1. The First Geneticist & Pea Plants
7D60 Mendel
The First Geneticist & Pea Plants

2. Background
Using a model for how inheritance works is based on the studies of Gregor Mendel.
He discovered the behavior of genes 40 years before scientists learned where genes are located in the cell AND almost 100 years before scientists discovered what the genes are made of.

3. Mendel’s Life: 1
In 1865, Gregor Mendel, an Austrian monk, published his work on the behavior of genes.
He studied math and botany for several years.
His agricultural research studied breeding of plants to make better food & other products.
He was interested in pea plants and wondered how the colors, shapes, & heights of offspring pea plants were related to the parent plants.

4. Mendel’s Life: 2
His trait discoveries depended on research, careful experiments, and creative thinking.
He understood that plant seeds are produced by sexual reproduction, and he understood the mathematics of probability, too.
Careful planning & recording showed patterns that apply to ALL organisms that reproduce sexually, and help us explain some diseases.

5. Stopping To Think: 1
What personal qualities do you think Mendel must have had that helped him in his work?
Write these in your lab book, and be prepared to discuss them with the class.

6. Mendel’s Experiments: 1
Pea plants were excellent to study because:
They grow from seeds in about days,
The produce numerous seed rapidly,
They have many observable characteristics that come in just 2 types (like purple or white flowers) and with no blending of these traits (like no lavender flowers).

7. Mendel’s Experiments: 2
He decided to breed large numbers of plants & search for simple patterns in the offspring.
He did hundreds of crosses and counted the offspring traits for thousands of pea seeds, pods, and flowers.
He then analyzed all these results and used statistics and probability to infer the behavior of individual genes.

8. Pea Plant Experiment

9. Mendel’s Results: 1 Generation Flower Color Seed Color Seed Surface
Pod Color
Original Cross Generation 1
purple x white
green x yellow
wrinkled x smooth
Generation 2 Offspring
all purple
all yellow
all smooth
all green
Generation 3 Offspring
705:224 purple:white
6022:2001 yellow:green
5475:1850 smooth:wrinkled
428:152 green:yellow

10. Stopping To Think: 2
Write these in your lab book and be prepared to discuss them with the class:
What are the advantages for Mendel in using pea plants for his breeding investigations?
Why did Mendel perform so many crosses for the same characteristics?

11. Mendel’s Results: 2 Mendel saw some interesting data.
If he calculated the ratio of the two traits in Generation 3 he saw a ratio close to 3:1.
For example, for seed color the ratio of yellow to green seeds can be calculated as:
6022 yellow = 3.01 yellow
2001 green green (whole number)
For every 1 green-seed plant there are 3 yellow.

12. Mendel’s Results: 3 The ratio is 3:1 yellow:green.
You could also call it 1:3 green:yellow.
Remember that ratios COMPARE one thing in the numerator to another in the denominator.
You could also express this as a fraction shown in the pie graph: ¼ green and ¾ yellow.
Fractions have the TOTAL in the denominator.

13. Mendel’s Results: 4
He saw that the green-seed trait disappeared in Generation 2 but came back in Generation 3
He also saw the probability of a Generation 3 plant having green-seed trait was ¼, so about 1 green-seed plant for each 4 plants produced.
He found the same ratio with other traits also. The 3:1 ratio was the clue to how the parents’ genes combine in their offspring.

14. Mendel’s Results: 5
Each trait appearing in Generation 2 is the DOMINANT version of the characteristic. The trait that is hidden is called RECESSIVE.
Every plant has 2 copies of the gene for each trait. These 2 copies are called ALLELES.
Plants receive only 1 allele for each trait from each parent & end up with 2 alleles of its own.
The 3:1 ratio is expected, like doing coin flips.

15. Stopping To Think: 3
Write in your lab book how D59 Gene Combo coin flips work just like Mendel’s explanation for his results with pea plants.
Be prepared to discuss it with the class.

16. D60 Mendel Analysis: 1 paragraph
Make a table of dominant and recessive pea plant traits from text Table 1 Page D36. Add an extra column for Question 2a. See next table.
A. Calculate to hundredths place the ratio of dominant to recessive traits in Generation 3. B. Why are the ratios not exactly 3:1?
Explain why ratio of 1:3 green-seed plants to yellow equals fraction of ¼ green-seed plants
Mendel checked more than just 4 traits. Why?

17. D60 Mendel Analysis: Table #1
Characteristic
Dominant Trait
Recessive Trait
Ratio Dominant : Recessive , 0.01
Flower color
Seed Color
Seed Surface
Pod Color