Chapter 6 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

S4.2.6 Chapter 6

Family trees and genetic disorders

Two healthy parents have a child with cystic fibrosis. How can that happen?

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

  • Read a family tree to find who carries an allele
  • Explain how sickle cell disease and cystic fibrosis are inherited

Some conditions run in families. Some are caused by a change in a gene or a chromosome. They are called genetic disorders. Doctors draw a family tree to follow an allele from one generation to the next.

In a family tree, a square stands for a male and a circle for a female. A line joins a couple, and a line down from it leads to their children. A person with the condition is hatched all over. A person with one copy of the allele and no signs is hatched on one half, and is a carrier. A tree hatches half a symbol only when a test has shown that the person is a carrier. A plain symbol means no signs of the condition, and the person may still be a carrier.

Cystic fibrosis is caused by a recessive allele. The gene makes a protein in the membranes of cells. Without it, the body makes thick, sticky mucus that clogs the lungs and the gut. People with the condition can have trouble breathing, many chest infections, and trouble digesting food. Doctors and health workers can help manage it.

We write the usual allele as F and the cystic fibrosis allele as f. FF is not affected. Ff is a carrier and is healthy. Only ff has cystic fibrosis. A carrier is a person with one copy of a recessive allele and one copy of the dominant allele.

Now look at a family tree. The daughter has cystic fibrosis. Her parents and her brother do not.

A family tree with two generations. Generation I has a father, I-1, a square, and a mother, I-2, a circle, joined by a line. Generation II has two children: II-1, a daughter, a hatched circle, and II-2, a son, a plain square. The key names male, female, affected and unaffected.I-2I-1II-1II-2IIImalefemaleaffectedunaffected
The daughter has cystic fibrosis. Her parents and her brother do not.

Reason it out. The daughter has the condition, so her genotype is ff. She got one f from each parent. Her parents are healthy, so each also has an F. So both parents are Ff, and both are carriers. Her brother is healthy, so he is FF or Ff. The tree cannot tell us which.

A rule follows. Two healthy parents may have a child with a condition like this. Suppose one gene on one of the 22 numbered pairs causes it, and the child got the allele from both parents. Then the allele must be recessive, and both parents must be carriers. Rarely, one of the two copies in the child is new, from a mutation in an egg or a sperm. Then only one parent is a carrier. Many carriers never know it until a child is born with the condition.

Draw it first

Cross two carriers

  1. A Punnett square. The mother, with F and f, is down the side and the father, with F and f, is across the top. In the first picture the four boxes are empty. In the second picture they read FF, Ff, Ff and ff. In the third picture the box ff is hatched and the key says cystic fibrosis.fathermotherFfFf

    1 Write the mother's alleles, F and f, down the side. Write the father's alleles, F and f, across the top.

  2. A Punnett square. The mother, with F and f, is down the side and the father, with F and f, is across the top. In the first picture the four boxes are empty. In the second picture they read FF, Ff, Ff and ff. In the third picture the box ff is hatched and the key says cystic fibrosis.fathermotherFfFfFFFfFfff

    2 Fill each box. The boxes read FF, Ff, Ff and ff.

  3. A Punnett square. The mother, with F and f, is down the side and the father, with F and f, is across the top. In the first picture the four boxes are empty. In the second picture they read FF, Ff, Ff and ff. In the third picture the box ff is hatched and the key says cystic fibrosis.fathermotherFfFfFFFfFfffcystic fibrosis

    3 Hatch the box with the condition. Only ff has cystic fibrosis.

Worked example

Both parents are carriers of cystic fibrosis. What is the chance that a child has the condition, is a carrier, or is not a carrier?

  1. Each parent gives F or f, so the square has 4 boxes. They read FF, Ff, Ff and ff.
  2. Boxes that read ff, so the child has the condition: 1.
  3. Boxes that read Ff, so the child is a carrier: 2.
  4. Boxes that read FF, so the child is not a carrier: 1.
  5. So ff is 1 out of 4, which is 1 in 4. Ff is 2 out of 4, which is 1 in 2. FF is 1 out of 4, which is 1 in 4.

