1. Chapter 6
Meiosis and Mendel

2. Section 6.1- Chromosomes
You have many different types of cells in your body, but there are only 2 general types.
The differences in these two is the amount of chromosomes in them.
Somatic Cells
Gamete Cells

3. Section 6.1- Chromosomes Somatic Cells Are also called body cells
These are the cell that make up your body tissues and organs
Everything from eyeballs to your heart
Your body cells are not passed on to offspring
Somatic cells are DIPLOID!
Skin Cells

4. Section 6.1- Chromosomes Gamete Cells These are sex cells
The sex cells are eggs in females and sperm in males
The two cells come together to make the 1st cell of a new human being.
Gamete cells are HAPLOID!

5. Section 6.1- Chromosomes
All organisms have a different number of chromosomes per cell.
The number of chromosomes; however, is not related to the complexity of the organism.
For example, yeast have 32 chromosomes and a fern plant has 1200 chromosomes.
Humans have 46 chromosomes that come in 23 pairs.
The diploid number for humans is 46 and the haploid number is 23.

6. Section 6.1- Chromosomes
So, in each cell in your body (except your sex cells) you have 46 chromosomes.
These 46 chromosomes come in 23 sets, and for each set one chromosome came from your mother and one came from your father.
Together, each pair of chromosomes is known as a homologous pair.

7. Section 6.1- Chromosomes
homologous chromosomes are two chromosomes that have the same general shape and structure.
One of these came from the mother and the other from the father.
Most importantly, these homologous chromosomes have copies of the same genes on them.
These copies may be different copies, but they are copies of the same gene.

8. Section 6.1- Chromosomes
Of the 23 pairs of chromosomes we have, the first 22 pairs are known as autosomes.
These are chromosomes that contain genes for characteristics not directly related to the sex of an organism.
The chromosomes in the first 22 pairs code for everything except sex.

9. Section 6.1- Chromosomes
The 23rd pair of chromosomes are called your sex chromosomes.
This pair controls what sex that organism is going to be.
The two possible sex chromosomes are “X” and “Y”.
If an organism has “XX” pair, that organism is a female.
If an organism has “XY” pair, that organism is a male.
If you are a male- your 23rd set of chromosomes are not homologous because they are not identical.

10. Section 6.1- Chromosomes
Males determine the sex of an offspring because they are the only one that can give a “Y” chromosome.
When sexual reproduction occurs, two gametes come together resulting in a genetic mixture of both parents.
This is called fertilization.

11. Section 6.1- Chromosomes
This 1st cell of the new organism must have the correct number of chromosomes (46 in humans).
This is why all sex cells are in the haploid condition.
This means that these cells only have one copy of the homologous pair.
If these cells were diploid, (which is the full number of chromosomes), when they came together during fertilization there would be double the number of chromosomes needed (92 in humans).
This would cause mass mutations and spontaneous abortion of the cell.

12. Section 6.1- Chromosomes
The correct number of chromosomes must be maintained for the organism to survive.
Also remember that having too many chromosomes is just as harmful as not enough.
Down Syndrome- caused by the presence of an extra chromosome 21
Patau Syndrome- caused by the presence of an extra chromosome 13
Turner’s Syndrome- children only have one X chromosome and no Y chromosome, so they have only 45 chromosomes.

13. Section 6.1- Chromosomes How do organisms maintain chromosome numbers?
How do sex cells have a different number of chromosomes than all other cells?

14. Section 6.1- Meiosis
Meiosis is similar to mitosis in that it is a form of nuclear division.
However, unlike mitosis, meiosis reduces the number of chromosomes to haploid numbers.
Sex cells undergo cellular division called MEIOSIS

15. Section 6.1- Meiosis
Mitosis and Meiosis both divide the nucleus but there are differences!
Meiosis’s most important feature is that it reduces the number of chromosomes in half so when a sperm and egg come together, the diploid number is restored.
MITOSIS:
Produces identical cells
Results in diploid cells
Takes place all through life
MEIOSIS:
Produces unalike cells
Results in haploid cells
Occurs only at certain times

16. Section 6.2- Process of Meiosis
One round of meiosis yields 4 unidenitcal sex cells from one diploid cell.
The chromosomes get reduced because meiosis goes through division twice.
Meiosis’s phases are the same in name as mitosis; however, there are two rounds of division without any replication between meiosis I and meiosis II.

