(8.12-8.24)
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(8.12-8.24)
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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
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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
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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
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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
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Meiosis I v. Meiosis II
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Meiosis in Males v. Females
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Meiosis Increases Genetic Variation:Independent Assortment
•Total # of chromosome combos = 2n •n = # chromosome pairs•For humans 223 or ~8 million combos
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Meiosis Increases Genetic Variation:Crossing Over & Genetic Recombination
Animation
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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
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Creating Karyotypes
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Non-Disjunction: Meiosis I v. Meiosis II
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Turner’s Syndrome
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Klinefelter’s Syndrome
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Jacob’s Syndrome/Super Male (XYY)• Acne • Agression• Tall stature• Superior muscle strength• Reduced muscle
coordination
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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
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Down Syndrome (Trisomy 21)
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Patau Syndrome (Trisomy 13)•Least common trisomy•More female fetuses survive to birth•Severe heart & kidney defects
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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
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Chromosomal Mutations