Answer: Cystic fibrosis 1 in 4, carrier 1 in 2, not a carrier 1 in 4

The chance is for each child, and each child is a fresh chance. Two carriers can have four healthy children, or two children with the condition. The ratio is a chance, not a promise.

Sickle cell disease is also caused by a recessive allele. The gene makes part of haemoglobin, the protein in red blood cells that carries oxygen. We write the usual allele as H and the sickle allele as h.

A person with hh makes a different haemoglobin. Their red blood cells can bend into a curved, sickle shape. These cells can block small blood vessels and cause pain. They also break easily, which can cause tiredness, and they raise the risk of infection. Doctors and health workers can help manage it.

A table with the columns Alleles and Result. HH is no disease. Hh is carrier. hh is disease, which means sickle cell disease.AllelesResultHHno diseaseHhcarrierhhdisease
Sickle cell disease needs two copies of h. A person with Hh is a carrier and is usually healthy.

The sickle allele is common in some parts of the world. The reason is malaria. Malaria is a disease caused by a tiny parasite, which mosquito bites pass to people. A carrier, with one h and one H, is less likely to become severely ill with malaria than a person with HH.

So in places where malaria is common, carriers were more likely to survive and to have children. The allele stayed common there. Two copies cause the disease, so the allele does not spread to everyone. The result is a balance.

Worked example

A man is a carrier of cystic fibrosis. His partner has cystic fibrosis. What is the chance that their child has it?

  1. The man is Ff. His partner is ff, so she can give only f.
  2. The four boxes read Ff, ff, Ff and ff. Boxes that read ff: 2.
  3. The chance is 2 out of 4, which is 1 in 2.

Answer: 1 in 2

A family tree with two generations. Generation I has a father, I-1, a square, and a mother, I-2, a circle, both with half the symbol hatched, which means carriers. Generation II has three children: II-1, a daughter with all of the symbol hatched, which means affected; II-2, a son who is a plain square; and II-3, a daughter who is hatched on one half, a carrier. The key names male, female, affected, carrier and unaffected.I-2I-1II-1II-2II-3IIImalefemaleaffectedcarrierunaffected
Sickle cell disease in one family. Tests show that both parents are carriers, Hh. II-1 has hh and II-3 is a carrier, Hh. II-2 has no signs, so he is HH or Hh. Only a test can tell which.

Try it

Draw your family tree

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

  1. Draw a square for each male and a circle for each female in your family. Put the oldest generation at the top.
  2. Join each couple with a line. Draw a line down from it to the children of the couple.
  3. Write a name under each symbol. Write "I" beside the top row, "II" beside the next row, and so on.
  4. Look at your tree. Which people are in the same generation as you?

What you will see: A family tree shows who is related to whom. Doctors add shading to show who has a condition and who is a carrier. Draw only names for now. Do not write a person's health on a tree without their wish.

If you cannot try it, this is what happens.
A family tree with two generations. Generation I, marked I beside the top row, has Kofi, a square, and Amina, a circle, joined by a line. Generation II, marked II beside the next row, has their two children, Mei, a circle, and Ravi, a square. The key names male and female.AminaKofiMeiRaviIIImalefemale

A wrong idea many people have

Cystic fibrosis and sickle cell disease can be caught from another person.

Why it feels right: Many illnesses are caused by germs that spread, so any illness may seem catchable.

The test that breaks it: Look at the family tree. The condition follows the alleles a child receives from their parents. No germ is involved, and touching or living with a person with the condition cannot pass it on.

What is true: These conditions are inherited. A child is born with the alleles. They are not infections, and nobody is to blame.

Key words

carrier
a person with one copy of a recessive allele. A carrier usually has no signs and can pass the allele on.
genetic disorder
a condition caused by a change in a gene or a chromosome. Many are passed on from parents to children.
family tree
a drawing that shows how people are related, and who has a condition.

