1. Sexual Reproduction
Meiosis

2. The problem with sexual reproduction
Each human cell has 46 chromosomes. If every egg and every sperm had 46 chromosomes, how many would be found in the fertilized egg?

3. The problem with sexual reproduction
92!

4. The problem with sexual reproduction
92! But the fertilized egg has 46 chromosomes, 23 from each parent. So what happened?

5. The problem with sexual reproduction
The number of chromosomes is reduced by half. This is a special type of cell division called meiosis.

6. The problem with sexual reproduction
Meiosis is the process by which the number of chromosomes is reduced by half. During this process the homologous chromosomes are separated.

7. The problem with sexual reproduction
Meiosis occurs in those tissues that give rise to gametes (eggs or sperm). Gametogenesis (the formation of gametes) occurs only in the gonads (testes or ovaries).

8. The problem with sexual reproduction
The typical human body cell contains 46 chromosomes. This is called the diploid or 2n state.

9. The problem with sexual reproduction
After meiosis, the homologous pairs are separated and the chromosome number is now haploid or 1n.

10. The problem with sexual reproduction
Homologous chromosomes contain similar types of genetic information.

11. The problem with sexual reproduction
Homologous chromosomes contain similar types of genetic information. Each member of the pair comes from one parent.

12. The problem with sexual reproduction
The human has 23 pairs of homologous chromosomes. During meiosis the pairs separate forming gametes with only one part of the pair.

13. Meiosis
Meiosis looks like two mitotic divisions. The major difference is that 4 cells instead of two are formed and the chromosome number is reduced by half.

14. Important Steps
In the first metaphase in meiosis, the homologous chromosomes pair up.

15. Important Steps
The homologous chromosomes pairing is called a tetrad.

16. Important Steps
During this phase, crossing over can occur. Crossing over is where chromatids can exchange pieces of genetic information.

17. Important Steps
This results in more genetic variation.

18. Important Steps
In the second metaphase of meiosis, the paired chromatids split. This looks very much like metaphase in mitosis.

19. Mitosis vs Meiosis

20. The problem with sexual reproduction
Meiosis can go wrong. During Metaphase I or 2, the chromosomes may not separate and this results in gametes having the wrong number of chromosomes.

21. The problem with sexual reproduction
Nondisjunction is when the chromosomes fail to separate properly during meiosis. In the human this can result in eggs or sperm having the incorrect number of chromosomes.

22. The problem with sexual reproduction
Common syndromes caused by nondisjunction include:
1 Downs Syndrome
2 Edwards Syndrome
3 Kleinfelters Syndrome
4 Turners Syndrome

23. Diagnosis
A karyotype is a pictorial presentation of an individual’s chromosomes, taken from photographs taken through a microscope.
The chromosomes can then be classified and paired up to find any mistakes.

24. Diagnosis
Chromosomes are divided into two types: Autosomes which are in homologous pairs and the Sex Chromosomes. These can be paired for the female XX or hemizygous for the male XY

25. Diagnosis
The chromosomes are arranged in a numeric sequence that aligns the chromosomes from largest to smallest.

26. Diagnosis
The karyotype shows the matching homologous chromosomes. There are 22 pairs of autosomes and 1 set of sex chromosomes.

27. Diagnosis
For example, Chromosomal pair 1, one is from the mother and the other from the father. This individual is 46, XX

28. Diagnosis
Karyotypes can also be used to determine the sex of the infant. The 23rd pair are the sex chromosomes.

29. Diagnosis
If this pair is a XX, the infant is a female.

30. Diagnosis
If this pair is a XY, the infant is a male. 46,XY

31. Preparation of a Karyotype
For prenatal diagnosis, an amniocentesis is performed. This takes a sample of amniotic fluid which contains cells from the infant.

32. Preparation of a Karyotype
Ultrasound Image
Amniocentesis

33. Preparation of a Karyotype

34. Preparation of a Karyotype
The karyotype can be analyzed for any chromosomal abnormalities.

35. Syndrome 1

36. Syndrome 1
What is the sex? Is there any abnormality?

37. Syndrome 1 Down’s Syndrome
This is a male with trisomy 21 (3 chromosome # 21) Downs syndrome is the most common nondisjunctional condition.

38. Syndrome 1 Down’s Syndrome
Signs and symptoms include:
Flatten face
Makes the tongue protrude
Loose joints & poor muscle tone
Upward slant to the eyes
Short stature
Mental retardation (varies)

39. Syndrome 2

40. Syndrome 2
What is the sex? Is there any abnormality?

41. Syndrome 2 Kleinfelter’s Syndrome
Signs and symptoms :
These individuals are males with a XXY genotype
Symptoms are not seen until puberty with a failure of the secondary sex characteristics to develop

42. Syndrome 2 Kleinfelter’s Syndrome
Signs and symptoms :
The extra X chromosome causes the testicles not develop adequately.
Some have learning disabilities.

43. Syndrome 2 Kleinfelter’s Syndrome
Signs and symptoms :
Estrogen and growth hormone are given to allow normal development.

44. Syndrome 3

45. Syndrome 3
What is the sex? Is there any abnormality?

46. Syndrome 3 Turner’s Syndrome
Signs and symptoms :
These individuals are females with a X O genotype
Symptoms are not seen until puberty with a failure of the secondary sex characteristics to develop.
Intelligence is normal

47. Syndrome 3 Turner’s Syndrome
Signs and symptoms :
The missing X chromosome causes the ovaries not to develop.
Some have learning disabilities.

48. Other Syndromes Patau, an extra chromosome 13
Edwards, an extra chromosome 18
Super females extra X chromosome
Super males extra Y chromosome