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Basic Concepts of Heredity Heredity - the transmission of traits from one generation to another Gregor Mendel in the 1860's published experiments that laid down the foundations for genetics. He is known for his garden pea experiments in which he was able to predict the outcome of the next generation. At this time no one had knowledge of chromosomes or the role of meiosis. Monohybrid Cross - simplest cross of two contrasting traits. Ex. Tall stems vs. Dwarf stems 1st generation (F1)= All tall stemmed plants When the F1 generation was crossed = 787 tall, 277 dwarf a ratio of 2.84 : 1 Mendel experimented with other traits and came up with similar findings. Ex. Seeds - Round vs. Wrinkled F1 = all round F2 = 5474 round, 1850 wrinkled Ratio of 2.96: 1 Flower color - Violet vs. Blue F1 generation = violet F2 generation = 705 violet, 224 white Ratio of 3.15:1 From this data, Mendel came up with 3 principles of Inheritance. 1. Unit Factors in Pairs - genetic characters are controlled by unit factors that exist in pairs in individual organisms 2. Dominance/Recessive - When 2 unlike unit factors are responsible for a single characteristic are present in a single individual, one is said to be dominant and one recessive. Dominant - the expression of a trait in a heterozygous condition. Ex. Tall stems, round seeds, violet colored flowers Recessive - term describes an allele that is not expressed Ex. dwarf stems, wrinkled seeds, white flowers 3. During the formation of gametes, the paired unit factors separate or segregate paired unit factors separate or segregate randomly so the gamete receives one or the other. - If an individual contains a pair of like unit factors - (ex. both specify for tall) then all gametes will receive one tall unit factor) - If individual contains unlike unit factors ( one dwarf and one tall) then each gamete has a 50% probability of receiving tall or dwarf unit factors. Example: Parental Cross Tall DD Fertilize D X X Dwarf dd d First Generation (F1) Dd Cross Dd Dd X D Second Generation (F2) DD F2 Genotype Tall Homozygous d = all Tall Dd D d Dd Dd Tall Tall Heterozygous dd Dwarf Homozygous Gene- a distinct hereditary unit found on a chromosome. Phenotype - the physical appearance - Ex. Tall verses Dwarf, Wrinkled verses Smooth Allele - single gene that determines the phenotype Ex. The allele for the phenotype tall vs dwarf is the height of the plant. A lowercase letter represents the allele for the recessive trait. An uppercase letter represents the dominant trait. Ex. d = Dwarf D = Tall Genotype - when 2 alleles are written in pairs to represent the 2 unit factor and is considered to be the genetic makeup of an individual (ex. DD, Dd, dd) Punnett Square - allows you to visualize genotypes and phenotypes resulting from the recombination of gametes Example: Dd D D D x Dd d