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Topic 4 Genetics

Topic 4 Genetics. 4.1 Chromosomes, genes, alleles, mutations. Eukaryotic chromosomes are structures made up of DNA and proteins. Animation of chromosome structure Second Animation. Some definitions.

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Topic 4 Genetics

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  1. Topic 4 Genetics

  2. 4.1 Chromosomes, genes, alleles, mutations • Eukaryotic chromosomes are structures made up of DNA and proteins. • Animation of chromosome structure • Second Animation

  3. Some definitions • Gene: a heritable factor that controls a specific characteristic. Coded by a portion of DNA. • Allele: Alternate form of a gene. Codes for the same characteristic and is at the same location (locus) on the chromosome but the DNA sequence is slightly different. • Genome: The entire set of genetic information of an organism • Gene Mutation: A change in the base sequence of a gene. This can be a small as one base change in an entire gene.

  4. Sickle Cell Anemia • An example of just one change in a base that causes a significant change in the organism and a life threatening disease. • The triplet code on the mRNA GAG is changed to GTG. • This means that the amino acid Valine is translated instead of Glutamic acid • These 2 amino acids are very different chemically and so affect the final protein, in this case hemoglobin. • The Hbs hemoglobin does not carry oxygen as well as normal, Hba hemoglobin

  5. Sickle Cell Anemia • This causes the red blood cell to have a sickled shape. • These RBCs do not pass through the small capillaries well and get stuck, occluding blood flow • This causes problems in many body systems and causes severe pain • Animation 1 • Animation 2

  6. 4.2 Meiosis • Meiosis is a reduction of the number of chromosomes by half and division of the nucleus. • A diploid nucleus (one with a pair of each chromosome) is divided into a haploid nucleus (one that has only one of each chromosomes. • So for humans meiosis reduces a gamete to 23 chromosomes from the original 46.

  7. Homologous chromosomes • In a diploid cell (all cells but gametes) the pairs of chromosomes are called homologous because they have the same genes on each chromosome. • Although the gene is the same the genetic information may be different. • Each gene has two possible types (alleles). • Homologous chromosomes have the same size and shape and have the same genes at the same place.

  8. Process of meiosis • Meiosis happens in 2 steps: meiosis I and II • In the first step the chromosome number is halved • In the second step the chromosomes are replicated, this is just like mitosis • The process is sometimes called reduction-division. • The parent cell is 2n and the result is 4 daughter cells with n # of chromosomes. • How is this different than mitosis?

  9. Meiosis I overview

  10. Prophase and Metphase I

  11. Prophase and Metphase I • In Prophase homologous chromosomes are paired together, called tetrads • Crossing over is the exchange of DNA between non sister chromatids • Points where crossing over occurs is called chiasmata • In metaphase the homologous pairs line up at the equator • The way a pair of chromosomes is oriented is by chance, this is called Independent Assortment. There is a 50/50 chance which allele will end up in which cell. (remember one allele is maternal, one is paternal)

  12. Anaphase, Telophase, Cytokinesis • Homologous chromosomes are pulled apart • Sister chromatids are still attached

  13. Anaphase I, Telophase I, Cytokinesis • In telophase each ½ of the cell has a haploid set of replicated chromosomes, each chromosome I made up of 2 sister chromatids. • One or both chromatids have regions of nonsister DNA • Cytokinesis happens during telophase • There is no additional replication between MI and MII

  14. Prophase II, Metaphase II

  15. Prophase II, Metaphase II • Spindle fibers reform in prophase • Single chromosome line up at the equator • The sister chromatids are NOT identical • The assortment of non identical sister chromatids further adds to genetic variation

  16. At the end of meiosis there are 4 daughter cells • Each cell is genetically different from the others and from the parent cell

  17. Meiosis II

  18. Animation/quiz • Animation 2/ questions

  19. Compare mitosis and meiosis

  20. Non-disjunction • Non-disjunction means that in either meiosis I or II, the chromosomes do not split apart. • What would happen in MI if the homologous pairs did not split? • What would happen if they didn’t split in MII? • In both cases this leads to disease or more often the fertilized egg does not even develop. • Some cells will be missing that chromosome and some will have an extra copy. • Cells without the chromosome do not develop at all.

  21. Trisomy 21 • If it has an extra copy of a chromosome it will also not usually develop. • In the case of chromosome #21 the fetus does develop but has significant problems. • Other trisomies: 13, XY

  22. Karyotyping • A karyotype is a display of condensed chromosomes arranged in pairs. • Cells are grown in a culture and then stopped when they are in metaphase. • They are then stained and photographed • Chromosomes are paired based on size and shape • A karyotype can be used to screen for abnormal number of chromosomes or for defective chromosomes • Sex can also be determined

  23. Human karyotype

  24. 4.3 Theoretical Genetics • Genotype: the alleles of an organism • Phenotype: the characteristic of an organism • Homozygous: 2 identical alleles • Heterozygous: 2 different alleles • Dominant allele: an allele that has the same effect on the phenotype whether it is present in homozygous or heterozygous state • Recessive allele: an allele that only has an effect when it is in homozygous state • Codominant allele: both alleles are expressed

  25. Apply Definitions • Using our Rebop creatures, give an example of each of the definitions: • Genotype: • Phenotype: • Homozygous: • Heterozygous: • Dominant allele: • Recessive allele: • Codominant allele

  26. Genotype: Tt • Phenotype: curly • Homozygous: TT or tt • Heterozygous: Tt • Dominant allele: T- curly • Recessive allele: t- straight • Codominant: QQ= red, Qq= orange, qq= yellow

  27. Punnet Square • A way to determine genotypes and phenotypes of genetic crosses. • We will follow some genetic crosses • Pure breeding individuals are homozygous • P generation is parent

  28. Practice with Punnet squares • Cross a true breeding pea plant with yellow seeds with a true breeding plant with green seeds. Yellow is dominant. • Cross one of the offspring from above with a green seeded plant. • You can deduce some rules: • Cross homozygous dominant with recessive and get • Cross 2 heterozygous and get • Cross a heterozygous with a recessive and get

  29. Test Cross • What is you have an individual with a dominant phenotype, how do you know if it is homozygous or heterozygous? • Cross it with a recessive phenotype. • Determine the genotypic and phenotypic ratios for the 2 possibilities

  30. Practice • problems

  31. Locus: position of gene on chromosome • Test cross: a way to test if a dominant phenotype is heterozygous or homozygous dominant. Cross the individual with a recessive phenotype if any offspring have recessive phenotype then the individual was heterozygous

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