Results and Conclusions of Mendel’s Experiments Mendel began his experiments by cross pollinating pure breeding plants • His studies focused on only one characteristic at a time for example, flower color. This type of cross is called a Monohybrid cross Mendel developed a mathematical vocabulary to identify different generations of pea plants • P1: Parent generation • F1: First (“filial”meaning offspring) generation Offspring of P1 cross P1 x P1 • F2: Second Generation - Offspring of F1 cross All F1 plants are purple (Ww )… Ww Ww Ww Ww Where did the WHITE form go? 3 out of 4 offspring with purple flowers WW Ww Ww ww What is the resulting ratio of purple to white flowers in the F2 generation? • Purple 3:1 white Express this answer as a percent • 75% purple: 25 % white Now let’s look at another… Mendel Concluded…. 1) Parents transmit information about traits to their offspring. • Mendel called this information a “factor.” Today we know that “factors” are really genes. 2) Each parent must be contributing a form of a gene to the offspring. • From our studies in the last unit, we know this is true because chromosomes came in pairs called homologous chromosomes. Each homolog contains the same genes, so a gene pair exists for each gene on the chromosome. Gene Pair Allele GTACCAGGA (Yellow) Gene locus Allele ATGCCGGGT (Green) 2) Each parent must be contributing a form of a gene to the offspring. • The two alleles in a gene pair can • Gene pairs are described in be: two ways: 1) identical or 1) homozygous: alleles in gene pair are identical Pure 2) different breeding plants are • For example, a gene pair which homozygous. codes for seed color in pea 2) heterozygous: alleles in plants could contain: gene pair are different 1) yellow, yellow (purebred or Hybrid plants are 2) green, green homozygous) hererozygous. 3) yellow, green (hybrid) heterozygous 3. Symbols can be used to represent alleles. • If you use an uppercase “W” to represent purple as the dominant form of pea color and use a lowercase “w” to represent white as the recessive form of pea color, you can illustrate the allele combinations for the following pea plants. (WW, Ww, ww) 4. Genetic makeup and observed characteristics are not always the same! GENOTYPE • the exact form of the alleles in a gene pair • Example: WW, Ww, ww PHENOTYPE • the observable or expressed form of a trait resulting from a gene pair • Example: yellow or green seed color In pea plants, Green pod color is dominant over yellow. Description GENOTYPE PHENOTYPE Homozygous dominant GG Green Homozygous recessive gg Yellow Heterozygous Gg Green Heterozygous recessive?? No such thing! As a result of his many years of careful study, Mendel’s work was summarized into three main laws. Mendel’s Laws 1. The law of Dominance • States that if two alleles in a gene pair are different, then one allele can control the trait and the other one can be hidden. • Example: Green pod color is seen in plants with the genotype Gg Mendel’s Laws 2. The law of Independent Assortment • States that gene pairs segregate into gametes randomly and independently of each other. • At what stage of meiosis does this occur? Metaphase I and Anaphase I of meiosishomologs randomly separate Mendel’s Laws 3. The law of Segregation • States that each pair of genes segregates, or separates, during meiosis and each gamete contains one gene from each gene pair. • What is the chance that a gamete will end up with a maternal homolog at any one chromosome pair? • ½ or 50% • The chance that all 23 homologs from one parent will end up in a single gamete is 1 in 8 million (1/2)23 CLOSER What is a monohybrid cross? • A monohybrid cross is when a scientist studies the inheritance of one trait at a time. How is the following statement “Each parent must be contributing a form of a gene to the offspring” supported by what you know about meiosis and homologous chromosomes? The chromosomes from each homologous pair come from the parents (one from each). Each homologous pair carries a pair of genes (one gene on each homolog). Therefore one copy of each gene comes from each parent. What is the difference between homozygous and heterozygous? • Homozygous means each gene has the same allele. • Heterozygous means that each gene in the pair has different alleles. What is the relationship between phenotype and genotype? • Genotype is what alleles are found in a pair of genes. Phenotype is the outward appearance of those alleles. What does dominant and recessive mean? • A dominant allele can control a trait and hide a recessive allele. A recessive allele will only show if the dominant allele is not present.