The Cell and Inheritance Chapter 4, Section 3

What structures in cells contained genes?... “The Clue in the Grasshopper’s Cells” Walter Sutton, an American geneticist who studied the cells of grasshoppers Wanted to know how sex cells form (sperm and egg) Chromosomes are the key to understanding how offspring have traits similar to those of their parents

Chromosomes in a Grasshopper… Body cells of grasshopper have 24 chromosomes Sutton found that the grasshopper’s sex cells have only 12 chromosomes Grasshopper’s sex cells have exactly half the number of chromosomes found in its body cells.

Chromosome Pairs When a sperm cell and an egg cell join during fertilization… The fertilized egg has 24 chromosomes The grasshopper offspring had same number of chromosomes in its cells as did each of its parents 24 chromosomes existed in 12 pairs ONE chromosome in each PAIR came from male parent, OTHER chromosome from female parent

Genes on Chromosomes Alleles: different forms of a gene, exist in PAIRS in an organism Ex: TT, Tt, tt One allele in a pair comes from male, one allele comes from female Chromosome Theory of Inheritance Genes are carried from parents to their offspring on chromosomes

How do sex cells end up with half the number of chromosomes as body cells? MEIOSIS The process by which the number of chromosomes is reduced by half to form sex cells– sperm and eggs

What happens during meiosis? Pg. 128-129 Before Meiosis Parent cell has four chromosomes arranged in two pairs Meiosis I A: Chromosomes line up at center of the cell B: The pairs separate and move to opposite ends of the cell C: Two cells form, each with HALF the number of chromosomes

Meiosis Continued… Meiosis II End of Meiosis A: The chromosomes move to the center of the cell....once again B: The centromeres split and move to opposite ends of the cell End of Meiosis Four sex cells form with half the number of chromosomes as the parental cells

Meiosis and Punnett Squares Punnett square is actually a way to show the events that occur in meiosis One allele from each pair goes to each sex cell (sperm and egg)

A Lineup of Genes The body cells in humans contain 23 chromosome pairs (or 46 chromosomes) Chromosomes are made up of many genes joined together like beads on a string Although you have only 23 pairs of chromosomes, your body cells each contain about 35,000 genes. Each gene controls a trait.

Genes on Chromosomes (pg. 130) Genes are located on chromosomes For which genes is this organism homozygous? For which genes is it heterozygous?