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Heredity Unit
LESSON PLAN: DAY TWO
Standards
LS 13: The student will investigate and understand that organisms reproduce and transmit
genetic information to new generations. Key concepts include…
c.) genotypes and phenotypes
Resources
Worksheets and materials required for this lesson are in a document called “Unit –
materials – genotypes and phenotypes”
Topic
Punnett Squares and predicting traits.
Instructional Objective(s)
Students will understand that…
 Probability is useful for determining the likelihood of events occurring in Science.
Students will know…
 The definitions of the following terms: probability, codominance, genotype and
phenotype.
 How to perform a single trait cross.
Students will be able to…
 Describe the difference between:
 Dominant and recessive traits.
 Genotype and phenotype
 Predict physical traits based using a Punnett Square single cross.
Materials & Resources
- PowerPoint presentation (see “engagement” section)
- A set of cards for groups of 4 students (so, 6 sets). Each card should have a fur colour
on one side (the phenotype) and, on the other side, a corresponding genotype. Each SET
of cards should have about 12-15 cards in them. Fur should be coloured grey and brown,
with more grey cards than brown, but not adhering to a certain ratio. 4 of the cards are
the “baby” furs – these are smaller and they should have no genotype on the back.
- A sheet with 4 empty Punnett Squares on the front and definitions on the back for each
student (see “exploration” section).
- A large Punnett Square on the Interactive Whiteboard (and so IW mat and pen).
- 2 probability worksheets that aid activity.
Engage
PowerPoint Presentation
To engage students, a PowerPoint presentation will be used that depict the genetics of
Marsupials. Marsupials – which happen to be predominantly Australian – vary in the gene
for coat colour. Normally this variation is from grey (dominant) to brown (recessive). The
slide show will simply expose students to many of these different animals... before looking
at animal in particular: the koala.
When this slide comes up, students will be asked:
- What is a trait?
- What trait(s) have you noticed that seem fairly common in the marsupials
shown?
- What genetically inherited trait do you see that is different in the two pictures of
the koala?
- Which of these traits, thinking about the general population of koalas, do you
think is more common?
- What is a trait called that is more common than another? (dominant)
- So, what would a trait be called that is less common than another? (recessive)
- So, what is the dominant/recessive trait in each picture? (i.e. dominant = grey,
recessive = brown)
- Judging by the pictures shown, do you think this can be generalised for all
marsupials?
Explore
Students should be given one of the packs of cards (see materials) to use as a group of 4.
Task 1: Exploring phenotypes
- Students are told that each card represents the coat of a father, a mother or a baby.
- Students are asked, ONLY using the picture side of the card, to make as many
combinations of a father, mother and baby as they think possible and lay them on their
tables. (Model one for students: for example, “I could make a guess that if the father had
a grey coat, the mother had a grey coat, then the baby would also have a blue grey
coat.”) Ask students to make four guesses in this way (see worksheet “What do you
think?”.
- After a few minutes ask each group to tell the class a combination they suggested and
WHY they suggested it.
- Say to students “if I told you that what you have been looking at are phenotypes, what
would you say a phenotype is?” (definition: the physical appearance of an organism).
- Ask students if they think that this is an accurate way to predict the colour coat of the
baby. What might be a better way?
Task 2: Exploring genotypes
- Have students flip their cards and discuss in their groups for a minute or two what the
letters on the back might mean. Tell students that some helpful words to use are allele,
dominant and recessive. (Students should have been introduced, in previous classes, to
alleles and to the idea that they can be represented by letters AND to the idea of recessive
and dominant alleles… but not to a combination of them as TRAITS.)
- Ask for student suggestions – stressing that EACH trait is made up of 2 alleles and that
one is often dominant and the other recessive. Ask students how they think that the code
on the back is related to the picture on the front. DON’T move on until they understand
how a genotype relates to a phenotype (without using those terms).
- Tell students that we can use that code (the two alleles for each trait) to predict more
accurately the physical traits of babies from their families.
- Hand out a sheet with four empty Punnett Squares on it (see worksheet labelled “. Ask
students to choose a mother and father from one of their combinations and add the
genotypes to the Punnett Square. (Show them how to do this on the Interactive
Whiteboard). Tell students that, when a baby is formed, they inherit one allele from one
parent and one from another. Tell them that we cannot say for sure which allele from the
father will be chosen, or which from the mother, but that we can make predictions).
REMIND students that these are all the POSSIBILITIES of the physical features of the
baby.
- Ask students to perform this procedure (producing their own Punnet Squares) for each of
the combinations they had made
- Write if the fur is grey or brown in the table underneath each possible genotype.
- Ask students if, after finding all the possibilities for the baby’s eyes, if the one they had
predicted was there or not.
Explain
This section should allow students to apply their knowledge as they consider, still, the
components of Punnett Squares.
- Hand 2 coins to each pair. On each of these coins, “heads” is marked with a “G” and
“tails” is marked with a “g”.
- Tell students that each coin represents a Koala (i.e. each of these koalas has a Gg
genotype).
- Tell students that this is a model for the characteristics of koala offspring. Students are
to toss the coin 25 times and record the combination each time (i.e. what the baby koala
inherited from the mother, and what from the father).
- Students should record this in the table (on the worksheet “in practice”)...
- And they should follow the directions to find the PROBABILITY of each genotype and
make some comment as to the most LIKELY genotype to occur.
Extend
- Students are then given the opportunity to find out whether this practical result reflects
the theory learnt through this lesson on Punnett Squares... This is done through producing
a Punnett square of the experiment, finding the probability and making a comparison
between this and the one “in practice” (conclusion).
- Students will be given time to discuss their results in class.
- A final discussion (comparison) of codominance and a lead in to mutations is done
through PowerPoint.
Evaluate
Hand students a piece of paper with the following questions to be turned in as an “exit
slip”:
Q.1. Fill in the following Punnett Square…
Father
B
Mother
B
B
B
If B = the allele for brown hair and if b = the allele for blonde hair…
Q.2. What colour hair (PHENOTYPE) does each parent have?
Q.3. What is the probability that a child will have blonde hair?
Connections
This lesson would have followed a basic study of Mendel and of his contributions to
science. This lesson assumes the knowledge of, in particular, the following key terms…
- Heredity: The passing of physical characteristics from a parent to a child.
- Trait: A physical characteristic.
- Genes: Factors that control a trait.
- Alleles: Different forms of a gene.
- Dominant allele: one whose trait ALWAYS shows up in the organism where the allele is
present.
- Recessive allele: one whose trait is hidden in the presence of dominant allele.
- Hybrid: 2 different alleles for a particular trait.
It is also assumed that the students know how the relevance of these terms to the field of
genetics and can use them to describe real traits in people.
Safety Considerations
There are no specific safety hazards that would be a concern in this lesson.