Pedigrees

Essential Question

How can a family tree show how a trait is inherited?

A pedigree is a family tree that follows one trait through several generations. By looking at who has the trait, you can often tell whether it is dominant or recessive and work out the chance that a future child will have it. In this lesson, you’ll learn the symbols, the clues for each pattern, and how to find probabilities from a pedigree.

Pedigree Symbols

• Squares are males and circles are females
• A filled symbol means the person has the trait
• A horizontal line joins two parents; a vertical line leads to their children
• Roman numerals show generations; numbers count people from left to right

A pedigree is a diagram of a family that follows one trait over several generations. People are shown as squares (males) and circles (females). A filled symbol means the person has the trait. A horizontal line joins two parents, and a line down from it leads to their children. Each generation is labeled with a Roman numeral, and each person has a number within the generation, so I-1 is the first person in the first generation. In this lesson, every trait is autosomal, which means its gene is on a numbered chromosome and not on the X or Y. Recessive conditions are usually uncommon, so assume that a person who married into the family is not a carrier unless the pedigree shows they must be.

Autosomal Recessive Traits

• Clue: two unaffected parents can have an affected child
• The trait can skip generations
• People with the trait are aa; unaffected parents of an affected child are both Aa
• Aa x Aa gives a 1/4 chance of an affected child

A recessive trait shows only in people with two copies of the allele (aa). Parents who do not have the trait but have an affected child must both be carriers (Aa), because the child got one a from each. In the pedigree, I-1 and I-2 do not have the trait, but II-2 does, so I-1 and I-2 are both Aa. Later, II-4 and II-5 do not have the trait, but their son III-3 does, so II-4 and II-5 are both carriers too. The trait skips generations: it is present in II-2, absent in III-1 and III-2, and appears again in III-3.

Autosomal Dominant Traits

• Clue: every person with the trait has a parent with the trait
• The trait usually appears in every generation
• Unaffected people are aa, so they cannot pass on the trait
• Aa x aa gives a 1/2 chance of an affected child

A dominant trait shows in a person with one or two copies of the allele (Aa or AA). Usually every person with the trait has an affected parent, so the trait appears in every generation. People without the trait are aa, so they cannot pass it on. In the pedigree, I-1 has the trait and passes it to two of his three children. Many people with a dominant trait are Aa, and an Aa parent with an aa partner has a 1/2 chance for each child to have the trait.

Probability from a Pedigree

• First work out the genotypes of the parents
• Then use a Punnett square for the cross
• Two carriers: 1/4 chance affected, and an unaffected child is a carrier with probability 2/3
• If a genotype is uncertain, multiply by the chance of that genotype

Parents’ genotypes	Chance a child is aa (recessive trait)	Why
Aa x Aa	1/4	1 of 4 boxes is aa
Aa x aa	1/2	2 of 4 boxes are aa
AA x Aa	0	No box is aa
aa x aa	1	All boxes are aa

To find a probability, start with the genotypes the pedigree tells you, then use a Punnett square. When a person’s genotype is uncertain, include the chance of each possibility. For example, an unaffected child of two carriers is AA, Aa, or Aa, so there is a 2/3 chance that this child is a carrier (a conditional probability, because we already know the child is not aa). If that child has a baby with an aa partner, the chance that the baby is aa is 2/3 x 1/2 = 1/3. The table shows the chance of an aa child for common crosses.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Find the Pattern, Dominant

The pedigree shows a trait. Is it autosomal dominant or autosomal recessive? Give two clues, and give the genotype of I-1.

Show the solution

1. Look for the clues: Every person with the trait (I-1, II-2, II-4, III-2, and III-4) has a parent with the trait. The trait appears in every generation.
2. Test recessive: A recessive trait would let two unaffected people have an affected child, but that never happens here. Also, II-2 (affected) has a partner without the trait, and a recessive trait would need that partner to be a carrier. Carriers are uncommon, so this is unlikely.
3. Conclusion: The trait is autosomal dominant.
4. Genotype of I-1: I-1 has the trait, so she has at least one A. Her child II-3 does not have it (aa), so II-3 got an a from I-1. I-1 is Aa.
5. Answer: Autosomal dominant: every affected person has an affected parent and the trait appears in every generation. I-1 is Aa.

Example 2: Probability from a Pedigree

A recessive trait appears in II-1. Neither parent, I-1 or I-2, has it. II-2 does not have the trait. (a) What are the genotypes of I-1 and I-2? (b) What is the probability that II-2 is a carrier? (c) II-2 has a child with a partner who has the trait (aa). What is the probability that the child has the trait?

Show the solution

1. (a) Parents’ genotypes: II-1 has the trait (aa), so each parent passed on an a. Neither parent has the trait, so each also has an A. I-1 and I-2 are both Aa.
2. (b) II-2: Aa x Aa gives AA, Aa, Aa, aa. II-2 does not have the trait, so aa is ruled out and 3 boxes remain. Two of the 3 are Aa. The probability that II-2 is a carrier is 2/3.
3. (c) The child: The child is aa only if II-2 is Aa (probability 2/3) and the child gets a from II-2 (probability 1/2, from Aa x aa). 2/3 x 1/2 = 1/3.
4. Answer: (a) Both Aa. (b) 2/3. (c) 1/3.

Example 3: Find the Pattern, Recessive

Is the trait in this pedigree autosomal dominant or autosomal recessive? Give the clue, and give the genotypes of II-4, II-5, and III-1.

Show the solution

1. The clue: II-2 and III-3 have the trait, but their parents do not. Two unaffected parents with an affected child means the trait is recessive.
2. Conclusion: The trait is autosomal recessive.
3. II-4 and II-5: III-3 has the trait (aa), so each parent passed on an a. Neither has the trait, so both are Aa.
4. III-1: II-2 has the trait (aa), so he can only pass on a. III-1 does not have the trait, so III-1 got an A from II-1. III-1 is Aa, assuming II-1 is not a carrier.
5. Answer: Autosomal recessive: unaffected parents have affected children. II-4 and II-5 are Aa. III-1 is Aa.

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Practice

Practice finding the pattern in a pedigree, writing genotypes, and finding the probability that a child has a trait. Try it on your own first, then check your work with the answer key.

• Pedigrees Practice Worksheet (PDF)
• Pedigrees Practice Answer Key (PDF)

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Lesson Slides

• 10.7 Pedigrees Slides (PDF)

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Big Ideas

• A pedigree follows one trait through a family
• Recessive: unaffected parents can have an affected child, and the trait can skip generations
• Dominant: every affected person has an affected parent
• Work out the parents’ genotypes, then use a Punnett square
• A genotype that is uncertain adds a chance to the calculation

Back to the essential question: A pedigree shows how a trait is inherited because the pattern of affected and unaffected people reveals whether the allele is dominant or recessive, and which people must carry it. Genotypes from the pedigree plus a Punnett square give probabilities for future children. Next, you’ll see how scientists copy, sort, and compare DNA.

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