Name: ____________________________
IB Biology SL Topic 3.4: The mystery of blond hair
AAC 2020-2021
Teacher: Ms. Beyyoudh
Aim: To apply the concept of inheritance to family trees and understand different patterns of inheritance.
Instructions:
1) You will review concepts in inheritance by first reading the background section in your group.
2) Next you will analyse the pedigree chart, look at who has the trait you are looking for and where
they got it from.
3) Then you will answer the questions below as a group after discussion.
4) Finally you will share your answers with the class and compare your conclusions.
Background:
What are Pedigree Charts?
Pedigree charts are used to show which individuals in a family have a specific trait. It uses symbols
to show the different people in different generations, a key is shown below in Figure 1. The chart includes
family ties and marriage as well as who has died. These were the first tools in trying to understand how
traits are inherited. These charts only show phenotype, you need to guess the genotypes from who has the
trait, so these can only be used to guess a pattern of inheritance. In Figure 2 a family pedigree is shown
where blond hair is tracked. All individuals affected by the trait have blond hair. You need to study the chart
then
analyse
it
to
determine
the
pattern
of
inheritance.
Name: ____________________________
IB Biology SL Topic 3.4: The mystery of blond hair
AAC 2020-2021
Teacher: Ms. Beyyoudh
What are Punnett Squares?
Geneticists soon understood that traits are passed on from parents to their children, especially after
the work of Gregor Mendel on breeding his peas. He discovered that genes come in different forms, now
called alleles, and that they are inherited independently of each other. We know now that that is because
gametes (egg and sperm) are haploid so only contain one copy of a gene. Since people can be
heterozygous for a gene, the children could inherit one of two types of alleles. An example is shown in
Figure 3 of a punnett square for eye color. The alleles each parent has are crossed to show the possible
combinations. From this we can predict the percent of children with each trait. These percentages are
especially true for large samples.
Questions:
1) Is the trait of having blond hair recessive or dominant? Explain your answer using evidence from the
pedigree chart.
2) Which individuals’ genotypes in the pedigree chart are known for sure? Explain your answer.
3) What are the possible genotypes for individual 2 of generation one? Explain your answer.
Name: ____________________________
IB Biology SL Topic 3.4: The mystery of blond hair
AAC 2020-2021
Teacher: Ms. Beyyoudh
4) What should the genotypes of individuals 1 and 2 of generation 2 be? Explain your answer.
5) What should the genotypes of individuals 1,2,3 and 4 of generation 3 be? Explain your answer.
6) What is the chance of individual 5 of generation 2 having a blond haired child if he married a blond
haired woman? Use a punnett square to explain your answer.
7) If both individuals 3 and 4 in generation 2 are heterozygous, what is the probability of them having
blond haired children? Is this the same as the percentage of blond haired kids they do have? If not,
why?
8) Challenge: Worldwide only about 2 % of people are naturally blond. In northern countries however
that number can go as high as 65% of the population. Suggest some reasons for this trend.