rr × Rr gives 3 round : 1 wrinkled.

Draw: A Punnett square for Rr × Rr, with the four boxes filled in and rr hatched.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (873 KB). It opens on any phone.