K327 / K328 / 2027

Lesson 7 of 9 / Inheritance

Pedigrees and human disorders

How can a family tree show whether a disorder is caused by a dominant or a recessive allele?

I can read pedigrees of human disorders, set out genetic diagrams for them and calculate combined probabilities such as an affected boy.

  • I can solve monohybrid problems using genetic diagrams.Syllabus

    Syllabus K327 / K328, Topic 12(f). Solve monohybrid problems using genetic diagrams.

  • I can describe usual XX/XY sex determination.Syllabus

    Syllabus K327 / K328, Topic 12(g). Describe usual XX/XY sex determination.

About 7 min

Make a guess. You are not marked.

A disorder appears in a grandparent, skips the parents and appears again in a grandchild. Is the allele more likely dominant or recessive?

  1. Dominant, because it keeps coming back.
  2. You cannot tell anything from a family tree.
  3. Recessive, carried hidden by the parents.
Show the answer

Recessive, carried hidden by the parents.

Carriers pass a recessive allele on without showing it, so the disorder can skip a generation.

The key idea

Two unaffected parents with an affected child show a recessive allele; two affected parents with an unaffected child show a dominant one. Multiply independent chances, such as 1/4 affected x 1/2 boy.

A pedigree is a family tree that shows which members of a family have a particular characteristic.

Read a family pedigree
Original teaching pedigree across three generations. Squares are males and circles are females. Shaded symbols are affected. A horizontal line joins two parents, and a vertical line leads down to their children. Roman numerals number the generations.IIIIIII-1I-2II-1II-2II-3II-4III-1III-2

male female shaded: affected

Squares are males and circles are females. Shaded symbols are affected. A horizontal line joins two parents, and a vertical line leads down to their children. Roman numerals number the generations.

Explanation

In a pedigree, squares are males and circles are females. A shaded symbol is an affected person. A horizontal line joins two parents, and a vertical line leads down to their children. Generations are numbered I, II, III, and people are numbered from left to right, so II-3 names one person.

Many human disorders are caused by a single gene. Cystic fibrosis is caused by a recessive allele: a person with two copies makes thick, sticky mucus that blocks the airways and the pancreatic duct. Sickle cell anaemia and phenylketonuria (PKU) are also caused by recessive alleles. Achondroplasia, a form of dwarfism, is caused by a dominant allele, so one copy is enough to show it. A carrier is a heterozygous person who has the recessive allele but does not show the disorder.

To decide whether the allele is dominant or recessive, find two parents with the same phenotype and a child who differs. If two unaffected parents have an affected child, the allele is recessive: both parents must be carriers. If two affected parents have an unaffected child, the allele is dominant: both parents must be heterozygous. Name the individuals you used, because the mark is for the evidence.

Then write the genotypes. For a recessive disorder, every affected person is homozygous recessive, and every unaffected parent of an affected child is heterozygous. If an unaffected person could be either, write both, such as FF or Ff. Show your working as a full genetic diagram, with labelled rows and circled gametes.

Some questions combine two chances. The chance of an affected child from Ff x Ff is 1/4. The chance that any child is a boy is 1/2, because half the sperm carry an X chromosome and half carry a Y. These two events are independent, so the chance of an affected boy is 1/4 x 1/2 = 1/8. Read the question carefully: "the chance that a boy is affected" means you already know it is a boy, so the answer is 1/4.

Step by step
  1. 1

    Dominant or recessive?

    Find two parents with the same phenotype and a child who differs. Name them.

  2. 2

    Write genotypes

    Start with people whose genotype is certain, such as affected people for a recessive disorder.

  3. 3

    Draw the cross

    Use a full genetic diagram with circled gametes for the parents in the question.

  4. 4

    Combine chances

    Multiply the chance of the phenotype by 1/2 if the sex is also specified.

Worked example

Cystic fibrosis in a family

Two unaffected parents have a son with cystic fibrosis. Using F and f, give the parents' genotypes and the chance that their next child is a girl with cystic fibrosis.

One way to explain it

The son is ff, so each parent passed on f. Both parents are unaffected, so each is Ff, a carrier. Ff x Ff gives FF, Ff, Ff and ff, so the chance of an affected child is 1/4. The chance of a girl is 1/2, so the chance of an affected girl is 1/4 x 1/2 = 1/8.

Why this answer works
  • Work backwards from the affected child.
  • Use the parents' unaffected phenotype to add the F allele.
  • Multiply by 1/2 only because the sex is specified.

Watch out for this

A student says: "If a disorder skips a generation, it must be caused by a dominant allele." What is wrong with this?

Show the answer

Skipping a generation is the sign of a recessive allele: carriers pass it on without showing it. A dominant allele shows in everyone who has it, so it cannot be hidden in a parent.

Check your understanding

Two parents who are both carriers of a recessive allele have a child. What is the chance that the child is an affected boy?

  1. 1/8, because 1/4 affected x 1/2 boy.
  2. 1/2, because half the children are boys.
  3. 3/4, because three in four children have the dominant allele.
  4. 1/4, because one in four children is affected.
Show the answer

1/8, because 1/4 affected x 1/2 boy.

Being affected and being a boy are independent, so multiply the two chances.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can a family tree show whether a disorder is caused by a dominant or a recessive allele?

You can return to this lesson any time.