Back to your guess

The answer is B: Both parents carry one copy of a hidden allele, and the child got a copy from each.

Cystic fibrosis is caused by a recessive allele. A parent with one copy is healthy, but can pass the copy on. A child who gets a copy from each parent has the condition.

Check yourself

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

  1. In the picture, II-2 has cystic fibrosis. His partner, II-3, and their two children have no signs of it. What are the genotypes of the two children?

    A family tree with three generations. Generation I has a father, I-1, a plain square, and a mother, I-2, a plain circle, joined by a line. Generation II has three people: II-1, a plain circle, the daughter of I-1 and I-2; II-2, a hatched square, their son; and II-3, a plain circle, his partner. Generation III has two plain children of II-2 and II-3: III-1, a circle, and III-2, a square. The key names male, female, affected and unaffected.I-2I-1II-1II-2II-3III-1III-2IIIIIImalefemaleaffectedunaffected
    Tap your answer.

    1. Not this one. A child with ff would have cystic fibrosis. Both children here have no signs of it. Try another.

    2. Yes, that is right. II-2 has cystic fibrosis, so he is ff and gives each child an f. Each child is healthy, so each also has an F. Both children are Ff, carriers.

    3. Not this one. II-2 is ff, so every child gets an f from him. No child can be FF. Try another.

    See the answer

    Both Ff

    II-2 has cystic fibrosis, so he is ff and gives each child an f. Each child is healthy, so each also has an F. Both children are Ff, carriers.

    • If you chose A: A child with ff would have cystic fibrosis. Both children here have no signs of it.
    • If you chose C: II-2 is ff, so every child gets an f from him. No child can be FF.

  2. What is a carrier?

    Tap your answer.

    1. Yes, that is right. A carrier has the genotype Ff or Hh. The dominant allele hides the recessive one, so a carrier is usually healthy. They can pass the recessive allele on.

    2. Not this one. That person has the condition. A carrier has only one copy. Try another.

    3. Not this one. Genetic disorders are not infections. They are passed on only in the sex cells, as alleles. Try another.

    See the answer

    A person with one recessive allele and one dominant allele

    A carrier has the genotype Ff or Hh. The dominant allele hides the recessive one, so a carrier is usually healthy. They can pass the recessive allele on.

    • If you chose B: That person has the condition. A carrier has only one copy.
    • If you chose C: Genetic disorders are not infections. They are passed on only in the sex cells, as alleles.

  3. Two carriers of cystic fibrosis have a child. What is the chance that the child has cystic fibrosis?

    Tap your answer.

    1. Not this one. That is the chance of a carrier child. Only one box in four reads ff. Go to S4.2.5. Try another.

    2. Not this one. Three in four is the chance that the child does not have the condition. Try another.

    3. Yes, that is right. Only one of the four boxes reads ff. So the chance is 1 in 4 for each child.

    4. Not this one. Both parents can pass on f. A child who gets f from each has the condition. Try another.

    See the answer

    1 in 4

    Only one of the four boxes reads ff. So the chance is 1 in 4 for each child.

    • If you chose A: That is the chance of a carrier child. Only one box in four reads ff. Go to S4.2.5.
    • If you chose B: Three in four is the chance that the child does not have the condition.
    • If you chose D: Both parents can pass on f. A child who gets f from each has the condition.

  4. Why is the sickle cell allele common in places where malaria is common?

    Tap your answer.

    1. Not this one. Malaria is spread by mosquitoes and it does not change a gene. Try another.

    2. Yes, that is right. One copy of the sickle allele gives some protection from severe malaria. Carriers did better where malaria is common, and the allele stayed common there.

    3. Not this one. The sickle allele is recessive. And dominant never means that an allele spreads. Go to S4.2.5. Try another.

    See the answer

    Carriers are less likely to become severely ill with malaria, so they live to pass the allele on.

    One copy of the sickle allele gives some protection from severe malaria. Carriers did better where malaria is common, and the allele stayed common there.

