1. Introduction to Genetics
Mendel and His Pea Plants

2. Think About It
Has anyone ever told you that you look like your Mom? Or your Dad? Or an uncle, aunt, or a grandparent?
In what ways do you not look like the people in your family?

3. Some Words You Need to Know
Genetics – the study of heredity
Heredity – the passing of traits to offspring (children, young, descendents)
Traits – characteristics that parents can pass to offspring (ex: eye color, hair color, shape of eyebrows)

4. What traits do you share…
with your parents?
with your brothers or sisters?
with your cousins?
with other people in this class?

5. Gregor Mendel (1822-1884) “Father of Genetics” Austrian monk
Studied the traits
of pea plants

6. Why pea plants? Pea plants are easy to grow
Pea plants have a lot of traits that can be studied
A lot of their traits have only two varieties:
Tall or short
Round seeds or wrinkled seeds
Yellow seeds or green seeds
Purple flowers or white flowers
Green pods or yellow pods

7. Some Pea Plant Traits

8. More about plants
In plants, male and female reproductive parts are found in every flower. (There are no boy plants or girl plants.)
The male reproductive organs produce pollen (plant sperm).
The female reproductive organs produce eggs (ovules).
Self-pollination – when a plant fertilizes itself
Cross-pollination – when a plant is fertilized by another plant

9. Mendel’s Experiment Mendel controlled how plants were fertilized by:
removing the male parts from some flowers so they could not produce pollen. This means they could not self-pollinate.
using a brush to move pollen from one flower to another
In other words, Mendel decided which plants were going to be the parents of the offspring.

10. Mendel’s Experiment Part 1
First, Mendel crossed (bred) these two plants. He called these plants the P1 Generation.
Purebred tall X Purebred short
plant plant

11. Mendel’s Experiment Part 1
What size of offspring do you think this produced?
All of the offspring were…
TALL!

12. Mendel’s Experiment Part 1
Mendel called them purebreds because the short one came from a family of all short plants, and the tall one came from a family of all tall plants. You will learn a different meaning for purebred later.

13. Mendel’s Experiment Part 2
Next, Mendel self-pollinated the offspring from the first generation. He called them the F1 Generation.
Tall plant X Tall plant X

14. Mendel’s Experiment Part 2
What size of offspring of the F2 Generation do you think this produced?
Three out of four of the offspring were…
TALL!

15. Mendel’s Experiment Part 2
But, one out of four was…
SHORT!

16. What happened?!

17. What did Mendel figure out?
Each parent contributes a substance that controls the traits of their offspring.
For example, each parent contributes a substance for the height, the seed color, the seed shape, etc.

18. What we now know
Those substances that control traits are called genes and they are part of chromosomes.
Chromosomes, you should remember, are found in the nucleus of cells.

19. What we now know
We have two genes for every trait. One comes from the male parent and one comes from the female parent.
The different forms of a
gene are called alleles.

20. Some examples of genes & alleles
Mendel’s pea plants had two genes for their height. The alleles for that gene are short and tall.
The pea plants had two genes for flower color. The alleles for that gene are purple and white.
The pea plants had two genes for their seed shape. The alleles for that gene are round and wrinkled.

21. What else did Mendel figure out?
One allele can mask, or hide, the other allele.
If one parent gives the offspring an allele that says a plant should have green seeds and the other parent contributes an allele that says the offspring should have yellow seeds, one of them will mask the other one. (one allele will mask the other)

22. Mask Think about someone wearing a mask. What do you see? The mask.
Is your face still there?
Of course, it is just hidden.

23. More words to know
Dominant allele – an allele that can mask another allele and always “shows up” if present (meaning you will see it)
Recessive allele – an allele that can be masked by a dominant allele and only “shows up” if both alleles for a trait are recessive

24. Symbols used for alleles
Dominant alleles are always represented by a capital letter. The first letter of the dominant trait is usually used. For example, if tall is dominant in pea plants, then T represents that allele.
Recessive alleles are always represented by the lower case letter of the dominant allele. Since short is recessive in pea plants, t represents that allele.

25. More words to know
Remember: dominant alleles can mask another allele and always “shows up” if present
this is the BIG GUY
the allele is a
CAPITAL letter
(ex. T) T

26. More words to know
Remember: recessive alleles can be masked by a dominant allele and only “shows up” if both alleles for a trait are recessive
this is the little guy
that hides behind the BIG
guy
the allele is a lowercase
letter (ex. t) t

27. Symbols used for alleles
Dominant allele for being tall is T.
Recessive allele for being short is t.
So, if a pea plant has one allele for tall and one allele for being short, you would write T t. T t

28. Symbols used for alleles
Remember any time you have a dominant allele present, you are going to see him, not the recessive allele. T t

29. Questions to ask yourself
1. What is the dominant trait?
2. What is the first letter of that trait?

30. Are you getting it so far?
How would you write the symbols for a pea plant that has one allele for purple flowers and one allele for white flowers if purple is dominant?
What color flowers would that plant have? Pp
purple

31. Are you getting it so far?
How would you write the symbols for a pea plant that has two alleles for purple flowers?
What color flowers would that plant have? PP
purple

32. Are you getting it so far?
How would you write the symbols for a pea plant that has two alleles for white flowers?
What color flowers would that plant have? pp
white

33. Phenotypes & Genotypes
Phenotype – a physical trait of an organism (what it looks like or how you describe it)
ex. purple flower
Genotype – the genetic makeup of an organism; it determines the phenotype (or appearance) of the plant; represented by letters
ex. PP
Next year switch the order --

34. Phenotypes & Genotypes
Examples:
Genotype Phenotype
P P PURPLE flowers
P p PURPLE flowers
p p WHITE flowers

35. Try these Fill in the missing information. Phenotype Genotype tall _ t
short _ _

36. Try these Fill in the missing information. Phenotype Genotype tall T t
short t t

37. A few more words homozygous or purebred –
2 of the same alleles for a trait
examples: P P, p p, T T, t t
(can be homozygous dominant or homozygous recessive)
heterozygous or hybrid –
2 different alleles for a trait
examples: P p, T t

38. YouTube - The Gregor Mendel Rap

39. YouTube - Mr. Lee - Genetics rap

40. Now, on to Punnett Squares
Punnett squares are tools to help geneticists predict the probability of certain traits in offspring by showing all of the ways the parents’ alleles can combine.
Probability is the relative possibility that an event will occur. It is usually expressed in a percentage or fraction.
This website will explain punnetsquares.