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Mendelian Genetics: The Laws of Inheritance By Lisa Marie Meffert, PhD Morpho plelides Mark T. Hark, MD BioEd Online Blending Model of Inheritance vs. Particulate Model of Inheritance Blending model: Offspring are a blend of the parents. The blend of characteristics is passed on to the next generation. Variation is washed out over time. Particulate model: Offspring are a combination of the parents. Parental contributions are passed to the next generation as separate entities. Variation is maintained over time. BioEd Online Some Terminology Genotype: the combination of alleles that an organism possesses in regard to a trait. Eg. Pp Genotypic ratio: the mathematical ratio of the allele pairs. Phenotype: the outward appearance of the trait in the organism (analogous to a photograph). Eg. Purple flower Phenotypic ratio: the mathematical ratio of the appearance of the traits. Note: Some genes may have three or more alleles, however, each individual can have a maximum of two alleles per gene. More Terminology P1 – Parental Generation F1 – The First Generation F2 – The Second Generation Pure Breeding Line Homozygous dominant (TT) or Homozygous recessive (tt) Hybrid Heterozygous (Tt) Mendel’s Research on Particulate Inheritance Made pure breeding (parental) lines for different phenotypes of pea plants: Gregor Mendel Crossed the pure lines to get F1 generation Plant height – tall (T) vs. short (t) Pea color – yellow (Y) vs. green (y) Pea texture – round (R) vs. wrinkled (r), etc. All of the F1 individuals looked the same Crossed F1 lines to get F2 generation ¾ of the F2 individuals looked like one of the parental lines ¼ of the F2 individuals looked like the other parental line Pisum sativum BioEd Online Punnett Squares (Text p 260) A bookkeeping tool devised to represent the independent assortment and segregation of the alleles. A chart that shows all the possible combinations of the alleles that can result when two organisms are crossed (bred). It allows geneticists to predict the probability of occurrence of a particular trait. Terminology (cont’d) Mendel’s Monohybrid Cross “Mono” means “one” and the parent plants differ from each other by a single trait – height. Dihybrid Cross “Di” means “two” and the parent plants differ from each other by two different traits. In a dihybrid cross, will the two traits stay together in the next generation or will they be inherited independently of each other? Working out Mendelian Genetics – Monohybrid Cross The allele is the specific form of hereditary particle passed on through male or female gametes. Allele capital letter T confers tallness. In this case, the tall parental line has all T’s. Allele lower case letter t confers shortness. The short parental line has all t’s. Cross a tall male (top) with a short female (side). BioEd Online Crossing the Parental Lines: Male Contribution First, the male passes on his alleles, one per gamete (reproductive cell). BioEd Online Crossing the Parental Lines: Female Contribution Then, the female passes on her alleles, one per egg. Crossing these two parents yields all Tt offspring. Since T is dominant to t, all of the offspring are tall pea plants. BioEd Online Crossing the F1 Generation Crossing two homozygous individuals, TT and tt, yields all heterozygous offspring, Tt. This generation is referred to as F1. Now, cross two individuals from the F1 generation (Tt x Tt). First, the male pea plant passes on its alleles, one per gamete. BioEd Online Crossing the F1 Generation Now, the female passes on her alleles, one per egg. Genotypes of the F1 Phenotypes of the F1 ¼ of the offspring are TT. ½ of the offspring are heterozygotes (one T and one t). ¼ of the offspring are tt. Genotypic ratio is 1:2:1 ¾ are tall (at least one T). ¼ are short (homozygous for t). Phenotypic ratio is 3:1 Because there are four boxes, the ratios become easy—these are expressed as fractions &/or percentages. BioEd Online – Dihybrid Cross Keep T and t for tall and short plants, respectively. Add R and r for round and wrinkled seeds, respectively. A double heterozygote male produces four types of gametes. BioEd Online Cross with a Female Double Heterozygote A double heterozygote female produces four types of eggs. BioEd Online Crossing the Double Heterozygotes The male passes on his alleles, two per gamete. BioEd Online Crossing the Double Heterozygotes The female passes on her alleles, two per egg. BioEd Online Double Heterozygote Cross: Assessing the Resultant Genotypes At least one T yields tall stature. At least one R yield round seeds. Thus, there are 9 ways (gray) to produce tall, roundseeded offspring. BioEd Online Double Heterozygote Cross: Assessing the Resultant Genotypes Again, at least one T yields tall stature. And one R is required for round seeds, so rr must produce wrinkled seeds. Thus, there are 3 ways (lighter gray) to produce tall, wrinkly seeded offspring BioEd Online The Third Phenotype in a Double Heterozygote Cross Two t alleles yield short stature. At least one R produces round seeds. Thus, there are 3 ways (lightest gray) to produce short offspring with round seeds. BioEd Online The Fourth Phenotype in a Double Heterozygote Cross Two t alleles yield short stature. Two r alleles produce wrinkled seeds. There is only 1 way to produce short offspring with wrinkled seeds. BioEd Online Assignment HEREDITY PROBLEM SET WORKSHEET Gene diagram – Flower colour parent gamete R Offspring genotype Rr phenotype red Male female Rr rr r rr r rr yellow yellow Red 50% yellow 50% r Rr red