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Genetic Variation: The Source of Evolutionary Diversity The five major vertebrate classes exist due to evolutionary change. This change is, in turn, caused by deterministic and stochastic factors according to the process of natural selection. Natural selection can be summarized in 3 basic steps: 1. Variation 2. Selection 3. Reproduction The source of variation is genetics. The source of this genetic variation is our present focus. 10.02 1 Genetic Variation: The Source of Evolutionary Diversity The evolution of diverse species within and among the vertebrate classes is due, originally, to genetic variation. What are the sources of genetic variation? I. Mutation in DNA. II. Sexual reproduction (meiosis and recombination of alleles as shown on Punnett squares). III. Unique inheritance patterns (as shown on Pedigrees). 10.02 2 I. Mutation in DNA. A lot goes into creating a living organism. Mutations are mistakes in DNA. Mistakes can happen at any of the steps listed below (and more!). gene: codes for a trait allele: form of a gene (A or a) genotype: genetic make-up (AA, Aa, or aa) phenotype: trait dominant: A recessive: a heterozygous: Aa homozygous: AA or aa gamete: reproductive cell (contains half of alleles) meiosis: how gametes form 10.02 Variation source: New alleles can form when a mutation occurs 3 II. Sexual reproduction: Meiosis: how gametes form. R r u u Z Z 6 genes (12 alleles) P p t t H h 4 chromosomes (2 pairs) MEIOSIS: Sex cells duplicate and divide twice to form daughter cells. R u Z Only half of alleles present in each cell. These cells can combine with other gametes to form the new cell (zygote) of a new organism. 10.02 R u Z P t H r u Z P t H r u Z p t h p t h Variation source: Offspring contain alleles from BOTH parents --not exactly like either parent 4 II. Sexual reproduction: Punnett squares . Possible alleles from male gamete (sperm) Possible alleles from female gamete (egg) R r R r r r r r Any one of these four combinations could result! Variation source: Many possible combinations of alleles form genetic traits of offspring 10.02 5 II. Sexual reproduction: Dihybrid Punnett squares . Parental (P generation) genotypes Possible alleles from female gamete (egg) 10.02 Possible alleles from male gamete (sperm) Possible offspring (F1 generation) Variation source: Even more possible combinations of alleles to form traits of new offspring 6 III. Unique inheritance patterns In addition to the classic recessive/ dominant inheritance pattern, traits can be passed on in other ways. These include: Incomplete dominance Codominance Polygenic traits Multiple alleles Sex linked traits Further details can be found in Biology (Miller, 2000), pp. 209-216 and 230-239. Inheritance patterns can be tracked on pedigrees. 10.02 Variation source: Alleles interact in various ways to produce genetic traits 7 III. Unique inheritance patterns Incomplete dominance Codominance Normal cell Sickled cell (diseased: Sickle Cell Anemia) Heterozygous individuals have enough sickled cells to be malaria-resistant, but not enough to have Sickle Cell Aniemia. 10.02 Variation source: Alleles interact in various ways to produce genetic traits 8 III. Unique inheritance patterns Polygenic traits More than one gene makes the trait Sex linked traits 10.02 Multiple alleles More than one allele makes the trait Can you see the numbers in each circle? If not, you are probably male. (See next slide) Variation source: Alleles interact in various ways to produce genetic traits 9 III. Unique inheritance patterns: Sex-linkage. Possible alleles from female gamete (egg) Possible alleles from male gamete (sperm) The Y chromosome codes only for “maleness”. So the linked alleles are always 0 and never show dominance (even over recessive alleles such as h). Dominant phenotype Dominant phenotype 10.02 Dominant phenotype Recessive phenotype 10 III. Unique inheritance patterns: Mapping with Pedigrees A pedigree is a multi-generational family tree that displays gender, mates, offspring, and one or more genetic traits. Parents Children Grandchildren n=normal allele (Dominant) 10.02 p=affected allele (recessive) 11