2. Genetic Variation
The purpose of meiosis is for sexual reproduction and passing on different combinations of genes to offspring
Meiosis of the germ cells forms multiple sperm (men) or egg (women)
A haploid sperm will fuse with a haploid egg forming a diploid zygote.
The zygote will grow and develop into the organism

3. Independent orientation
Independent orientation at metaphase I
Each pair of chromosomes independently aligns at the cell equator.
Number of combinations = 2n (n= number of tetrads)
This cell there are four potential outcomes

4. Two equally probable arrangements of chromosomes at
Figure
Possibility A
Possibility B
Two equally probable arrangements of chromosomes at
metaphase I
Metaphase II
Figure Results of the independent orientation of chromosomes at metaphase I (step 2)

5. Two equally probable arrangements of chromosomes at
Figure
Possibility A
Possibility B
Two equally probable arrangements of chromosomes at
metaphase I
Metaphase II
Figure Results of the independent orientation of chromosomes at metaphase I (step 3)
Gametes
Combination 1
Combination 2
Combination 3
Combination 4

6. Random Fertilization
The combination of each unique sperm with each unique egg increases genetic variability.

7. Cross over
Shuffles the genes around from homologous chromosomes creating different combinations than the parents

9. X and Y chromosome Humans have 23 pairs of homologous chromosomes
22 are numbered and 1 pair are the sex chromosomes – X and Y
Females – XX
Males - XY

10. Errors in Meiosis
Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis.
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.
Normal nondisjunction

11. Meiosis I Nondisjunction Figure 8.18-1-1
Figure Nondisjunction in meiosis I (part 1, step 1)

12. Meiosis I Nondisjunction Meiosis II Normal meiosis II Figure 8.18-1-2
Figure Nondisjunction in meiosis I (part 1, step 2)

13. Meiosis I Nondisjunction Meiosis II Normal meiosis II Gametes
Figure
Meiosis I
Nondisjunction
Meiosis II
Normal meiosis II
Figure Nondisjunction in meiosis I (part 1, step 3)
Gametes
Number of chromosomes
n + 1
n + 1
n − 1
n − 1
Abnormal gametes

14. Meiosis I Normal meiosis I Meiosis II Nondisjunction Figure 8.18-2-2
Figure Nondisjunction in meiosis II (part 2, step 2)

15. Meiosis I Normal meiosis I Meiosis II Nondisjunction n + 1 n − 1 n n
Figure
Meiosis I
Normal meiosis I
Meiosis II
Nondisjunction
Figure Nondisjunction in meiosis II (part 2, step 3)
n + 1
n − 1 n n
Abnormal gametes
Normal gametes

16. Errors in Meiosis
Abnormal number of chromosomes usually leads to miscarriage
Down Syndrome is caused when there is an extra copy of chromosome 21
trisomy 21

17. Infants with Down syndrome (per 1,000 births)
Figure 8.20b 90 80 70 60
Infants with Down syndrome (per 1,000 births) 50 40 30
Figure 8.20b Maternal age and incidence of Down syndrome 20 10 20 25 30 35 40 45
Age of mother

18. Mutations of Chromosomes
Deletion
Inversion
Duplication
Reciprocal translocation
Homologous chromosomes
Nonhomologous chromosomes
Figure 8.23a-0 Alterations of chromosome structure

19. Recall Some mutations harmful, some beneficial
Chromosomes are passed on from parents to offspring
Chromosomes contain genes
Genes code for protein
Proteins carry out the cell processes

20. Lactose Intolerance

21. Fill out the review sheet – 2 pts extra credit onto the test