Genotype Phenotype Activities of genes & gene products Environment & development Genotype = collection of genes (and alleles) in an organism Phenotype = observable properties of an organism
Mendelian Genetics: Gregor Johann Mendel (1822-1884) Augustinian monk, Czech Republic. Foundation of modern genetics. Studied segregation of traits in the garden pea (Pisum sativum) beginning in 1854. Published his theory of inheritance in 1865. Mendel was “rediscovered” in 1902.
Mendel’s Experiments: 1.Began by self-fertilizing 34 different pea strains (phenotypes) so that they bred true (selfing, the opposite of cross-fertilization). 2.Focused on 7 well-defined garden pea traits by crossing different phenotypes one at a time: Flower/seed coat color: purple vs. white flowers grey vs. white seed coats (*controlled by single gene) Seed color: yellow vs. green Seed shape:smooth vs. wrinkled Pod color:green vs. yellow Pod shape:inflated vs. pinched Stem height:tall vs. short Flower position:axial vs. terminal 3.Counted offspring of each phenotype and analyzed the results mathematically.
Some basic terminology: Generations: P = parental generation F 1 = 1st filial generation, progeny (offspring) of the P generation F 2 = 2nd filial generation, progeny of the F 1 generation (F 3 and so on) Crosses: Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait. Reciprocal cross = sexes for the two strains are reversed (and if the results are the same, trait is not sex-linked). Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits. *Genetics etiquette - female conventionally is written first
Results of Mendel’s monohybrid parental cross: “Mendel’s Principle of Uniformity (the same) in F 1 ” F 1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents. Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (allele s). Fig. 10.5
Smooth and wrinkled parental seed strains crossed. Punnett square F 1 genotypes 4/4 Ss F 1 phenotypes 4/4 smooth
F 1 x F 1 crosses: Mendel also discovered that traits that disappear in the F 1 generation reappear in the F 2 generation in a 1:3 ratio.
F 1 x F 1 Punnett square: F 2 genotypes 1/4 SS 1/2 Ss 1/4 ss F 2 phenotypes 3/4 smooth 1/4 wrinkled
Crosses also can be represented with branching diagrams.
What about the six other phenotypic traits? 1.Results of reciprocal crosses always were the same. 2.F 1 progeny always resemble the parental strain. 3.In the F 2 progeny, parental strain lost in the F 1 generation always reappeared at a ratio of 1:3. “Mendel’s Principle of Segregation”: Recessive characters masked in the F 1 progeny of two true-breeding strains, reappear in a specific proportion of the F 2 progeny. Modern formulation of Mendel’s Principle of Segregation: Two members of a gene pair segregate (separate) from each other during the formation of gametes.
Confirming the Principle of Segregation with test-crosses: SS x SS true breeding (100% homozygous dominant) ss x ss true breeding (100% homozygous recessive) How do you determine whether an individual with the dominant phenotype is homozygous or heterozygous? Cross it with homozygous recessive: SS x ss 4/4 dominant trait Ss x ss 1/2 dominant trait + 1/2 recessive trait
Mendel’s dihybrid crosses: 1.Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green). 2.If alleles sort independently, four possible phenotypes (2 n ) appear in the F 2 generation in a 9:3:3:1 ratio. “Mendel’s Principle of Independent Assortment”: Alleles for different traits assort independently of one another. Modern formulation of independent assortment: Genes on different chromosomes behave independently in gamete production.
Summary of Mendel’s Principles: Mendel’s Principle of Uniformity in F 1 : F 1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents. Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (allele s). Mendel’s Principle of Segregation: Recessive characters masked in the F 1 progeny of two true-breeding strains, reappear in a specific proportion of the F 2 progeny. Two members of a gene pair segregate (separate) from each other during the formation of gametes. Inheritance is particulate, not blending as previously believed. Mendel’s Principle of Independent Assortment: Alleles for different traits assort independently of one another. Genes on different chromosomes behave independently in gamete production.
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