Journal Entry: 5/10 Write a sentence to describe each of the following terms: heredity, genotype, and phenotype. Is heredity necessarily a factor in both genotype and phenotype? Why or why not? • Objectives: • Explain the difference between mitosis and meiosis. • Describe how chromosomes determine sex.
Asexual Reproduction • Asexual reproduction • Only one parent cell is needed. • The structures inside the cell are copied, and then the parent cell divides, making two exact copies. • This type of cell reproduction is called mitosis. • Most of the cells in your body and most single-celled organisms reproduce this way.
Cell Cycle • Interphase • G1 • S • G2 • Mitosis • Prophase • Metaphase • Anaphase • Telophase
Sexual Reproduction • In sexual reproduction, two parent cells (sex cells) join together to form offspring that are different from both parents.
Chromosomes • Chromosomes: Structure consisting of DNA that transmits genetic information from the previous generation of cells and organisms to the next generation. • Chromosomes that carry the same sets of genes are called homologous chromosomes.
Sexual Reproduction, Continued • Body Cells • Human body cells have 46 chromosomes or 23 homologous chromosome pairs. • Human body cells are referred to as somatic cells. • Examples: Liver cells, heart cells • Sex cells • Sex cells have only one of the chromosomes from the homologous pair (total of 23 chromosomes per cell) • Sex cells are made during meiosis. • Sex cells are referred to as gametes. • Examples: Sperm and Egg
Sexual Reproduction, Continued • Genes and Chromosomes • Walter Sutton studied meiosis in sperm cells in grasshoppers. • Using his observations and his knowledge of Mendel’s work, Sutton proposed that: • Genes are located on chromosomes.
Meiosis • Meiosis is a copying process that produces sex cells (sperm and egg) with half the usual number of chromosomes. • Why? When the sperm (23 chromosomes) combines with the egg (23 chromosomes) it produces a cell with 46 chromosomes.
DO NOW: 5/9 What do you think would happen if something went wrong during cell division and the sperm or egg cell ended up with either too few or too many chromosomes? • Objectives: • Explain why sex-linked disorders occur in one sex more often than in the other. • Interpret a pedigree.
The Steps of Meiosis • During meiosis, chromosomes are copied once, and then the nucleus divides twice. • The resulting sex cells (sperm and eggs) have half the number of chromosomes of a normal body cell.
Meiosis and Mendel • The steps of meiosis explain Mendel’s results. The following slide shows what happens to a pair of homologous chromosomes during meiosis and fertilization.
Meiosis and Mendel, Continued • Sex chromosomes carry genes that determine sex. • Human females have two X chromosomes. • Human males have one X chromosome and one Y chromosome.
Meiosis and Mendel, Continued • Sex-Linked Disorders • The genes for certain genetic disorders, such as colorblindness, are carried on the X chromosome. • Since the gene for these disorders is recessive, men are more likely to acquire the genetic disorder.
Meiosis and Mendel, Continued • Genetic Counseling • Genetic counselors use pedigrees to trace traits through generations of a family. These diagrams can often predict if a person is a carrier of a hereditary disease. • Selective Breeding • In selective breeding, organisms with desirable characteristics are mated.