1. Sexual Reproduction & Meiosis

2. What is Sexual Reproduction?
Occurs when two different parents reproduce to form unique offspring (different from their parents and siblings).

3. What determines the TRAITS of the offspring?
DNA codes for all traits
The code for each trait is called a GENE
Offspring receive a randomized combination of genes from their parents
Some genes will be expressed, while others will not
All genes that are passed on from one generation to another are called HEREDITARY

4. What determines the TRAITS of the offspring?
Even traits that are not expressed in either parent, but are coded for in their DNA, are hereditary
blond??

5. Which traits do you get?
Before we examine how and why some traits are expressed while others are not, we must first understand how traits are passed on from one generation to another
Let’s review sexual reproduction and the process of meiosis

6. Gametes
sperm
egg
Woo hoo!

7. Zygote

8. develops into body cells
Sexual Reproduction
Gametes join to form a Zygote
egg
DNA
Gametes
sperm
DNA
develops into body cells
Zygote
Combined
DNA

9. What does DNA do?
DNA is a nucleic acid containing 4 different nitrogenous bases (A, T, G, C)
The sequence of bases is called the GENETIC CODE!
this code tells your cells what to do and contributes to all of your physical traits
Eg. hair/eye colour, shoe size, determines if you are male or female

10. What does DNA do?
Scientists also believe that your DNA has an impact on how you think and how you behave.

11. The code base S S G P S A P S T P S C P P S G P T C A S G P A T C
Sugar
Phosphate P S
base
Nucleotide S G P T C A
Genetic Code S G P A T C
Template Strand

12. DNA with associated proteins as it is found in the nucleus.
Naming the DNA
Chromosome
Chromatin
DNA with associated proteins as it is found in the nucleus.

13. Naming the DNA Chromosome
One particular bundle of DNA (every species has it’s own number of chromosomes)

14. Naming the DNA Chromosome
Can be coiled up during cell division. (easier to move around without getting tangled up)
One particular bundle of DNA (every species has it’s own number of chromosomes)

15. Naming the DNA
Chromosome
copy
copied during cell division

16. Identical chromosomes bound together by a centromere
Naming the DNA
Sister Chromatids
copy
Identical chromosomes bound together by a centromere

17. 46 Sorting out the DNA Chromosomes are bundles of DNA
Every species has a different number of chromosomes
How many chromosomes do humans have? 46
Chromosomes

18. Sorting out the DNA
If you got 46 chromosomes from your mother and you got 46 chromosomes from your father you would have… 92
Chromosomes!!

19. Sorting out the DNA
If you then reproduced, your offspring would get 92 chromosomes from you and 92 chromosomes from your partner… that’s…
184
See the problem?
Chromosomes!!

20. Sorting out the DNA 20 Question: Answer:
If we combine the chromosomes from each parent during sexual reproduction, why doesn’t the chromosome number double?
example (not human)
Answer:
Gametes must have half the number of chromosomes as body cells
egg
sperm 10 10 20

21. Sorting out the DNA Try these 46 78 24
How many chromosomes in each gamete? 46
Human 78
Chicken 24
Rice
Gametes
Zygote 23 39 12
This means that EACH cell in your body has one copy of DNA from your mother and one copy of DNA from your father… BUT ONLY HALF OF THEIR GENES!!!!

22. Meiosis How? How do gametes get half the number of chromosomes?
By the process of
Meiosis

23. Homologous Chromosomes
Gamete from mother
Gamete from father

24. Homologous Chromosomes
Homologous chromosomes are NOT identical, but code for the same traits
Zygote
Homologous chromosomes

25. Meiosis I
Prophase I

26. Line up homologous chromosomes Completely unique DNA created
Meiosis I
Metaphase I
Line up homologous chromosomes
Completely unique DNA created
Crossing Over

27. Meiosis I
Metaphase I

28. Meiosis I
Anaphase I

29. Meiosis I
Telophase I

30. Meiosis II
Prophase II
DNA is not copied

31. Meiosis II
Metaphase II

32. Meiosis II
Anaphase II

33. Meiosis II
Telophase II

34. Meiosis… the end result
4 unique cells created from one body cell
Each cell has half the normal number of chromosomes (gametes)

35. Key Features of Meiosis
Meiosis I
Copy chromosomes
Line up and separate homologous chromosomes
Crossing over between homologous chromosomes
Meiosis II
Separate sister chromatids (normal cell division)

36. Why are you random?
There are BUILT IN MECHANISMS in meiosis to create variation:
Crossing over events which generate an infinite number of new random chromosomes
Random assortment of your parents homologous chromosomes in their gametes during meiosis I and sister chromatids during meiosis II

37. Why are you random?
Your specific combination of genes is a result of…
One of many sexual encounters on a particular day at a particular time
One of millions of sperm which meet with one of thousands of eggs

38. Why are you random?
DNA doesn’t define you…
Your environment, choices and random chance all play a large role in your
Gene expression
Physical Development
Experiences
Health
Abilities

39. Identical?

40. Errors in Meiosis

41. Nondisjunction – Down Syndrome

42. Down Syndrome

43. Down Syndrome

44. Nondisjunction – Turner Syndrome

45. Nondisjunction – Turner Syndrome