The Chromosomal Basis of Inheritance - WS AP...
Transcript of The Chromosomal Basis of Inheritance - WS AP...
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•The Chromosomal Basis of Inheritance
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The Chromosomal Theory of Inheritance
• Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment
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Chromosomal Linkage
• Morgan
• Drosophilia melanogaster
• XX (female) vs. XY (male)
• Sex-linkage: genes located on a sex chromosome
• Linked genes: genes located on the same chromosome that tend to be inherited together
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Genetic recombination
• Crossing overAllows genes to assort
independently
• Genetic mapsThe further apart 2 genes are,
the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
• Linkage mapsGenetic map based on
recombination frequencies
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Gene Mapping
• If traits assort independently, they will follow patterns predicted in a Punnett Square.
• Linked genes are more likely to be found together because they are carried together on the same chromosome
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Practice Problem
•Parents: NnPp x nnpp
•Use a Punnett square to predict the Expected number of offspring out of 1000.
• If your data shows: NP = 400, np = 400, Np = 100 and nP = 100, use a chi square to determine whether your expected data matches these observations
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• Sturtevant, an undergrad student of Morgan’s (21 old), invented a way to quantify the relationship between the rate of recombination and the distance between genes.
# recombinants = (RF=Recombination
# total Frequency)
• RF X 100 = = % recombination recombination
• 1% recombination = 1 map unit(mu)
• = 1 cM (centiMorgan)
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Human sex-linkage• SRY gene: gene on Y chromosome that triggers the development of
testes
• Fathers= pass X-linked alleles to all daughters only (but not to sons)
• Mothers= pass X-linked alleles to both sons & daughters
• Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia
• X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in cats)
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Chromosomal errors, I
• Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II
• Aneuploidy: chromosome number is abnormal• Monosomy- missing chromosome
• Trisomy- extra chromosome (Down syndrome)
• Polyploidy- extra sets of chromosomes
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Chromosomal errors, II
• Alterations of chromosomal structure:
• Deletion: removal of a chromosomal segment
• Duplication: repeats a chromosomal segment
• Inversion: segment reversal in a chromosome
• Translocation: movement of a chromosomal segment to another
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Genomic imprinting
• Def: a parental effect on gene expression
• Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
• Expression of genes varies as they are modified during development
• One copy of a gene is silenced by methylation
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Methylation
• Genes are silenced by addition of methyl groups to DNA or histones
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Epistasis
•Other genes impact the expression of traits.
•Genes in combination act differently due to background genes
•Genes in identical twins can be expressed differently
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Homeobox Genes
• Regions of the gene carry out similar function in different organism
• These regions have evolved from common ancestors and are homologous