17. Section 6.2- Process of Meiosis
Meiosis I:
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II:
Prophase II
Metaphase II
Anaphase II
Telophase II

19. Section 6.2- Process of Meiosis
In Meiosis I, paired homologous chromosomes split and in meiosis II the duplicated chromosomes split so you end up with 4 unidentical cells in the haploid condition.
The initial cell has 4 chromosomes and the 4 cells in telophase II have 2 chromosomes.

20. Section 6.2- Process of Meiosis
Haploid cells are the end result of meiosis
However, these cells are not yet ready to fertilize and must go through more changes that “mature” the cell.
In sperm production, all sperm cells formed from meiosis are functional and ready to fertilize.
In egg production, only 1 out of every 4 cells become an egg because as meiosis occurs the cytoplasm divide unevenly.

21. Section 6.3- Mendelian Genetics
When we talk about differences among organisms, we talk about their traits.
TRAITS are characteristics that are inherited.
GENETICS is the study of how these traits get passed on from parent to offspring

22. Section 6.3- Mendelian Genetics
Initial studies in genetics started in the mid 1800’s by an Austrian monk named GREGOR MENDEL.
Most scientist believed at this time that offspring were a blend of their parents.
For examples, if one parent was tall and one was short, then the offspring would be medium in size.

23. Section 6.3- Mendelian Genetics
Mendel didn’t believe this theory because there were too many traits that remained “undiluted”
meaning they were not blends.
So, Mendel started doing thousands of test crosses breeding plants.
He was a mathematician by trade so he analyzed all of these results.
Mendel primarily used the pea plant because
It reproduced quickly
Its traits were easy to see

24. Section 6.3- Mendelian Genetics
As Mendel started his studies, he took over the monastery garden.
He made sure to use only purebred pea plants to start with
This way he knew what the traits were for all his starting pea plants.
Purebred means both copies of a gene are the same
We represent genes with letters.
Purebred: TT Tt Rr RR

25. Section 6.3- Mendelian Genetics
Remember Mendel chose pea plants because they reproduce quickly.
He could also control how they mated.
He allowed certain plants to self-pollinate over and over again until he was sure he had purebred plants.
It was vital that Mendel start his experiments with purebreds so he knew what he had at the beginning.

26. Section 6.3- Mendelian Genetics
Mendel than removed the male part of the plant of his purebreds
he then could control what plant pollinated what plant.
In doing this Mendel knew any variation in pea plant offspring was a result of his experiment and not any other random crossing.

27. Section 6.3- Mendelian Genetics
Mendel chose 7 specific traits to follow:
Pea Shape
Pea Color
Pod Shape
Pod Color
Plant Height
Flower Color
Flower Position

28. Section 6.3- Mendelian Genetics
All of the traits Mendel looked at were simple “Either-Or” traits.
Plant height= tall or short
Pod shape= smooth or constricted

29. Section 6.3- Mendelian Genetics
In genetics, when you mate two organisms, this is called a cross.
The 1st plants Mendel started with is the P1 generation or parental.
The offspring of the P1 is called F1 (filial).
Offspring of F1 is the F2 and so on….

30. Section 6.3- Mendelian Genetics
Mendel took all 7 of the traits he indentified and crossed the two purebred conditions for each trait.
Round x Wrinkled
Yellow x Green
Smooth x Constricted
Green x Yellow
Tall x Short
Purple x White
Axial x Terminal

31. Section 6.3- Mendelian Genetics
What Mendel found in the (F1) was that all the offspring looked like one of the two parents and the trait of the other parent seemed to disappear.
Tall x Short = ALL TALL
Purple x White = ALL PURPLE

32. Section 6.3- Mendelian Genetics
Mendel then allowed his F1 plants to self- pollinate and what he discovered was that most of the offspring (F2) looked like the parents, but a small percentage of them looked like the P1 parent whose characteristic disappeared in the F1.
There was about ¾ of the offspring that looked like F1 and ¼ that looked like P1 parent that disappeared.

33. Section 6.3- Mendelian Genetics
Summary of what Mendel has done to this point:
P1 Cross:
Tall Plant x Short Plant
F1:
All tall plants
F1 plants are allowed to self-fertilize
F2:
¾ Tall Plants
¼ Short Plants

34. Section 6.3- Mendelian Genetics
From these initial crosses, Mendel came up with 3 important conclusions:
Traits are inherited separately as discrete units
Organisms inherit 2 copies of a gene, one from each parent
Organisms donate only one copy of each gene in their gametes, so genes separate upon gamete formation

35. Section 6.3- Mendelian Genetics
The last two points make up the LAW OF SEGREGATION
Genes of traits don’t stick together
We will learn why this is in the following sections when we start doing crosses!