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Aim: How do organisms create offspring through sexual reproduction? What is Sexual Reproduction? It involves the joining of two cells to begin the development of a new individual. Special cells = Gametes Female cells = Egg Male cells = Sperm Joining of sperm & egg cell = fertilization. How many chromosomes do we have in our cells? We have 46 chromosomes in each our body cells. We can also say that we have 23 pairs of Chromosomes. Karyotype-the number and appearance of chromosomes in the nucleus of a eukaryotic cell. Why do we have 2 of each chromosome? We get one chromosome from each parent Each pair of chromosomes are called: Homologous Chromosomes Homologous Chromosomes… Mom Dad Eye Color Hair Color B G B R Gene Part of a chromosome that codes For a trait Have genes that code for the same trait, but may have different types of that trait. How are chromatids different than homologous chromosomes? Chromatids are exact duplicates of each other Homologous chromosomes have genes that code for the same trait, but they may code for different types of that trait How many chromosomes do the offspring have after mitosis? 46X Mitosis 46X 46X Do our sex cells have 46 chromosomes? NO!!! What would happen if our sex cells did have 46 chromosomes? 46 + Sperm 46 92 Egg Offspring The offspring would have 92 chromosomes! TOO MANY!!! Sexual Reproduction: Zygote 23 + 23 Egg 46 Offspring Sperm Characteristics of Sexual Reproduction: 1) Is the creation of a new individual from 2 parents. 2) Offspring are not genetically identical to the parents. Sex Cells (Gametes) • Sex Cells: SPERM and EGG • Sex cells should have 1/2 the normal number of chromosomes Chromosome Number a) Diploid cells: (2n) normal number of chromosomes (Body Cells) b) Haploid cells: (n) ½ the normal number of chromosomes (Sex Cells) Diploid vs Haploid ________ ________ Meiosis •The process by which haploid (sex) cells are produced with half the number of chromosomes. Meiosis has 2 Cell Divisions 1st Cell Division 2nd Cell Division 1 Diploid cell 4 Haploid Cells What is the end result of meiosis in humans? 46X Meiosis 23X 23X 23X 23X End Results of Meiosis: • Four Cells are made (gametes). which have the haploid or monoploid number of chromosomes (half). • All of the cells are DIFFERENT from each other. Where does meiosis occur? Gonads – Ovaries and Testis Why is meiosis important? • Maintains the number of chromosomes from generation to generation • Provides genetic variation • Only during fertilization (union of sperm and egg) is the full (diploid) set of chromosomes restored. Practice Problems: