1. 4.3 Meiosis and Inheritance

2. POINT > Describe Walter Sutton’s work
POINT > Describe meiosis
POINT > Compare meiosis and mitosis

3. POINT > Describe meiosis
Punnett squares show us that each parent gives only one allele to the offspring
But most cells have two alleles for each trait Pp
Purple Parent P p P PP Pp
Purple Parent p Pp pp

4. POINT > Describe meiosisPp
Meiosis is the process that produces sex cells (egg and sperm cells) that have only one copy of each gene
Purple Parent P p P PP Pp
Purple Parent p Pp pp

5. POINT > Describe Sutton’s work
In 1903 Walter Sutton was working with grasshopper cells
Sutton thought that chromosomes were the key to understanding how offspring inherit their traits

6. POINT > Describe Sutton’s work
Sutton knew that grasshopper body cells have 24 chromosomes
He discovered that grasshopper sex cells (eggs and sperm) only have 12 chromosomes

7. POINT > Describe Sutton’s work
This made sense! When egg and sperm join, the fertilized egg contains 24 chromosomes (12 pairs)
One chromosome in each pair came from the male parent, and the other from the female parent
Sutton knew from Mendel’s work that alleles exist in pairs – one from each parent
He realized that paired alleles were carried on paired chromosomes

8. CHECK:
How many alleles for one trait does one parent contribute to an offspring?
How many alleles for one trait are in most cells?
How many chromosomes in a normal grasshopper body cell?
How many chromosomes in a grasshopper egg or sperm cell?
What does homozygous mean?

9. POINT > Describe Sutton’s work
Sutton’s work led to the Chromosomal Theory of Inheritance: Genes (and therefor traits) are passed from parents to offspring on chromosomes Each individual gets two alleles for one trait: one allele from mother and one allele from father This confirmed Mendel’s work at the molecular level

10. POINT > Describe meiosis
Human body cells have 23 pairs of chromosomes = total chromosomes

11. POINT > Describe meiosis
How is it that egg and sperm cells only have half as many chromosomes? This happens through the process of meiosis Remember that mitosis produces two identical daughter cells from one parent cell Meiosis also involves cell division, but with different results…

12. POINT > Describe meiosis
Meiosis is the formation of sex cells – eggs and sperm cells Meiosis is like mitosis, but with one extra cell division In meiosis, one cell produces 4 unique daughter cells Each daughter cell (egg or sperm) has only one of each chromosome (23 total in humans), instead of two like all other body cells

13. CHECK: How many chromosomes in a human body cell? What is meiosis?
What is produced by meiosis?
How many chromosomes in a human egg or sperm cell?
What are alleles?
How many alleles does an individual have for one trait?

14. POINT > Describe meiosis
Parent cell – showing one chromosome pair
Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

15. POINT > Describe meiosis
Parent cell – showing one chromosome pair
Chromosomes copied during interphase (S phase)
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

16. POINT > Describe meiosis
Parent cell – showing one chromosome pair
Chromosomes copied
1st cell division – chromosome pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

17. POINT > Describe meiosis
Parent cell – showing one chromosome pair
Chromosomes copied
1st division - pairs split
2nd cell division – produces 4 gamete cells with ½ the original number of chromosomes

19. MITOSIS:
Occurs in body cells (skin, liver, stomach, etc)
DNA is replicated
Cell divides once into 2 identical cells with identical DNA
MEIOSIS:
Occurs in special cells: ovaries produce eggs and testes produce sperm cells
DNA is replicated
Cell divides twice into unique cells with half the number of chromosomes

22. Study Figure 13 in your book!! Pg 128-129

23. Homework:
Read pages
S. A. 1-3 page 130
GRAS pages 61-62