5-1: Mendel’s Work

EQ: What controls the inheritance of traits in organisms?

Previously, in life science…

• You learned that DNA in the nucleus of a cell contains the cell’s genetic material. Now, you will see that this genetic material occurs in the form of paired genes.

1. What is heredity?

• The passing of traits from parent to offspring.

• Ex: parent with blue eyes child with blue eyes.

2. What is a trait?

• A physical characteristic

• How you look

• Ex: brown eyes or blue eyes

3. What is genetics?

• The study of how traits are inherited.

4. Who was Gregor Mendel?

• Known as the Father of Genetics (1850)

5. Why pea plants?

• Grow quickly and there are many varieties

6. What is purebred?

• Always produces the same trait.

• “true-breeding”

7. What was Mendel’s first experiment?• Crossed purebred plants

with opposite forms of a trait.

• Ex: tall × short

• Results: next generation all tall (no short)

• Ex: purple × white

• Results: next generation all purple (no white)

8. What was Mendel’s second experiment? • Then crossed the first

generation with itself

• Ex: tall X tall

• Results: short trait reappeared!

• 75% tall, 25% short

• 3:1 ratio!

• Ex: purple X purple

• Results: White flowers reappeared!

• 3:1 ratio

• What did Mendel observe about the F2 plants?

The “lost” form of the trait reappeared in about one-fourth of the plants.

Summary of Mendel’s Results.

• In every one, only one form of the trait appeared in the F1 generation.

• However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

• Is yellow seed color controlled by a dominant allele or a recessive allele?

dominant!

9. What is a gene?

• A factor that controls a trait

• Located on DNA

• Ex: gene for plant height

gene for eye color

10. What are alleles?

• Different forms of a gene

• Ex: tall or short plant

brown or blue eyes

11. How many copies do organisms have of each gene?• Two!

12. Why do genes exist in pairs?

• Each parent gives you one set of genes. One from mother, one from father.

13. What controls the inheritance of traits in organisms?• Traits are determined by the alleles it

inherits from its parents.

• Some alleles are dominant, while other alleles are recessive.

14. What is a dominant allele?

• Trait that always appears when the allele is present.

• Example: tall plants or brown eyes

15. How is a dominant allele represented?• With a capital letter.

• Example: Brown eyes = B

Tall Plant = T

16. What is a recessive allele?

• Traits that can be masked or covered up when the dominant allele is present.

• Example: short plants or blue eyes

17. How is a recessive allele represented?• With a lower case letter of the dominant

allele.

• Example: blue eyes = b

short plants = t

18. How do you show the alleles?

• By writing them side-by-side

• BB

• Bb

• bb

• If an allele is represented by a capital letter, what does this indicate?

It is dominant!

What combination of alleles must the white rabbit have?

Two recessive alleles!

Purebred tall stem height = TT

Purebred brown eyes = BB

BB

• Purebred short stem height = tt

• Purebred blue eyes = bb

bb

• Hybrid tall stem height = Tt

• Hybrid brown eyes = Bb

Bb