in different plants the gene pair can be of the same

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Unformatted text preview: 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 What we learned from Mendel’s monohybrid crosses • Mendel’s Model Continued: 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. Mendel’s First Law of Segregation Principle of segregation The two alleles (i.e. A and a) segregate during gamete formation… the fusion of gametes at fertilization creates allele pairs again. More Nomenclature • 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 genotype Lets put everything together: Monohybrid Cross Example: Crossing a 2 pure-breeding peas plants: Phenotypically Smooth X Wrinkled Pea Plant Homozygous for Dominant Allele (S S) Pea Plant Homozygous for Recessive Allele (s s) (Genotype) (Genotype) Parental Gamete Formation Patenal Gamete Formation* (*Carries only one allele; in this case only one type of gamete formed from a homozygote) Heterozygous Pea Plant (S s) (Genotype) F1 Gamete Formation F1 Gamete Formation (2 types of gametes formed from a heterozygote) (2 types of gametes formed from a heterozygote) : Explains Phenotypic ratio… 3 smooth : 1 wrinkled What about the 1:2:1 ratio? Punnett Square • Named after British scientist Reginald Punnett… F1 male gametes F1 female gametes -they show you the combination of different genotypes -summarizes all the possibilities -determines the portion of the possibilities Punnett Square: Genotypic Ratio → Phenotypic Ratio F1 Male Gametes F2 = Smooth Ss 1 F1 Female Gametes SS + 2 : 3 + ss : 1 1 wrinked Phenotypic and Genotypic Ratios Resulting from a monohybrid cross Phenotypic Classes Smooth : wrinkled 3:1 Genotypic Classes SS : Ss : ss 1:2 :1 Mendel’s model put to the test Using a...
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