1. Organisms that reproduce Sexually are made up of two different types of cells.
Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.
Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.
n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set)

2. Two haploids make a diploid.

3. Gametes
The Male Gamete is the Sperm and is produced in the male gonad the Testes.
The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries.

4. Chromosomes
If an organism has the diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA).
Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).

5. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

6. 10 chromosomes (haploid)
Answer:
10 chromosomes (haploid)

7. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

8. Answer:
10 chromosomes

9. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes

10. Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
eye color
locus
hair color
Paternal
Maternal

11. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

12. Cell division that makes haploid cells.
Meiosis
Cell division that makes haploid cells.

13. Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
eye color
locus
hair color
Paternal
Maternal

14. Sex Chromosomes 23 “Sex Chromosomes” …….the 23rd set
This person has 2 “X” chromosomes… and is a female.
“Sex Chromosomes” …….the 23rd set 23

17. Produces haploid cells (from diploids)
Meiosis
2 divisions
Produces haploid cells (from diploids)

18. MEIOSIS 1 Tetrad forms Homologous chromosomes separate 2 cells formed
1 diploid splits into 2 haploid cells

19. Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
eye color
locus
hair color
Paternal
Maternal

21. Crossing Over Increases genetic diversity
Creates new & novel chromosomes
Happens during Meiosis 1

22. Crossing Over creates variation (diversity) in the offspring’s traits because it creates novel chromosomes with new combinations of traits.
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

23. INDEPENDENT ASSORTMENT OCCURS NEXT:
1. Orientation of homologous pair to poles is random. Which homologue moves to which pole is a random event for each pair.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations

24. Independent Assortment

25. Independent Assortment

26. Sister Chromatids separate
MEIOSIS 2
Sister Chromatids separate
4 daughter cells (haploid/n) formed

28. Non-disjunction
Non-disjunction is one of the Two major occurrences of Meiosis
(The other is Crossing Over)
Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis.
Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring.

29. Non-disjunctions usually occur in one of two fashions.
The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set.

30. Common Non-disjunction Disorders
Down’s Syndrome – Trisomy 21
Turner’s Syndrome – Monosomy 23 (X)
Kleinfelter’s Syndrome – Trisomy 23 (XXY)
Edward’s Syndrome – Trisomy 18

31. Nondisjunction

32. It’s different for males and females.

33. Meiosis in males is called spermatogenesis and produces sperm.
Meiosis in females is called oogenesis and produces ova.

34. 4 sperm cells are produced from each primary spermatocyte.
Spermatogenesis
n=23
sperm
haploid (n)
meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
meiosis I
4 sperm cells are produced from each primary spermatocyte.

35. 1 viable egg cell produced from each primary oocyte.
Oogenesis
n=23
Polar bodies
haploid (n)
meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
meiosis I
1 viable egg cell produced from each primary oocyte.

36. Spermatogenesis vs. Oogenesis
Spermatogenesis results in 4 haploid cells
Oogenesis results in 1 viable haploid cell
The egg provides the cytoplasm and the organelles for the first cell

37. Spermatogenesis vs. Oogenesis

38. MEIOSIS
Summary

39. Chromosomes duplicate before dividing.