chapter 15 chromosomes. chromosome theory of inheritance genes located on chromosomes = gene locus...
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Chapter 15 Chromosomes
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Chromosome theory of inheritance
• Genes located on chromosomes = gene locus• Thomas Hunt Morgan, Columbia Univ. “Fly room”
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• Drosophila– 100s of offspring– 2n = 8• 3 prs autosomes• X and Y sex chromosomes
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Bithorax Wildtype White eye
Wingless eyeless
Wildtype = normalMutant = abnormal
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Drosophila genetics
White eye allele = wWildtype allele = w+
gene locus onX chromosome
Sex-linked!
Genotype? ww, w+w w+w+
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Sex-linked genes
Cross a red female with a white male (pg. 288)
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Xw+ Xw+ X Xw Y
Punnett square results
Cross f1 females with f1 males
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f1 Xw+ Xw X Xw+ Y
f2
Some genes are located on the X chromosome
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The chromosomal basis of sex
In humans, Y chromosome determines sex
XX X XY Gametes?p(son)p (daughter)
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Humans have an X/Y system
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Development<2 months gestation – embryo has rudimentary
gonads
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2 mos. SRY gene on Y chromosome active-> testes develop
-> testosterone -> male
If no SRY female default pathway ovaries
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X-linked genes in humans
Female genotypes Male genotypes
XCXC XCXc XcXc XC Y XcY
Terms: homozygous, heterozygous, hemizygousConcept check:• From whom do males obtain the Y chromosome?• From whom do females obtain the X chromosomes?• Why are X-linked disorders more prevalent in males?
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Example: colorblindness
• 1/1O males• Colorblindness is an X-linked trait. A man is
colorblind. What % of his sons and daughters are expected to be colorblind. His wife does not carry the colorblind allele on either X chromosome.
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No green photoreceptors
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X-chromosome inactivation in females
• One X inactivated during embryonic development Barr body (see nuclear envelope)
• # in female cells? male cells?
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• Genes on this X are not expressed• Lyon hypothesis– Females mosaics for X-linked traits
– Allele key
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XCXc
• Patches of colorblind cells in retina
• Normal phenotype
Blue colorblindness is rare
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• Genotype of orange female?• Genotype of black female?
• Genotype of orange male?
• Genotype of black male?
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LinkageUnlinked genes
-On different chromosomes-Independent assortment!
Example in humans:Blood type (Chromosome 9) Lactose intolerance (Chromosome 2)
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Linked Genes -on same chromosome-do not assort independently
Example humans:Freckles (Chromosome 16)Red hair (Chromosome 16)
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Genetic recombination of linked genesCrossing over
Meiosis Non-sister chromatids of homologous chromosomes
25% of each
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Crossing over results in recombinants
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Chromosomal abnormalities• Disjunction anaphase of meiosis
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If chromosome do not disjoin
• Non-disjunction– Meiotic spindle error– Sister chromatids do not separate in Anaphase II– Gametes ?
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Fertilization (human)
Aneuploidy – abnormal number of chromosomes
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1. Monosomy (2n-1)
Ex. Turner syndrome 45, XOnly viable monosomy in humans
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2. Trisomy (2n+1)
Ex. Trisomy 21 (Down syndrome)
Higher risk in women > 35Abnormal meiosis
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• Nondisjunction anaphase I• Nondisjunction anaphaseII
• Fertilization animation at Learn Genetics
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Chromosome structure abnormalities
1. Deletion 2. Duplication 3. Translocation 4. Inversion
Univ. Wisc. Genetics center tumors
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Inheritance of mitochondrial DNAand chloroplast DNA (plants)
• Extranuclear genes
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• Maternal inheritance (cytoplasm)
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