Lecture 1 April 3, 2012 “Mendelian Genetics” *Explained Syllabus I. Introduction a. What is a gene i. DNA Sequence in Chromosome that code for a specific trait b. What is an allele i. Variation of Gene c. What is genotype i. Collection of allele d. What is Phenotype i. Expressed trait shown II. Mendel’s First Law: The Principle of Segregation - Deduced the fundamental principles of genetics… including that parents pass on to their offspring discrete heritable factors (later known as genes) that are responsible for inherited traits. o Mendel Used Garden Peas. - Secret s of Mendel’s Success o Limited the number of variables (Unlike the humans, who have different variables) o Started with pure breeding parents o Quantified results. (Could be Counted and easily analyzed) - Mendel chose the garden pea for Two main reasons: o Available in many variant shapes and colors that could easily be identified and analyzed. o Peas can either a) self-pollinate (“self”), where the pollen of a plant pollinates its own ovules- Mendel carried out Cross Pollination - Transfer pollen from one plant to the ovule of the second plant Procedure of Mendel’s Experiments - Mendel isolated pure breeding lines (true breeding)… identical phenotypes are observed from generation to generation - 7 pairs of pure lines for 7 characteristics, with each pair differing in only one characteristic… Many different phenotypes. P (parental) generation: true-breeding varieties of plants, pure for one specific trait -> Mendel then crossed two different true-breeding varieties, creating hybrids F1 (filial) generation -> He then allowed the F1 to self-fertilize, which resulted in the F2 generation - In all cases, one parental phenotype disappeared in the F1 and reappeared in one-fourth of the F2 - How could one explain the 3:1 ratios observed in monohybrid crosses? P yellow X green  F1 all yellow (dominant) (self)  3/4 yellow (519) 1/4 green (170) 689 total F2Mendel further tested the members of the F2 generation. F2  3/4 yellow (519) 1/4 green (170) - Green F2 plants were grown and selfed All green F3 peas What does this tell us about the F2 green pea plants? - F2 • Next Mendel took a sample of 519 yellow F2 peas and grew plants from them. -These 519 yellow-pea F2 plants were selfed individually to produce F3: - a) 166 of the yellow F2 plants bore only yellow peas (Pure breeding) - b) 353 of the yellow F2 plants bore a mixture of yellow and green peas in the F3, again in a 3:1 ratio. (Similar to the original F1 generation) - - Of the F2 yellow peas: - 1/3 (166/519) were like the pure breeding yellow parent - 2/3 (353/519) were like the F1 yellow peas…producing yellow and green seeds again in a 3:1 ratio - All F2 green peas appeared pure-breeding (like green parental line) - Therefore, the underlying 3:1 phenotypic ratio in the F2 generation was a more fundamental 1:2:1 ratio Mendel’s Model: 1) The existence of genes: Traits are determined by “discrete heritable factors.” (Genes) 2) Genes are in pairs: In the adult pea plant, each type of gene is present twice in each cell (gene pair). - The gene may have different forms (alleles), each corresponding to alternative phenotype of a particular characteristic. - In different plants, the gene pair can be of the same alleles or of different alleles. Alleles can be dominant or recessive (properties of the phenotype)… (i.e. When “T” and “t” co-exist in a plant, one observes the T phenotype) - T = dominant; t = recessive - 3) Halving of gene pairs in gametes: Each gamete carries only one member of each gene pair. To prevent the number of genes from doubling every time gametes fused (fertilization), Mendel proposed that during gamete formation the gene pair halved (Principle of Segregation). 4) Equal segregation: The members of the gene pairs segregate (separate) equally into the gametes (Principle of Segregation). -50% of the gametes will carry one member of the gene pair, and 50% will cary the other (i.e. Tt = Tall, 50% of the sperm will have a “T” and the other 50% will have a “t”). 5) Random fertilization: The union of one gamete from each parent to form the zygote is random.Principle of Segregation - Genotype: The genetic make-up of an individual (allelic make-up) - Homozygote: an individual that carries two identical alleles of a particular gene TT tt - Heterozygote: an individual that carries two different alleles of a particular gene Tt • Phenotype: The physical appearance of an individual as a result of its