1. Mendel and Punnett Squares
Genetics &Heredity:
Mendel and Punnett Squares

2. Standard: S7L3
Students will recognize how biological traits are passed on to
successive generations.
a. Explain the role of genes and chromosomes in the process of inheriting a specific trait.

3. Heredity The passing of traits from parents to offspring.
Traits- The characteristics of an organism.
**(any description)

4. Alleles A different form of a gene.
One allele may code for dimples, and the other for no dimples.

5. Parents
You get one allele from mom and one from dad.

6. Genetics
The study of how traits are inherited through the interactions of alleles.

7. Gregor Mendel
The first to study how traits were passed from one generation to the next.
He studied pea plants.

8. Mendel’s Experiments
Mendel crossed a pure breeding white flowered plant with a pure breeding purple flowered plant.
He got all purple flowers in the offspring.
BUT, then he crossed two of the second generation purple flowers and he got ¾ purple flowers and ¼ white flowers!

10. So….what does that mean?
From the results of his second cross, Mendel determined that one allele was dominant and the other allele was recessive.

11. Dominant vs. Recessive
Dominant Alleles- An allele that covers, or dominates another allele.
**the dominant allele is ALWAYS expressed when a dominant allele is present.
Recessive Allele- An allele that is covered, or hidden by another allele.
**TWO recessive alleles must be present for the recessive allele to be expressed.

16. Representing Alleles Letters! Upper case= dominant alleles. (R)
Lower case =recessive alleles.
(r)

18. Phenotype- Outward appearance, or behavior of an organism.
Genotype vs. Phenotype
Genotype- An organism’s genetic makeup-combination of alleles that determine the organism’s phenotype.
Phenotype- Outward appearance, or behavior of an organism.

21. Punnett Squares
A tool to predict the probability of certain traits in offspring.
Punnett squares show the different ways alleles can combine.

22. Homozygous
If the two alleles that you get from your mom & dad are the same, it is homozygous
(RR, rr) R R

23. Heterozygous
An organism that has two different alleles for a trait is called heterozygous. (Rr) R r

26. The tongue rolling allele is R.
Example:
The tongue rolling allele is R.
R is dominant
r is recessive

27. Your mom is homozygous dominant for the tongue rolling gene.
Her genotype is RR

28. Your dad is heterozygous for the tongue rolling gene.
His genotype is Rr

29. Probability
What is the probability that you will be able to roll your tongue?
Do a Punnett Square!

30. Making a Punnett Square
To set up a Punnett Square, draw a large square.
Then, divide it into four equal sections.

31. Dad’s Alleles go on top
Mom’s Alleles go on the side

32. Sperm r R R R
Eggs

33. Sperm r R RR Rr R RR Rr R
Eggs

34. Your genotype could be:
50% RR- Homozygous
Dominant
50% Rr- Heterozygous
Your phenotype would be:
100% Tongue Roller
0% No Tongue Roller

35. Hybrid vs. Purebred
Hybrids=Heterozygous- Have two different alleles for a gene: Rr
Purebreds=Homozygous- Have the same allele for a gene: RR or rr

36. Let’s Do Another One! The Big Nose Allele is N. Big Nose is N
Little nose is n

37. Mom is Heterozygous for Big Nose: Nn
Dad is Heterozygous for Big Nose: Nn
What is the chance you’ll have a big nose? Do the genetic cross to find out!

38. Sperm n N NN Nn N Nn nn n
Eggs

39. What is the genotype?
25% NN- Homozygous Dominant
50% Nn- Heterozygous
25% nn- Homozygous Recessive

40. What is the phenotype?
75% Big Nose
25% little nose

41. Incomplete Dominance
When the offspring of two homozygous parents show an “in-between” phenotype.

42. Example: Purebred white flowers Purebred red flowers Makes
“in-between” pink colored flowers