1. Unit 5: Mendelian Genetics
Chromosomes & Meiosis
Unit 5:
Mendelian Genetics

2. Learning Goals
1. Define homologous chromosomes and explain diploid and haploid.
2. Describe the overall process of meiosis.
3. Explain crossing over including the step in meiosis where it takes place and why it is important.
4. Describe the two types of gamete formation and how fertilization occurs.

3. Chromosome Structure Made of DNA wrapped around histones (proteins)
Sections of DNA make up genes

4. Homologous Chromosomes
Each body cell has 2 sets of chromosomes
Homologous Chromosomes = a pair of chromosomes that have the same genes at the same locations
1 came from mom
1 came from dad

5. Homologous chromosomes
From mom
From dad

8. Chromosome Number
Diploid (2n): cells that have 2 sets of homologous chromosomes (2n)
Human body cells are diploid (46 chromosomes or 23 pairs)
Haploid (n): cells that have 1 set of chromosomes (n)
sex cells (sperm & egg) are haploid (23 chromosomes total)

9. 1) A bug has a haploid number n=5. What is the diploid number (2n)?
2) A crocodile has a diploid number 2n=50. What is the haploid number (n)?
meiosis
Haploid gamete
Diploid 2n n n

10. Karyotypes
A karyotype shows an individual’s complete set of chromosomes
Autosomes = Chromosome #1-22
Sex chromosomes = Chromosome #23
XX = Female
XY = Male
Karyotypes are studied to determine irregularities in the chromosomes and can show genetic disorders.

11. Human Female Karyotype

12. Human Male Karyotype

13. Down Syndrome Female
Trisomy

14. On the karyotype you were given circle and label the following:
Autosomes
Sex chromosomes
1 set of homologous chromosomes
*Determine and label the gender!

15. Meiosis Cell division that results in the formation of gametes:
Gametes = Sex Cells (sperm & eggs)
Human gametes have 23 chromosomes (22 autosomes + 1 sex chromosome)
Egg 23 23

16. Steps of Meiosis

17. Interphase DNA is copied before the start of Meiosis I.
This makes 2 identical sister chromatids

18. Meiosis 1
Four steps:
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1

19. Prophase 1
The homologous pairs of chromosomes line up side by side. (tetrad = 4)

20. Crossing-over
Homologous chromosomes can Cross-over and exchange a piece of the chromosome randomly

21. Crossing-over is very important because it creates genetic variation (new combinations of genes), which makes us all different.
This is why you do not look exactly like your siblings.

22. Crossing over happens randomly.
However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together.
Red hair
Freckles
Chromosome 16

23. After crossing-over, the homologous chromosomes separate & the cell divides

24. Meiosis II
Four steps:
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2

25. Sister chromatids separate & the cells divide.
Result is 4 genetically different haploid daughter cells

26. Meiosis Overview
Gametes produced during meiosis are needed for sexual reproduction
Cells (diploid) divide twice resulting in 4 daughter cells (haploid)
Each cell has half the number of chromosomes as the original cell
Each new cell is genetically different

27. Mendel’s Law of Segregation
Genes segregate (separate) into different cells during meiosis
Each gamete only gets one possible gene

28. Principal of Independent Assortment
Genes for different traits can segregate (separate) independently during the formation of gametes.
The inheritance of one trait is not determined by the inheritance of another trait
Ex: The gene for eye color is not connected to the gene for hair color.

29. Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms.
Assume there was only one cross-over in a man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children

30. Spermatogenesis In males meiosis occurs in the testes produces 4 sperm23 23 23 23

31. Spermatogenesis

32. Oogenesis In females meiosis occurs in the ovaries
produces only 1 egg and 3 polar bodies
Egg 23
Polar Bodies

33. Oogenesis

34. First cell of a new organism
Fertilization: the combining of sperm and egg to form a zygote
Zygote 46 23
First cell of a new organism

35. Only one sperm and one egg come together
The first sperm to reach the egg will be the one to fertilize it.

36. Learning Goals
1. Define homologous chromosomes and explain diploid and haploid.
2. Describe the overall process of meiosis.
3. Explain crossing over including the step in meiosis where it takes place and why it is important.
4. Describe the two types of gamete formation and how fertilization occurs.