Chapter 15: The Chromosomal Basis of Inheritance
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Transcript of Chapter 15: The Chromosomal Basis of Inheritance
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Chapter 15: 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
Thomas Morgan (early 20th century)Drosophilia melanogaster(fruit flies)Associated a specific gene with a specific chromosome
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Morgan’s Experiment
P1: Mated white eyed male with red eyed female
F1: 100% red eyedF1 generation mated
F2: 3 red : 1 whiteHowever???
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Morgan’s Experiment:
All females were red eyes:Half the males were red
The other half were white
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Morgan’s Conclusion:
1.1. Eye color was linked to sex2.2. specific genes are carried on
specific chromosomes3.3. genes located on sex
chromosomes exhibit unique inheritance patterns
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Linked Genes:
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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Another Morgan Experiment:
This time he observed body color and wing size:
Wild type = gray body (b+) and normal wings (vg+)
Mutant type = black body (b) and vestigial wings (vg)
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First Cross: true breeding wild type wit black vestigial
wingsb+b+vg+vg+ X bbvgvg
F1 = all wild type phenotype(b+bvg+vg)
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Second Cross: female dihybrids vs
true breeding reeessive males
b+ bvg+vg X bbvgvg(test cross)
2300 offspring were scored
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Results:-High proportion of parental phenotypes(965 wild type, 944 black vestigial)
-Low proportions of non- parental phenotypes
(206 gray vestigial, 185 black normal)
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Conclusion: 1. Body color and wing size are
usually inherited together(genes must be on the same
chromosome??)
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Conclusion: 2. Body color and wing size are
only partially linked:
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Explaining Morgan’s Results:Recombination of unlinked genes vs.
linked genes: Unlinked genes = independent
assortmentLinked genes = crossing over
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Recombination:Production of offspring with
combinations of traits different from those found in
either parent!
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Genetic Mapping:
Genetic maps are an ordered list of the genetic loci along a
particular chromosome
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Genetic mapping:
Recombination frequencies depend on distances between genes on a
chromosome
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Recombination frequency refers to the percentage of
recombinants occurring in the
offspring
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Alfred Sturtevant: hypothesis
*crossing over is a random event*the farther apart the genes on a
chromosome, the higher the probability that crossing over will
occur, so the higher the recombination frequency
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Sturtevant Reasoning:
The further apart two genes are, the more points between them where crossing over can occur.
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Linkage Map:
Probability of crossover between two genetic loci is proportional to the distance separating the two
loci. *experimental crosses reveal recombination frequencies
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Example: Drosophila
Body color (b)Wing size (vg)Cinnabar (cn)
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Map units: Distance between genes on a
chromosome1 map unit = 1% recombination
frequency
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Seed and flower color in pea
plants:Genes that are very far apart on
the chromosomeCrossing over is almost certain.
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Frequency of crossing over is not
uniform over the length of the chromosome
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Map units do portray order of genes on a chromosome
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Human sex-linkage
• The X-Y system: • Sex of offspring is determined by the
sperm.• Fathers pass Y chromosome to sons• Fathers pass X chromosome to daughters
• Mothers donate the X chromosome to sons and daughters
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Human sex-linkage
• SRY gene: gene on Y chromosome that triggers the development of testes• Sex determining region of
the Y chromosome• If Y chromosome is present,
gonads (first two months are generic) will develop into testes.
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Sex lined genes:
• Genes that are located on the sex chromosome• Genes that may code for
characteristics unrelated to sex• Recessive sex linked traits:
• Females must be homozygous• Males are hemizygous
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Sex-Linked Disorders:
•Color-blindness• female- must have a color
blind father and carrier mother.
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Sex-Linked Disorders:
•Duchenne muscular dystropy (MD); hemophilia• Defective dystrophin
protein
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Sex-Linked Disorders:
•X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in cats)• Except in ovaries where it
becomes reactivated
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Sex-Linked Disorders:
•Hemophilia: absence of one or more of the proteins responsible for blood coagulation• Queen Victoria pedigree
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Chromosomal Errors:
• Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II
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Chromosomal Errors:
• Aneuploidy: chromosome number is abnormal• Monosomy: missing chromosome
• Turner Symdrome -XO • Trisomy : extra chromosome
• Down syndrome- Trisomy- 21• Kleinfelters Syndrome- XXY
• Polyploidy: extra sets of chromosomes
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Chromosomal Errors:
• 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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Point mutations: affect protein structure and
function• Base pair substitution: one
nucleotide pair replacing another
• Missense vs. Nonsense mutations• Missense = altered codon still codes for an
amino acid – not necessarily the right one• Nonsense = changes the codon to a stop
codon• Premature termination leading to malfunctional
proteins.
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Insertions and Deletions:
• Adding or losing a nucleotide pair
• Disastrous effect on the protein
• Causes a Frame Shift:• Nucleotides down stream of the
mutation will be improperly grouped into codons that will likely produce a non- functional protein
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Genomic imprinting
• 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.
• Fragile X syndrome: higher prevalence of disorder and retardation in males
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Inheritance of Organelles:
• Some genes are considered extranuclear:• That is not found in nucleus• But, in organelles such as mitochondria
and chloroplasts
• These genes do not follow mendelian inheritance patterns• Randomly assorted to gametes and
daughter cells
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Inheritance of Organelles:
• Organelles are inherited maternally• Sperm only contributes
genetic information
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Mutations:
• Plant variegation: due to mutations in the genes that control plant pigments• Pattern of variegation is determined by
the ratio of wild type allele vs. mutant type allele for pigmentation
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Mitochondria DNA mutations:
• Heteroplasmy: when a cell contains both wild type and mutant type mtDNA.• Disorders usually affect nervous
and muscular systems• They require the most energy from
ATP
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Mitochondria DNA mutations:
• Disorders of the optic nerve (leber’s neuropathy) and other eye defects.
• Kearns- Sayre Syndrome- abnormal heart rate and central nervous system disorder
• Mitochondrial myopathy: muscle deterioration, intolerance to exercise
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