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(8.12-8.24). Chromosomes Exist in Homologous Pairs. There are 46 chromosomes in a human somatic (body) cell. These chromosomes exist in 23 homologous pairs The two homologs carry genes controlling the same inherited traits (chromosome theory of heredity)

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(8.12-8.24)

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  1. (8.12-8.24)

  2. Chromosomes Exist in Homologous Pairs • There are 46 chromosomes in a human somatic (body) cell. • These chromosomes exist in 23 homologous pairs • The two homologs carry genes controlling the same inherited traits (chromosome theory of heredity) • Although each homolog may have a different version (allele) for the gene, the locus is the same for each

  3. Autosomes v. Sex Chromosomes • Only the first 22 of the 23 chromosome pairs are definite homologs. (The autosomes) • The 23rd pair contains the sex chromosomes (X/Y). • Females have two Xs; Males have an X and Y • Only small portions of X and Y are homologous

  4. Gametes Have a Single Set of Chromosomes • A cell with two homologous chromosomes is diploid (2n) • Gametes (egg & sperm) are haploid (1n) because they only have a single set of chromosomes • Fusion of the haploid cells produces a diploid zygote during fertilization

  5. Meiosis Reduces the Chromosome Number from Diploid to Haploid • Often referred to as “reduction division” meiosis is a cell division that forms haploid gametes in diploid organisms • Many stages closely resemble corresponding stages in mitosis • Preceded by duplication of chromosomes, followed by two consecutive divisions, Meiosis I and II Animation

  6. Meiosis I v. Meiosis II

  7. Meiosis in Males v. Females

  8. Meiosis Increases Genetic Variation:Independent Assortment • Total # of chromosome combos = 2n • n = # chromosome pairs • For humans 223 or ~8 million combos

  9. Meiosis Increases Genetic Variation:Crossing Over & Genetic Recombination Animation

  10. Linkage & Gene-Mapping • Genes that are located close together have a lower frequency of crossing over • By evaluating relative frequencies, geneticists can map the locations of genes on chromosomes

  11. Creating Karyotypes

  12. Non-Disjunction: Meiosis I v. Meiosis II

  13. Turner’s Syndrome

  14. Klinefelter’s Syndrome

  15. Jacob’s Syndrome/Super Male (XYY) • Acne • Agression • Tall stature • Superior muscle strength • Reduced muscle coordination

  16. Triple X/Super female Syndrome (XXX) • No unique physical features apart from tall stature. • It may be asymptomatic and diagnosed incidentally • There is an increased incidence of motor and cognitive impairment, and psychological problems • Fertility of affected females is normal • There is a slight increased risk of offspring with chromosomal abnormalities

  17. Down Syndrome (Trisomy 21)

  18. Patau Syndrome (Trisomy 13) • Least common trisomy • More female fetuses survive to birth • Severe heart & kidney defects

  19. Edward’s Syndrome (Trisomy 18) • 25% die before within first month, only 10% survive past first year • Growth deficiency • Feeding difficulties • Breathing difficulties • Developmental delays • Kidney malformations • Structural heart defects

  20. Chromosomal Mutations

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