    • If you chose A: Malaria is spread by mosquitoes and it does not change a gene.
    • If you chose C: The sickle allele is recessive. And dominant never means that an allele spreads. Go to S4.2.5.

  5. A man with sickle cell disease (hh) has children with a woman with the genotype HH. What is true of their children?

    Tap your answer.

    1. Yes, that is right. The father gives h and the mother gives H. Every child is Hh, which is a carrier.

    2. Not this one. To have the disease a child needs h from both parents. The mother has only H, so no child has hh. Try another.

    3. Not this one. The father has only h to give, so every child gets one h. Try another.

    See the answer

    All of them are carriers.

    The father gives h and the mother gives H. Every child is Hh, which is a carrier.

    • If you chose B: To have the disease a child needs h from both parents. The mother has only H, so no child has hh.
    • If you chose C: The father has only h to give, so every child gets one h.

  6. Two healthy parents have a child with a genetic disorder caused by one gene on one of the 22 numbered pairs. The child has two copies of the allele, one from each parent. What does this tell us about the allele?

    Tap your answer.

    1. Not this one. A dominant allele shows in anyone who has one copy. Healthy parents would not have it. Try another.

    2. Yes, that is right. The child must have two copies, one from each parent. Both parents are healthy, so the allele is hidden in them: it is recessive.

    3. Not this one. The question says the gene is on one of the 22 numbered pairs, and the child got a copy from each parent. So both parents carry it. Try another.

    See the answer

    It is recessive, and both parents are carriers.

    The child must have two copies, one from each parent. Both parents are healthy, so the allele is hidden in them: it is recessive.

    • If you chose A: A dominant allele shows in anyone who has one copy. Healthy parents would not have it.
    • If you chose C: The question says the gene is on one of the 22 numbered pairs, and the child got a copy from each parent. So both parents carry it.

  7. From an earlier chapter

    Two plants are crossed, Tt with tt. T is the dominant allele for tall and t is for short. What is the chance of a short offspring?

    Tap your answer.

    1. Not this one. That is the chance when two Tt plants are crossed. Here the square has two boxes of tt. Go to S4.2.5. Try another.

    2. Yes, that is right. Two of the four boxes read tt, which is short. So the chance is 2 out of 4, which is 1 in 2.

    3. Not this one. Three in four is the chance of a tall plant when two Tt plants are crossed, not this cross. Go to S4.2.5. Try another.

    See the answer

    1 in 2

    Two of the four boxes read tt, which is short. So the chance is 2 out of 4, which is 1 in 2.

    • If you chose A: That is the chance when two Tt plants are crossed. Here the square has two boxes of tt. Go to S4.2.5.
    • If you chose C: Three in four is the chance of a tall plant when two Tt plants are crossed, not this cross. Go to S4.2.5.

  8. Look at the empty square for a carrier mother and a carrier father of sickle cell disease. How many boxes will read Hh when it is filled in?

    An empty Punnett square of four boxes. The mother, with H and h, is down the side and the father, with H and h, is across the top.fathermotherHhHh
    See the answer

    2 boxes

    The mother's H meets the father's h, and the mother's h meets the father's H. Two boxes read Hh, so the chance of a carrier child is 1 in 2.

  9. Among 400 children whose parents are both carriers of cystic fibrosis, about how many are expected to have the condition?

    See the answer

    About 100 children

    One child in four has ff. 400 ÷ 4 = 100. The real number will be near 100, but not exactly 100.

  10. Many couples are each a carrier (Ff) and a person with cystic fibrosis (ff). They have 120 children in all. About how many have the condition?

    See the answer

    About 60 children

    Two of the four boxes read ff, so half of the children have the condition. 120 ÷ 2 = 60.

Keep it

Copy this into your exercise book.

Carriers have one copy of a recessive allele and are usually healthy. Two carriers: 1 in 4 affected, 1 in 2 carriers, 1 in 4 not carriers. Squares are males, circles are females.

Draw: A family tree of two carrier parents and their children, with the Punnett square beside it.

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.