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Where we’ve been: 16.1 GENES AND VARIATION I. “Gaps” in Darwin’s Theory 1. How do VARIATIONS arise? -Variation was one of the key “ingredients” of NATURAL SELECTION - Remember: variation, selection, time - Some variations are better than others, the environment SELECTS those. 2. How are FAVORABLE variations (traits) passed on to offspring? What scientist’s work could have help Darwin with these questions? MENDEL!!! (Remember, he described INHERITANCE of “factors” in pea plants). + II. Putting it all together! By the 1930s, Mendel and Darwin’s work were combined. Molecular biologists had also discovered that: 1) DNA was the molecule of heredity. DNA determines phenotype. 2) Sources of variation include MUTATION and GENE SHUFFLING We can now describe evolution in GENETIC terms. We can be more specific than “change over time.” Evolve: change over time Evolution (in genetic terms) is any CHANGE in the relative FREQUENCY of ALLELES in a population. OR AKA: biological evolution or microevolution A change in genetic composition within a population over generations. Genetic Vocabulary Review Gene Trait Genotype Allele Phenotype Vocab: 1. A POPULATION is a group of individuals of the same species that interbreed and reproduce. 2. A GENE POOL is all genes, including different alleles, that are in a population. 3. RELATIVE FREQUENCY is the # of times an allele appears in a population. General Equation: Relative frequency of an allele= # of the certain allele in the population # of TOTAL alleles in the population MAIN IDEA! Allele Frequency is about finding (# of A’s) (total # of A’s + a’s) Application What is the allele frequency in the mice population Which mice genotypes contain the dominant allele for fur color (B)? 2, the BB and Bb mice How many dominant alleles are in the homozygous dominant black mouse? The heterozygous mouse? 2, 1 allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown ***Relative frequency of a DOMINANT allele = (# homozygous dominant x 2) + (# heterozygous x 1) (# individuals in population x 2) Why multiply the # of individuals by 2? Each individual has two alleles for a trait, one from mom, one from dad. allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown Which mice genotypes contain the recessive allele (b)? the are Bb inand bb mice mouse? The homozygous recessive How many recessive2, alleles the heterozygous mouse? 1, 2 allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown ****Frequency of a RECESSIVE allele= (# homozygous recessive x 2) + (# heterozygous x 1) (# individuals in population x 2) allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown III. Example: Calculating the relative frequency Example 1: Let's consider a gene with only two alleles. In mice, Black fur color (BB or Bb) is dominant to brown fur color (bb). In a population of 100 mice, 36 mice are homozygous dominant (BB), 48 mice are heterozygous (Bb) and 16 are brown (bb). Relative frequency of B= # of B alleles in the population # of TOTAL alleles in the population Relative frequency of a dominant allele: = (# homozygous dominant x 2) + (# heterozygous x 1) (# individuals in population x 2) Relative frequency of B = (36 x 2) + (48 x 1)= .60 = 60% (100 x 2) = 200 or 36+36+48 = 120 = 60% 100+100 200 How could we figure out the frequency of “b” without doing the big equation? 0.6 + 0.4 =1.0 Note that the allele frequencies add up to 1. *This is a law of population genetics: The sum of all allele frequencies will always be equal to 1. This is because 1 represents the frequency of all possible alleles within the population. Frequency of b? Frequency of b = # of b alleles in the population # of TOTAL alleles in the population Frequency of a recessive allele: = (# homozygous recessive x 2) + (# heterozygous x 1) (# individuals in population x 2) Frequency of b= (16 x 2) + (48 x 1) = (100 x 2) = 200 80 = 0.4 = 40% 200 or (16 + 16 + 48) 40% (100 +100 = )= 80 200 Note that the two allele frequencies add up to ONE. 0.6 + 0.4 =1.0 = 0.4 = Let’s say that these mice have several generations of offspring. We want to know if EVOLUTION has occurred. How would we know? If the relative frequencies of the alleles CHANGE, then evolution has occurred! 2. Sexual Reproduction (Gene Shuffling) Q: Why don’t you look exactly like your parents? (what were the sources of genetic variation you learned in meiosis?) Meiosis: Random assortment of genes, random separation of chromosomes (the Law of Independent Assortment). Crossing Over: exchanging parts of homologous chromosomes REMEMBER! Some variations are better than others, the environment SELECTS those. The source of variation is on the DNA level!