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Heredity Overview

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  1. Heredity Overview How are genetic characteristics passed on from one generation to the next?

  2. Cell Division • Chromosomes • Contains the genetic information that is passed on (made of DNA) • Hominids = 46 chromosomes (23 pairs) • Mitosis • Body (“somatic”) cells  replaces damaged cells • Meiosis • Sex cells (sperm and egg)  creates variation within the species because of the random assortment of chromosomes in the sex cells

  3. Genetic Vocabulary Review • Fertilization  joining of egg and sperm • True-breeding  produces offspring identical to themselves • Trait  specific characteristic inherited • Hybrid  offspring of parents with different traits • Gene  chemical factors (DNA) that determine traits • Allele  the different forms of a gene

  4. Principle of Dominance • States that some alleles are dominant and others are recessive • Dominant allele  will always exhibit that form of the trait (A) • Recessive allele  will only exhibit if no dominant allele is present; need 2 recessive alleles to show a recessive trait (a) • Homozygous  individuals 2 of the same alleles • Homozygous dominant: AA • Homozygous recessive: aa • Heterozygous  individuals with 2 different alleles (Aa)

  5. http://faculty.southwest.tn.edu/jiwilliams/Human_Traits.htm

  6. Punnett Squares

  7. Segregation of Genes • Alleles segregate during gamete production • Phenotype = physical characteristics (tall vs. short • Genotype = genetic makeup (AA, Aa, or aa) • Pure cross  2 parents: one homozygous dominant and one homozygous recessive • Results in F1 offspring • F1 cross  2 parents from the F1 offspring (both are heterozygous) • Results in a phenotypic ratio of 3 tall:1 short • Results in a genotypic ratio of 1 AA:2 Aa:1aa

  8. Genes and Probability • Probability – likelihood that a particular event will occur • Flipping a coin once: heads (50% or 1/2); tails (50% or 1/2) • If flipped 3 times in a row = ½ * ½ * ½ = 1/8 (or 0.125) • Probability predicts possible genetic outcomes based on the way alleles segregate in meiosis

  9. Mendelian Genetics • In Mendelian genetics, Genes must be able to independently assort • Genes must not influence each other’s inheritance • Independent assortment accounts for the genetic variation in living things

  10. Beyond Dominant and Recessive Alleles • Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or genes • Incomplete dominance One allele is not completely dominant over the other • Codominance Both alleles contribute to phenotype • Multiple alleles (ex. blood type) • Polygenic traits  controlled by more than one gene (ex. Skin color, eye color, hair color)

  11. Genetics and the Environment • The characteristics of organisms  not solely determined by inherited genes • Determined by interaction between genes and the environment • Genes provide a plan for development  how the plan unfolds also depends on the environment

  12. Genes and Variation • 2 main sources of genetic variation: • Mutations • Gene shuffling during sexual reproduction • Gene pool  all genes (and alleles) that are present in a population • Relative frequency  # of times an allele occurs in a gene pool (given as a percentage) • Evolution is any change in the relative frequency of alleles in a population

  13. Evolution as Genetic Change • Evolutionary fitness  an organism’s success in passing genes to the next generation • Evolutionary adaptation  any genetically controlled physiological, anatomical, or behavioral trait that increases ability to pass on genes

  14. Natural Selection and Evolution • Natural Selection never acts directly on genes • Works on the entire organism…can only affect which individuals survive and reproduce • If an individual dies  does not pass on genes • ALSO…Individuals do not evolve…Populations evolve as the gene pool changes because of the relative frequency of the alleles changes

  15. How did Hominids Evolve? • Natural Selection in not the only source of evolutionary change • Genetic drift  • Individuals that carry a particular allele may leave more descendents BY CHANCE. • Over time, a series of chance occurrences can cause an allele to become common in a population • A new species can result if there is enough of a genetic difference between the original population and the new population

  16. The Process of Speciation • Formation of a new species (organisms that breed and produce fertile offspring) • As new species evolve, populations become isolated from one another • Reproductive isolation  members of 2 populations can no longer interbreed and produce fertile offspring

  17. Isolating Mechanisms • Behavioral Isolation  differences in courtship or other reproductive strategies that involve behavior • Geographic Isolation  2 populations are separated by geographic barriers • Temporal Isolation  2 or more populations reproduce at different times of the year