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Chromosomes and Sex-Chromosomes Inheritance

Chromosomes and Sex-Chromosomes Inheritance. Genetics Spring 2014. Outline. The life cycle of a typical animal. Mitosis - Overview. Mitosis is the process of nuclear division in which replicated chromosomes divide.

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Chromosomes and Sex-Chromosomes Inheritance

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  1. Chromosomes and Sex-Chromosomes Inheritance Genetics Spring 2014

  2. Outline

  3. The life cycle of a typical animal

  4. Mitosis - Overview Mitosisis the process of nucleardivision in which replicated chromosomes divide. Daughter cells have the same chromosome number and genetic composition as the parent nucleus. Cytokinesisisphysical division of cell into two daughter cells; usually accompanies mitosis. (Be able to draw mitosis for organism with 2 or 3 chromosomes) Checkpoints control transitions for G1 into S; G2 into M

  5. Mitosis – Different Phases Prophase Metaphase Anaphase Telophase • Separation of sister chromatides • Chromosome number does not change • Two genetically-identical daughter cells are produced

  6. Meiosis - Overview Meiosis: two successive nuclear divisions after only one round of DNA replication Meiosis in an organism with two pairs of homologous chromosomes: Separation at Anaphase I is physical basis of gene segregation

  7. Meiosis – looking at one chromosome • First nuclear division: • Homologous chromosomes closely associate (synapse), and then separate from each other. • Cell has half the number of chromosomes (each daughter chromosome consists of two identical chromatids). • The two cells are genetically non-identical. • The chromosome number is halved in the first division (reductional division). • Second nuclear division: • Resembles mitotic division of a cell with half the number of chromosomes. • The sister chromatids separate; four haploid daughter cells are produced (two each of two genetic types). • After second division (equational division), two each of two genetically non-identical gametes . Prophase I

  8. Meiosis – Independent Assortment Nonhomologous chromosomes assort independently during meiosis I: One meiotic event in 1 cell has 2 each of 2 different types But overall ratio of all gametes is 1:1:1:1

  9. Sex-Chromosome Inheritance – Sex Determination • Females are homogametic (X); males are heterogametic (X or Y). • Autosomes are all the other chromosomes. • X an Y associate during meiosis, at a small region of homology.

  10. Recombination between X and Y chromosomes in males occurs through pseudoautosomal regions (PAR) located at the ends of both short and long arms. • The Y chromosome contains genes related to gonadal and sexual development.

  11. X: female determinants Autosomes: Male determinants Y: not male determinants but sperm maturation XX: Female XY: Male XO: Sterile male or Y

  12. Sex-Chromosome Inheritance – X-linked Inheritance in Drosophila (reciprocal crosses) • Reciprocal crosses for X-linked eye color gene (w+ or w allele) give different results (not the same progeny). • Eye color gene is on the X chromosome • Males therefore exhibit recessive traits carried on • the X ch: • X-linked traits

  13. Sex-Chromosome Inheritance – X-linked Inheritance in Humans More diseased males than females Hemophilia (recessive) transmission among Queen Victoria's descendants

  14. Rett syndrome (dominant) is a progressive neurodevelopmental disorder almost exclusively affecting females. No male-to-male transmission because the male has to pass on its Y chromosome to its son. More diseased females than males Pedigree showing full penetrance

  15. Sex-Chromosome Inheritance – Nondisjunction Rare progeny from Nondisjunction of the X-chromosomes during meiosis in Drosophila provide proof for chromosomal theory

  16. Nondisjunction: chromosomes not separatingat meiosis (often anaphase I) and both chromosomes go to one pole. • (Gametes have too few or too many chromosomes. • At fertilization, zygote gets wrong number of chromosomes: • Nondisjunction of X chromosome result in some eggs with XX and some no X: • Fertilization -> XXX, XXY, XO and YO (YO is nonviable) Trisomy 21 (Down’s syndrome) results from non-disjunction of chromosome 21 (usually in female)

  17. Testing Goodness of Fit to a Genetic Hypothesis Laboratory Drosophila wild-type Drosophila mutant: vestigial wing Drosophila mutant: ebony body Drosophila mutant: white eye

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