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POINT > Describe Walter Sutton’s work POINT > Describe meiosis POINT > Compare meiosis and mitosis POINT > Describe meiosis Punnett squares show us that each parent gives only one allele to the offspring But most cells have two alleles for each trait Pp Purple Parent Pp P p P PP Pp p Pp pp Purple Parent POINT > Describe meiosis Meiosis is the process that produces sex cells (egg and sperm cells) that have only one copy of each gene Pp Purple Parent Pp P p P PP Pp p Pp pp Purple Parent POINT > Describe Sutton’s work In 1903 Walter Sutton was working with grasshopper cells Sutton thought that chromosomes were the key to understanding how offspring inherit their traits POINT > Describe Sutton’s work Sutton knew that grasshopper body cells have 24 chromosomes He discovered that grasshopper sex cells (eggs and sperm) only have 12 chromosomes POINT > Describe Sutton’s work This made sense! When egg and sperm join, the fertilized egg contains 24 chromosomes (12 pairs) One chromosome in each pair came from the male parent, and the other from the female parent Sutton knew from Mendel’s work that alleles exist in pairs – one from each parent He realized that paired alleles were carried on paired chromosomes CHECK: How many alleles for one trait does one parent contribute to an offspring? How many alleles for one trait are in most cells? How many chromosomes in a normal grasshopper body cell? How many chromosomes in a grasshopper egg or sperm cell? What does homozygous mean? POINT > Describe Sutton’s work Sutton’s work led to the Chromosomal Theory of Inheritance: Genes (and therefor traits) are passed from parents to offspring on chromosomes Each individual gets two alleles for one trait: one allele from mother and one allele from father This confirmed Mendel’s work at the molecular level POINT > Describe meiosis Human body cells have 23 pairs of chromosomes = 46 total chromosomes POINT > Describe meiosis How is it that egg and sperm cells only have half as many chromosomes? This happens through the process of meiosis Remember that mitosis produces two identical daughter cells from one parent cell Meiosis also involves cell division, but with different results… POINT > Describe meiosis Meiosis is the formation of sex cells – eggs and sperm cells Meiosis is like mitosis, but with one extra cell division In meiosis, one cell produces 4 unique daughter cells Each daughter cell (egg or sperm) has only one of each chromosome (23 total in humans), instead of two like all other body cells CHECK: How many chromosomes in a human body cell? What is meiosis? What is produced by meiosis? How many chromosomes in a human egg or sperm cell? What are alleles? How many alleles does an individual have for one trait? POINT > Describe meiosis Parent cell – showing one chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes POINT > Describe meiosis Parent cell – showing one chromosome pair Chromosomes copied during interphase (S phase) 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes POINT > Describe meiosis Parent cell – showing one chromosome pair Chromosomes copied 1st cell division – chromosome pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes POINT > Describe meiosis Parent cell – showing one chromosome pair Chromosomes copied 1st division - pairs split 2nd cell division – produces 4 gamete cells with ½ the original number of chromosomes MEIOSIS: MITOSIS: Occurs in special cells: Occurs in body cells (skin, liver, stomach, etc) ovaries produce eggs and testes produce sperm cells DNA is replicated DNA is replicated Cell divides once into 2 identical cells with identical DNA Cell divides twice into 4 unique cells with half the number of chromosomes Study Figure 13 in your book!! Pg 128-129 Homework: Read pages 126-130 S. A. 1-3 page 130 GRAS pages 61-62
