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Chapter 6

Chapter 6. Meiosis and Mendel. 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. Section 6.1- Chromosomes. Somatic Cells

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Chapter 6

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  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 II: • Prophase II • Metaphase II • Anaphase II • Telophase II • Meiosis I: • Prophase I • Metaphase I • Anaphase I • Telophase I

  18. 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.

  19. 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.

  20. 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

  21. 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.

  22. 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

  23. 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

  24. 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.

  25. 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.

  26. 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

  27. 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

  28. 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….

  29. 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

  30. 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

  31. 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 P1parent that disappeared.

  32. 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

  33. 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

  34. 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!

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