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 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

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!

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

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.

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

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.

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.

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

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.

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

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

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.

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.

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

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

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….

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

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

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.

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

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

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!