Not much was known about it.A. One idea was that traits came from a mixing of factors from the parents B. Variation among populations also seemed to randomly appear and disappear C. Old Ideas of HeredityI. Monk who established many basic ideas on inheritanceA. Mendel paired traits that different in a single variablea. They were very specific traits that went either one way or another b. Good Experimental System1. He set up his crosses very wella. His records were extremely well organizedb. He also prevented any unwanted fertilization of the pea plantsc. Good technique2. He integrated many existing disciplines to solve his questionsa. Specifically, he used his knowledge of horticulture and stats.b. Good Thinking3. Why He was SuccessfulB. He took a yellow pea and a green pea and crossed them, resulting in a yellow pea 1. When this resulting yellow pea was self-fertilized, the following resulted: 2. Note that the green phenotype reappears from an all yellow progeny a. The ratio is very specific: 3 yellow to one greenb. When each of these resulting progeny is self-fertilized, we see the following pattern emerges: 3. Note that when the green is self-fertilized, all yield all green peas. a. On the other hand, when the yellow peas are self-fertilized, 1/3 of the plants lead to an all-yellow progeny, while 2/3 lead to a 3:1 ratio. b. Mendel performed crosses between true-breeding strainsC. MendelII. Mendel explained his work by hypothesizing that there are two different A. Explaining Mendel's ResultsIII. True Breeding Strains are those that keep their characteristics when self-fertilized: the resulting off-spring have no changes in phenotype. Mendelian Genetics (1/23/08) Transmission Genetics in Eukaryotes Page 1 Mendel explained his work by hypothesizing that there are two different factors controlling the traits A. There are two different alleles (different forms of a gene) for each trait he chose B. "G" symbolizes the dominant allele (the yellow)1. "g" symbolizes the recessive allele (the green)2. Mendel created a nomenclature to track this. Each trait is assigned a letter. (For the color of the pea plants, we'll use g.) C. With this nomenclature, we can look at the transmission of the genes over time and the pattern of transmission makes sense D. Heterozygous: hybrid individuals who carry two different alleles.1. Homozygous: individuals carrying two copies of the same allele: true- breeding plants are homozygous. 2. Principle of Segregation- alleles segregate from each other during the formation of gametes and reunite at fertilization. 1. Principle of Dominance- in a heterozygote, the dominant allele conceals the presence of the recessive 2. Alleles of different genes segregate independently of one anothera. This is relevant in dihybrid crosses (see below)b. The only exception to this rule is linked traits, which are found on the same chromosome c. Principle of Independent Assortment3. Rules of Phenotype TransmissionE. Transmission of alleles can be predicted using a Punnett Square1. Male gametes are place on one side, female on the other2. Following TransmissionF. Genotype can be determined using a test cross1. A test cross is a crossing of individuals of an unknown genetic makeup with homozygous-recessive individuals to determine this unknown genetic makeup 2. Unknown:a. If the individual is homozygous-dominant:b. Example:3. Determining GenotypeG. Dominant allele: symbolized by a capital letter, the dominant allele is the controlling allele: it masks the recessive. Recessive Allele: the allele corresponding to a trait that is only revealed when no dominant allele is present. Shown by a lower- case letter. Phenotype- the visible, outward aspect of a trait Genotype: the genetic characteristics of a trait Transmission Genetics in Eukaryotes Page 2 If the individual is heterozygous:c. A dihybrid cross is a cross studying two different phenotypesA. Example:B. A Punnett Square can also be used in dihybrid crosses. Remember to sort the alleles according to the principles listed above. C. Dihybrid CrossesIV. Note this ratio! Transmission Genetics in Eukaryotes Page 3 Transmission Genetics in Eukaryotes Page 4
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