Introduction In 1865 Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas Early in the 20th century Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance Proposes that meiosis causes the patterns of inheritance that Mendel observed Genetics is the branch of biology that focuses on inheritance 2014 Pearson Education Inc Mendel s Experimental System Gregor Mendel was A 19th century monk Active member of his city s agricultural society Mendel was interested in heredity Heredity Is the transmission of traits from parents to their offspring A trait Is any characteristic of an individual 2014 Pearson Education Inc What Question Was Mendel Trying to Answer Mendel was addressing the basic questions Why offspring resemble their parents How transmission of traits occurs In his time two hypotheses had been formulated to try to answer this question 1 Blending inheritance Parental traits blend Such that their offspring have intermediate traits 2 Inheritance of acquired characteristics Parental traits are modified Then passed on to their offspring 2014 Pearson Education Inc Garden Peas The First Model Organism in Genetics Genetics The branch of biology that focuses on the inheritance of traits Uses model organisms Because the conclusions drawn from them can be applied to other species 2014 Pearson Education Inc Garden Peas The First Model Organism in Genetics Mendel chose the common garden pea Pisum sativum as his model organism because It is easy to grow Its reproductive cycle is short It produces large numbers of seeds Its matings are easy to control Its traits are easily recognizable 2014 Pearson Education Inc How Did Mendel Arrange Matings Peas normally pollinate themselves This process is called self fertilization Mendel could prevent self pollination by Removing the male reproductive organs containing pollen from each flower He then used this pollen To fertilize the female reproductive organs of flowers on different plants Thus performing cross pollination 2014 Pearson Education Inc Figure 14 1 a Self pollination Female organ receives pollen Eggs Male organs produce pollen grains which produce sperm cells b Cross pollination Collect pollen from one individual and transfer it 2014 Pearson Education Inc to the female organs of an individual whose male organs have been removed What Traits Did Mendel Study Mendel worked with pea varieties that differed in seven easily recognizable traits Seed shape seed color pod shape pod color flower color flower and pod position and stem length An individual s observable features comprise its phenotype Mendel s pea population had two distinct phenotypes for each of the seven traits 2014 Pearson Education Inc What Traits Did Mendel Study Mendel worked with pure lines They produced identical offspring when self pollinated He used these plants to create hybrids He mated two different pure lines that differed in one or more traits 2014 Pearson Education Inc Inheritance of a Single Trait Mendel s first experiments involved crossing pure lines that differed in just one trait The adults in the cross were the parental generation The offspring are the F1 generation The F indicates first filial 2014 Pearson Education Inc The Monohybrid Cross Mendel first experimented with crossing plants that differed in only one trait When Mendel crossed plants with round seeds and plants with wrinkled seeds All of the F1 offspring had round seeds This contradicted the hypothesis of blending inheritance The genetic determinant for wrinkled seeds seemed to have disappeared Mendel allowed the F1 progeny to self pollinate The wrinkled seed trait reappeared in the next F2 generation 2014 Pearson Education Inc Figure 14 2 a Results of Mendel s single trait monohybrid cross Male parents produce round seeds F1 generation Female parents produce wrinkled seeds All produce round seeds Plant grow and allow to self fertilize F2 generation 5474 1850 3 1 b Prediction of blending inheritance hypothesis Male parents produce round seeds F1 generation Female parents produce wrinkled seeds All produce slightly wrinkled seeds Plant grow and allow to self fertilize F2 generation All produce slightly wrinkled seeds 2014 Pearson Education Inc Dominant and Recessive Traits Mendel called The genetic determinant for wrinkled seeds recessive The determinant for round seeds dominant The terms dominant and recessive Identify only which phenotype is observed in individuals carrying two different genetic determinants Mendel repeated these experiments with each of the other traits In each case the dominant trait was present in a 3 1 ratio over the recessive trait in the F2 generation 2014 Pearson Education Inc Summary Table 14 1 2014 Pearson Education Inc A Reciprocal Cross Mendel wanted to determine if gender influenced inheritance He performed a reciprocal cross The mother s phenotype in the first cross is the father s phenotype in the second cross The father s phenotype in the first cross is the mother s phenotype in the second cross The results of the two crosses were identical Established that it does not matter whether the genetic determinants are located in the male or female parent 2014 Pearson Education Inc Figure 14 3 Is the inheritance of seed shape in peas affected by whether the genetic determinant is in a male or female gamete The type of gamete does affect the inheritance of seed shape The type of gamete does not affect the inheritance of seed shape The reciprocal cross A cross Pollen from roundseeded parent Male parent to female organ of wrinkled seeded parent Round seeded parent receives pollen Female parent Female parent from wrinkledseeded parent Male parent Offspring phenotypes will be different in the two crosses Offspring phenotypes will be identical in the two crosses Results are identical First cross All progeny have round seeds Reciprocal cross All progeny have round seeds It makes no difference whether the genetic determinant for seed shape comes from the male gamete or from the female gamete 2014 Pearson Education Inc Table 14 2 2014 Pearson Education Inc Particulate Inheritance Mendel proposed a hypothesis called particulate inheritance Suggests that hereditary determinants maintain their integrity From generation to generation Directly contradicts The blending of inheritance The inheritance of acquired characteristics hypotheses 2014 Pearson Education Inc Genes Alleles and Genotypes Hereditary