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Punnett Square

Punnett Square. Demonstrates how alleles can be combined when the F1 plants are self-fertilized to produce an F2 generation. Shows that 1/4 of the F2 plants should be homozygous dominant (TT), 1/2 heterozygous (Tt), and the remaining 1/4 to be homozygous recessive (tt). Genotype.

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Punnett Square

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  1. Punnett Square • Demonstrates how alleles can be combined when the F1 plants are self-fertilized to produce an F2 generation. • Shows that 1/4 of the F2 plants should be homozygous dominant (TT), 1/2 heterozygous (Tt), and the remaining 1/4 to be homozygous recessive (tt).

  2. Genotype • The genetic makeup of an individual. • Genotype can refer to an organism’s entire genetic makeup or to the alleles at a particular locus. The Genotype of the pea pods here would be either “RR”“Rr”“Rr” or “rr”

  3. Phenotypes • The observable or detectable physical characteristics of an organism; the detectable expressions of genotypes, frequently influenced by environment.

  4. Codominance • The expression of two alleles in heterozygotes. • In this situation, neither allele is dominant or recessive; thus, both influence the phenotype. (AB bloodtype)

  5. Exercise…

  6. Microevolution • Small genetic changes that occur within a species. • A human example is the variation seen in the different ABO blood types.

  7. Principle of Independent Assortment • The distribution of one pair of alleles into gametes does not influence the distribution of another pair. • The genes controlling different traits are inherited independently of one another.

  8. Mendelian Traits • Characteristics that are influenced by alleles at only one genetic locus. • Examples include many blood types, such as ABO. • Many genetic disorders, including Achondroplasia (dominant), Huntington disease (dominant), and Sickle-cell anemia (recessive) are also Mendelian traits.

  9. Huntington Disease • Huntington disease affects about 1 out of every 100,000 people and is caused by a dominant mutation on chromosome 4. • A person with the allele has a 50% chance of passing it their offspring. • There is no cure and symptoms most often occur between ages 35 and 45. • By this time, most people who want children have had them and may have passed the mutant allele on to their offspring.

  10. Huntington’s Disease… • Suppose one of your parents was diagnosed with Huntington disease. • Would you be tested to determine whether you carried the allele for the disease? • If you have the test, you will either be relieved by the results or know that you’ll develop a neurological disease that will ultimately kill you.

  11. Non-Mendellian traits:Polygenic Inheritance • Polygenic traits, or continuous traits, aregoverned by alleles at two or more loci, and each locus has some influence on the phenotype. • Hair, eye and skin color are polygenic traits

  12. Polygenic traits cont…. • Stature, shape of face, fingerprint patterns are polygenic traits • Most can be measured, i.e. height in feet and inches or meters and centimeters

  13. Mitochondrial Inheritance • All cells contain mitochondria that convert energy into a form that can be used by the cell. • Each mitochondrion contains copies of a ring-shaped DNA molecule, or chromosome. • Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers. • All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time.

  14. Gene Flow • The exchange of genes between populations. • If individuals move temporarily and mate in the new population (leaving a genetic contribution), they don’t necessarily stay there. • Example: “Sex Tourism”

  15. Genetic Drift • In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. • The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. That, in a nutshell, is genetic drift. It happens to ALL populations—there’s no avoiding the vagaries of chance. • Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.

  16. Founder Effect • Occurs when a small band of “founders” leaves its parent group and forms a colony elsewhere. • A new population is established and as long as mates are chosen within this population, all the members will be descended from the founders. • A once rare allele that was carried by even one of the founders can eventually become common.

  17. Bottleneck • Cheetahs, like many other species, have passed through a genetic bottleneck. • As a species they have little genetic variation.

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