1. Biology 1-3a & 1-3b
Mendelian Genetics

2. 1-3: Mendelian Genetics
Genetics is the study of genes, variation in genes throughout generations, and heredity (the ways in which genes are past on from generation to generation) in living things.

3. 1-3: Mendelian Genetics
Gregor Mendel ( ) was an Austrian Monk with scientific training in mathematics, physics and biology.
His experiments laid the foundation for everything we know about genetics today.
He is often called the Father of Genetics

4. 1-3: Mendelian Genetics
Before we take a closer look at exactly what Mendel did to receive such a major title, we need a little more background…

5. 1-3: Mendelian Genetics
We inherit 23 chromosomes from our mother and 23 chromosomes from our father.
These chromosomes contain genes for the same type of information, so essentially we have two versions of every gene in our DNA.
Each version of the gene we inherit is called an allele.
So you inherit one allele from your mother and one from your father

6. 1-3: Mendelian Genetics

7. 1-3: Mendelian Genetics Mendel’s Experiments:
In order to understand what Mendel did in his experiments, we need a basic understanding of plant reproduction:

8. The male reproductive part of the flower is called the stamen.
From the anther located at the top of the stamen, pollen (plant sperm) is released.

9. The female reproductive part of the flower is called the pistil.
The sticky top of the pistil, called the stigma, collects the pollen which moves down into the ovary to create the
next generation of
pea plants.

10. Now, because pea plants have BOTH male and female reproductive parts, it is possible for them to self-pollinate themselves.
Also, because the pollen grains are usually distributed by either the wind or pollinators, such as bees, it is very difficult to know which plants are reproducing with each other
For his experiments, Mendel needed to control exactly what plants were reproducing with each other…

11. In order to control the breeding, Mendel cut the stamen off of half of the flowers
He then brushed the pistil of these stamen-less flowers with pollen that he had collected from a chosen plant
Finally, the flowers were covered with a bag to prevent stray pollen grains from finding their way in.

12. 1-3: Mendelian Genetics Mendel’s Experiments – Phase One
The first thing Mendel did was to isolate true breeding strains of peas with distinctive traits.
To do this he crossed pea plants with, for example, purple flowers with other purple flower pea plants until all of their offspring were consistently purple.

13. 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two
Once he had isolated his true breeding strains, he began to cross-breed them and analyze what their offspring looked like.
The first cross was between a true breeding white flowering plant and a true breeding purple flowering plant
ALL of their offspring (the F1 generation) were purple!

14. 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two
Next, he took two of the offspring from this first breeding test and bred them to each other
Yes, it’s technically incest, but they’re flowers!
75% of the new offspring were purple, but 25% were white again!

15. Now keep in mind that the prevalent thought of the day was that if a purple flower and a white flower were bred, they should create a light purple flower
For Mendel to have produced white flowering plants by breeding two purple flowering plants….well, these were very strange results indeed!

16. 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two
Mendel’s next step was to take plants from the F2 generation and breed them with each other to try and determine if they were “true breeding plants” or not
He discovered that the white plants WERE true breeding, but only 1/3 of the purple plants were.
The other 2/3 of the purple plants would product a mix of purple and white offspring

17. Mendel repeated this type of experiment with six other traits, examining thousands of pea plants
In all cases, one trait disappeared in the F1 generation and then reappeared in the F2 generation.

18. What in the world does this all mean?

19. 1-3: Mendelian Genetics Mendel’s Logic:
If Mendel crossed purple peas with white peas he got all purple peas
When these purple peas were crossed with each other he got 3 purple to 1 white
He decided to use symbols to represent what happened. He used a P for purple and a W for white. He represented the cross as follows:
In the parent generation P x W
In the F1 generation all W
In the F2 generation P to 1W

20. 1-3: Mendelian Genetics Mendel’s Logic:
Mendel reasoned that if there were no “W” traits present in the F1 generation, they must have died out and should never reappear
When the W trait reappeared in the next generation, Mendel reasoned that it must have been present but not seen in the previous generation.
He concluded that there must be two factors for every trait present in each individual, but that one factor could be masked by another
His new theory was represented in the cross as follows:
In the parent generation PP x PW
In the F1 generation all PW
In the F2 generation PP to 2PW to 1WW

21. Ok, confusion about all the letters aside, what Mendel had discovered was the existence of two “factors” (what we now know as genes or alleles) for each genetic trait!!
Even Mendel realized that all the lettering was confusing, so he developed some new terminology for genetics…

22. 1-3: Mendelian Genetics Genetics Terminology:
The appearance of an organism is its phenotype
i.e. purple or white
The genetic makeup of an organism is its genotype
i.e. WW or PP or PW
The original, true-breeding strain is called the parental (P) generation
The first generation of offspring are the F1 generation
The second generation of offspring are the F2 generation

23. 1-3: Mendelian Genetics Genetics Terminology:
The ability of one factor to mask the appearance of another makes it dominant.
The ability of a factor to be masked by another makes it recessive.
To reduce the amount of letters used, when choosing letters for the genotype use the letter of the dominant characteristic
True breeding purple = PP
True breeding white = pp

24. Now that we know the terminology, how do we predict the genetic makeup of future generations?
By using a PUNNET SQUARE

25. 1-3: Mendelian Genetics The Punnet Square:
Maternal and paternal genotypes are written on the outsides
Fill in the boxes based on the alleles present

26. 1-3: Mendelian Genetics The Punnet Square:
Show the P generation cross between a true breeding purple and a true breeding white plant
What would the phenotypes of the offspring be?
What would the genotypes of the offspring be?

27. 1-3: Mendelian Genetics The Punnet Square:
Show an F1 generation cross from the previous question
What would the phenotypes of the offspring be?
What would the genotypes of the offspring be?

28. A bit more vocabulary…
If an individual has two copies of the same allele, they are said to be homozygous
Eg. PP or pp
If an individual has two different copies of an allele, they are said to be heterozygous
Eg. Pp

29. A bit more vocabulary…
If an individual has two copies of the same DOMINANT allele, they are said to be homozygous dominant
Eg. PP
If an individual has two copies of the same RECESSIVE allele, they are said to be homozygous recessive
Eg. pp