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The Chromosomal Basis of Inheritance

The Chromosomal Basis of Inheritance. Chapter 15. Review. Mitosis Meiosis Chromosome Genotype and Phenotype Mendelian Genetics. Thomas Hunt Morgan. Studied Drosophila melanogaster Large number of offspring Small, easy to care for 4 chromosomes with easily observable phenotypes.

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The Chromosomal Basis of Inheritance

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  1. The Chromosomal Basis of Inheritance Chapter 15

  2. Review • Mitosis • Meiosis • Chromosome • Genotype and Phenotype • Mendelian Genetics

  3. Thomas Hunt Morgan • Studied Drosophila melanogaster • Large number of offspring • Small, easy to care for • 4 chromosomes with easily observable phenotypes

  4. Drosophila melanogaster Phenotypes • Wild type: red eyes • Variations: eye color, body color, wing shape • Males and females are easy to tell apart

  5. Morgan’s Cross • Red eyed female x White eyed male (w+) (w) • F1  100% red eyed (wild type) • F2  3:1 ratio BUT only males had white eyes

  6. Morgan’s Conclusion • The gene for eye color must be carried on the X chromosome and NOT an autosome • Higher probability of a male having the recessive phenotype

  7. Sex Linked Traits

  8. Color Blindness • Xb = No color blindness • XB = color blindness • Determine the crosses (both phenotype and genotype) for the following crosses • Color blind father x normal mother yields one color blind son. • Normal father, carrier mother. • Normal father, color blind mother. • What cross will yield a color blind daughter?

  9. Sex Linked Traits • Called hemizygous • Do occur in females but males have a higher probability of inheriting the trait • Duchenne muscular dystrophy, hemophilia, color blindness

  10. X Inactivation in Females • Having two X chromosomes is a lot of genes! • One X chromosome will be turned off • Barr body: X chromosome condenses and will be near the nuclear envelope • Ovaries – Barr body will be duplicated for viable egg cells • In development, different X chromosomes could be turned off • About ½ of the cells display the mother’s traits and half display the father’s

  11. Linked Genes • Genes on the same chromosome that tend to be inherited together • Morgan believed body color and wing shape were inherited together

  12. Genetic Recombination • Offspring show combinations of traits not found in the parents • How does this happen?

  13. Recombinants • Mendel’s peas • YyRr x yyrr • Which genotype and phenotypes are recombinants? Which are parental types?

  14. Crossing Over • During Meiosis I (Prophase I) • Homologous chromosome pairs come together forming a tetrad • Crossing over • Each chromosome will cross with the other in the pair • Parts of the chromosome will be exchanged

  15. Recombination Frequency • How likely is it that the two genes will be linked? • Based on how close they are on the chromosome • Closer they are, more likely they will be linked • Linkage map: genetic map based on recombination frequency

  16. Linkage Map for Drosophila melanogaster

  17. 15.4 Alterations of chromosome number or structure cause some genetic disorders

  18. Abnormal Chromosome Number • Nondisjunction: chromosomes do not separate correctly • Meiosis I or II

  19. Aneuploidy • Gamete with abnormal number of chromosomes unites with a normal gamete • Offspring will have abnormal number of chromosomes • Monosomic (2n – 1) • Trisomic (2n + 1) • Polyploidy: triploidy (3n), tetraploidy (4n)

  20. Chromosomal Mutations • Involve changes in the number or structure of chromosomes • 4 types: • Deletion • Duplication • Inversion • Translocation

  21. Deletion and Duplication • Deletion: • Loss of all or part of a chromosome • Duplication: • Produce extra copies of the chromosome

  22. Inversion and Translocation • Inversion: • Reverse direction of parts of the chromosome • Translocation: • Part of one chromosome breaks off and attaches to another chromosome

  23. Human Disorders caused by Chromosomal Alterations • Down Syndrome (Trisomy 21) • Kleinfelter Syndrome (XXY) • Turner Syndrome (monosomy of the X chromosome)

  24. Inheritance Patterns • Not all fall into set patterns of equal inheritance • Genomic Imprinting • Effect of the allele for a certain trait depends on which parent passed on the trait • Could be expressed in different strengths

  25. Genetic Imprinting in Humans Prader-WilliSyndrome AnglemanSyndrome Deletion of part of chromosome 15 Learning difficulties, speech problems, seizures, jerky movements, and an unusually happy disposition Defective chromosome came from mom • Deletion of part of chromosome 15 • Learning difficulties, short stature, and compulsive eating • Defective chromosome came from dad

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