10 1 Mendel s Laws 10 1 Mendel s Laws cont Gregor Mendel was a 19th century Austrian monk who derived a series of laws that described the genetic patterns of heredity from one generation to the next These laws account for the variability between members of a family Mendel derived these laws from his experiments with pea plants Mendel s Experimental Procedure The study of peas was advantageous because they can both selfpollinate and cross pollinate When peas self pollinate the progeny are genetically identical to the parent When peas cross pollinate variation occurs Mendel tracked traits such as seed color seed shape and flower color Mendel s Experimental Procedure cont Mendel s Experimental Procedure cont Mendel applied mathematical laws of probability to interpret the results From the results Mendel derived the particulate theory of inheritance The particulate theory of inheritance is based on the inheritance of particles from each parent which we now know are genes Mendel s Experimental Procedure cont 1 Mendel s Experimental Procedure cont Mendel s Experimental Procedure cont One Trait Inheritance One Trait Inheritance cont Mendel performed crosses with true breeding pea lines to observe the results The original parents were called the P generation The first generation of offspring were called the first filial F1 generation The second generation of offspring were called the second filial F2 generation The results of genetic crosses can be predicted using a Punnett square In a Punnett square the possible male and female gametes of each parent are arranged on the horizontal and vertical axes The squares represent every possible combination of gametes that could combine to form a zygote One Trait Inheritance cont One Trait Inheritance cont The Punnett square can be used to understand the results of one of Mendel s crosses When true breeding tall plants were crossed with true breeding short plants the F1 generation was all tall When the F1 generation self pollinated of the progeny were tall and were short 3 1 Thus the F1 character for shortness was passed on by the tall F1 generation Mendel s mathematical approach offered an explanation for the 3 1 pattern The F1 parents each contained one copy of the hereditary factor for height one dominant and one recessive they are heterozygous for the factor The factors separated when the gametes were formed during meiosis each gamete would get either the tall or short gene When random fusion of the gametes occurred during fertilization the combinations were brought together in a 3 1 ratio as indicated by the Punnett square 2 One Trait Inheritance cont One Trait Testcross A testcross can be performed to determine if the F1 progeny carries a recessive factor A testcross crosses the F1 generation with true breeding tall plants to observe the distribution of height in the progeny A 1 1 ratio of tall short in the progeny confirms that a recessive factor is present One Trait Testcross cont One Trait Testcross cont It is also possible that the F1 generation carries two dominant factors homozygous for the factor If true then the testcross would produce progeny that were all tall One Trait Testcross cont One Trait Testcross cont From the results of these one trait crosses Mendel formed the law of segregation Each individual carries two factors for each trait The factors segregate separate when gametes form during meiosis Each gamete contains only one factor from each pair Fertilization gives each new individual two factors for each trait 3 The Modern Genetics View cont The Modern Genetics View Mendel s results can be stated in more modern terms Traits are controlled by two alleles The dominant allele has the ability to mask the recessive allele The dominant allele is typically designated with a capital letter and the recessive a lowercase letter Alleles occur on homologous chromosomes at a specific location called the gene locus Genotype Versus Phenotype It is possible that two organisms with different allelic combinations can have the same outward appearance For example if the dominant allele for finger length is S for short and the recessive allele is s for long the individuals that are SS and Ss both have short fingers Genotype Versus Phenotype cont Genotype Versus Phenotype cont Separate terms are needed to describe each condition The genotype of an organism describes the two alleles that are present and the condition that this combination creates For example if an organism has two S alleles the genotype is SS or homozygous dominant for finger length Genotype Versus Phenotype cont The phenotype of an organism refers to the physical appearance For example organisms that were either SS or Ss would have short fingers 4 Two Trait Inheritance Two Trait Inheritance cont Mendel also performed crosses between plants that differed in two traits The progeny from the cross the F1 generation were allowed to selfpollinate to generate the F2 generation One example was a cross between tall plants with green pods and short plants with yellow pods Tall green pod plants are homozygous dominant for both traits TTGG Short yellow pod plants are homozygous recessive for both traits ttgg From this cross two possible patterns would be expected in the F2 generation If the dominant factors T and G segregate during meiosis together progeny will all be tall with green pods If the factors segregate separately then four possible phenotype could result Two Trait Inheritance cont Two Trait Inheritance cont The results of the cross produced four phenotypes Tall with green pods Tall with yellow pods Short with green pods Short with yellow pods Based upon these results Mendel formulated the law of independent assortment Each pair of factors segregates assorts independently of other factors All possible combinations of factors occur in the gametes These results demonstrated that the two factors segregated independently Two Trait Inheritance cont Two Trait Inheritance cont 5 Two Trait Testcross Because the fruit fly Drosophila melanogaster has a variety of heritable mutations this insect has been used extensively in genetic research Two mutations displayed by this fly are wing length and body color Wild type normal flies have long wings and gray bodies Mutant flies can have short wings ebony bodies or both Both of these mutations are recessive traits Two Trait Testcross cont In a two trait testcross a dominant L G fly is crossed with a recessive fly of known genotype llgg A