Mendel discovered basic principles of heredity by breeding garden peas in carefully planned experiments Advantages of pea plants for genetic study 1 There are many varieties with distinct heritable features or characters such as flower color character variants such as purple or white flowers are called traits 2 Mating of plants can be controlled 3 Each pea plant has sperm producing organs stamens and eggproducing organs carpels 4 Cross pollination fertilization between different plants can be achieved by dusting one plant with pollen from another 5 Pea plant are capable of true breeding plants that produce offspring of the same variety when they self pollinate Fig 14 3 3 EXPERIMENT P Generation true breeding parents Purple flowers White flowers F1 Generation hybrids All plants had purple flowers F2 Generation 705 purple flowered 224 white flowered plants plants In an experiment via cross pollination Mendel mated two contrasting true breeding varieties a process called hybridization The true breeding parents are the P generation When Mendel crossed contrasting true breeding white and purple flowered pea plants all of the F1 hybrids referred as F1 generation were purple When F1 individuals self pollinate the F2 generation is produced When Mendel crossed the F1 hybrids many of the F2 plants had purple flowers but some had white Mendel noticed that only the purple flower factor was affecting flower color in the F1 hybrids Whereas there was a ratio of about three to one purple to white flowers in the F2 generation Mendel called the purple flower color a dominant trait and the white flower color a recessive trait He observed the same pattern of inheritance in six other pea plant characters each represented by two traits What Mendel called a heritable factor is what we now call a gene Table 14 1 Mendel s Model Mendel developed a hypothesis to explain the 3 1 inheritance pattern he observed in F2 offspring Four related concepts make up this model 1 The first concept is that alternative versions of genes account for variations in inherited characters For example the gene for flower color in pea plants exists in two versions one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome Fig 14 4 Allele for purple flowers Locus for flower color gene Homologous pair of chromosomes Allele for white flowers 2 The second concept is that for each character an organism inherits two alleles one from each parent The two alleles at a locus on a chromosome may be identical as in the true breeding plants of Mendel s P generation Alternatively the two alleles at a locus may differ as in the F1 hybrids 3 The third concept is that if the two alleles at a locus differ then one the dominant allele determines the organism s appearance and the other the recessive allele has no noticeable effect on appearance In the flower color example the F1 plants had purple flowers because the allele for that trait is dominant 4 The fourth concept now known as the law of segregation states that the two alleles for a heritable character separate segregate during gamete formation and end up in different gametes Thus an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis The possible combinations of sperm and egg can be shown using a Punnett square a diagram for predicting the results of a genetic cross between individuals of known genetic makeup Fig 14 5 3 P Generation Purple flowers White flowers Appearance Genetic makeup PP pp Gametes p P F1 Generation Appearance Genetic makeup Gametes Purple flowers Pp 1 2 P 1 2 Sperm F2 Generation P p PP Pp Pp pp P Capital letter represents a dominant allele and a lowercase letter represents a recessive allele Eggs p 3 1 p Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes heterozygotes are not true breeding Because of the different effects of dominant and recessive alleles an organism s traits do not always reveal its genetic composition Therefore we distinguish between an organism s phenotype or physical appearance and its genotype or genetic makeup In the example of flower color in pea plants PP and Pp plants have the same phenotype purple but different genotypes Fig 14 6 3 Phenotype Genotype Purple PP homozygous Purple Pp heterozygous 1 2 1 Purple Pp heterozygous White pp homozygous Ratio 3 1 Ratio 1 2 1 1 The Testcross How can we tell the genotype of an individual with the dominant phenotype Such an individual must have one dominant allele but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype the mystery parent must be heterozygous Fig 14 7 TECHNIQUE Dominant phenotype Recessive phenotype unknown genotype known genotype PP or Pp pp Predictions If PP Sperm p p P Pp Eggs P Pp Eggs P Pp If Pp Sperm p p or p Pp Pp Pp pp pp RESULTS or All offspring purple 2 offspring purple and 1 offspring white 2 1 The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Mendel identified his second law of inheritance by following two characters at the same time Crossing two true breeding parents differing in two characters produces dihybrids in the F1 generation heterozygous for both characters Fig 14 8 EXPERIMENT yyrr YYRR P Generation A dihybrid cross a cross between F1 dihybrids can determine whether two characters are transmitted to offspring as a package or independently Gametes YR F1 Generation yr YyRr Hypothesis of dependent assortment Predictions Hypothesis of independent assortment Sperm or Predicted offspring of F2 generation 4 YR 1 Sperm 1 2 YR 1 2 yr 1 2 YR 4 YR 4 Yr 1 1 4 yR 4 yr 1 YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr 16 3 1 2 1 YyRr YYRR Eggs 1 4 Yr Eggs