chapter 15 notes the chromosomal basis of inheritance
TRANSCRIPT
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Chapter 15 notes
The Chromosomal Basis of Inheritance
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Concept 15.1
In 1902 Sutton noted the parallels between the behavior of chromosomes and the behavior of Mendel’s factors.
Chromosomal theory of inheritance: genes have specific loci on chromosomes, and the chromosomes undergo segregation and independent assortment
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Concept 15.1
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Concept 15.1
Morgan traced a gene to a specific chromosome
- Morgan used Drosophila “fruit flies” for his experiments
- only 4 pairs of chromosomes - wild type flies have red eyes- white eyes is a mutant phenotype
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Concept 15.1
- Morgan mated the white-eyed male to a red-eyed female
- all F1 offspring were red-eyed- The F2 offspring were not 3:1; instead all females were red-eyed while half of the males had red and half had white eyes- eye color was linked to the fly’s sex
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Concept 15.1
Genes located on sex chromosomes are called sex-linked genes- Morgan’s evidence that a specific gene is carried on the X chromosome helped confirm the chromosomal theory of inheritance.
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Concept 15.1
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Concept 15.2
linked genes: genes that are located on the same chromosomes and tend to be inherited together- linked genes deviate from expected Mendelian ratios-ex. in flies body color and wing shape are inherited together
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Concept 15.2
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Concept 15.2
Genetic recombination: the production of new combinations of traits inherited from two parents
yellow-round x green-wrinkledYyRr x yyrr
Parental types: when the offspring phenotypes are identical to the parents- ex ¼ YyRr, ¼ yyrr
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Concept 15.2
Recombinants: when the offspring phenotypes are new combinationsex ¼ Yyrr, ¼ yyRr
When 50% of all offspring are recombinants, we say there is a 50% frequency of recombination- 50% frequency is observed for genes located on different chromosomes
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Concept 15.2
Geneticists can use recombination data to map a chromosomes genetic loci
Genetic map: an ordered list of the genetic loci along a particular chromosome
Linkage map: a genetic map based on recombination frequencies
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Concept 15.2
Map of body-color (b), wing-size (vg) and cinnabar (cn)- cn and b is 9%- cn and vg is 9.5%- b and vg is 17%
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Concept 15.2
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Concept 15.3
Sex-linked genes have unique patterns of inheritance- fathers pass sex-linked alleles to daughters, but not sons- mothers pass sex-linked alleles to both sons and daughters
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Concept 15.3
Because males only have one locus, they cannot be heterozygous
Sex-linked disorders in humans- muscular dystrophy: 1/3500 males in the US- hemophilia: absence of proteins for blood clotting
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Concept 15.3
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Concept 15.4
Nondisjunction: the members of a pair of homologous chromosomes do not move apart properly during meiosis I or II.- gametes are (n+1) or (n-1) if they have too many or too few chromosomes
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Concept 15.4
Aneuploidy: having an abnormal chromosome number
Trisomic: a chromosome in triplicate (2n+1); ex. trisomy 21 (Down’s syndrome)
Monosomic: if a chromosome is missing (2n-1)
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Concept 15.4
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Concept 15.4
Polyploidy: organisms that have more than two complete chromosome sets- triploidy (3n)- tetraploidy (4n)
Polyploids are more normal in appearance than aneuploids
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Concept 15.4
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Concept 15.4
Breakage of a chromosome can lead to four types of changes in chromosome structure
Deletion: occurs when a chromosomal fragment lacking a centromere is lost during cell division
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Concept 15.4
Duplication: a fragment becomes attached as an extra segment to a sister chromatid
Inversion: a chromosomal fragment can reattach to the original chromosome in reverse order
Translocation: the fragment joins a nonhomologous chromosome
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Concept 15.4
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Concept 15.4
Alterations in chromosome number and structure are associated with several human disorders- Down syndrome: 1/700 children born; extra 21st chromosome- Klinefelter syndrome: XXY male- Turner syndrome: XO female
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Concept 15.4
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Concept 15.4
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Concept 15.4