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Chapter 15: The Chromosomal Basis of Inheritance. The Chromosomal Theory of Inheritance. Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment. Chromosomal Linkage. Thomas Morgan (early 20 th century) Drosophilia melanogaster(fruit flies) - PowerPoint PPT Presentation
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Chapter 15: The Chromosomal
Basis of Inheritance
The Chromosomal Theory of Inheritance• Genes have specific loci on
chromosomes and chromosomes undergo segregation and independent assortment
Chromosomal Linkage
Thomas Morgan (early 20th century)Drosophilia melanogaster(fruit flies)Associated a specific gene with a specific chromosome
Morgan’s Experiment
P1: Mated white eyed male with red eyed female
F1: 100% red eyedF1 generation mated
F2: 3 red : 1 whiteHowever???
Morgan’s Experiment:
All females were red eyes:Half the males were red
The other half were white
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
Linked Genes:
Sex-linkage: genes located on a sex chromosome
Linked genes: genes located on the same chromosome that tend
to be inherited together
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)
First Cross: true breeding wild type wit black vestigial
wingsb+b+vg+vg+ X bbvgvg
F1 = all wild type phenotype(b+bvg+vg)
Second Cross: female dihybrids vs
true breeding reeessive males
b+ bvg+vg X bbvgvg(test cross)
2300 offspring were scored
Results:-High proportion of parental phenotypes(965 wild type, 944 black vestigial)
-Low proportions of non- parental phenotypes
(206 gray vestigial, 185 black normal)
Conclusion: 1. Body color and wing size are
usually inherited together(genes must be on the same
chromosome??)
Conclusion: 2. Body color and wing size are
only partially linked:
Explaining Morgan’s Results:Recombination of unlinked genes vs.
linked genes: Unlinked genes = independent
assortmentLinked genes = crossing over
Recombination:Production of offspring with
combinations of traits different from those found in
either parent!
Genetic Mapping:
Genetic maps are an ordered list of the genetic loci along a
particular chromosome
Genetic mapping:
Recombination frequencies depend on distances between genes on a
chromosome
Recombination frequency refers to the percentage of
recombinants occurring in the
offspring
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
Sturtevant Reasoning:
The further apart two genes are, the more points between them where crossing over can occur.
Linkage Map:
Probability of crossover between two genetic loci is proportional to the distance separating the two
loci. *experimental crosses reveal recombination frequencies
Example: Drosophila
Body color (b)Wing size (vg)Cinnabar (cn)
Map units: Distance between genes on a
chromosome1 map unit = 1% recombination
frequency
Seed and flower color in pea
plants:Genes that are very far apart on
the chromosomeCrossing over is almost certain.
Frequency of crossing over is not
uniform over the length of the chromosome
Map units do portray order of genes on a chromosome
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
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.
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
Sex-Linked Disorders:
•Color-blindness• female- must have a color
blind father and carrier mother.
Sex-Linked Disorders:
•Duchenne muscular dystropy (MD); hemophilia• Defective dystrophin
protein
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
Sex-Linked Disorders:
•Hemophilia: absence of one or more of the proteins responsible for blood coagulation• Queen Victoria pedigree
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
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
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
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.
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
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
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
Inheritance of Organelles:
• Organelles are inherited maternally• Sperm only contributes
genetic information
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
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
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