ch. 15 the chromosomal basis of inheritance...human disorders due to chromosomal alterations down...
TRANSCRIPT
1Nov 12 12:58 PM
Ch. 15The Chromosomal Basis of
Inheritance
2Nov 12 1:00 PM
Essential Question:
Are chromosomes the basis of inheritance?
3Nov 12 1:01 PM
1902 Walter S. Sutton, Theodor Boveri, et al Chromosome Theory of Inheritance
There are specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment
4Nov 12 1:06 PM
Thomas Hunt Morgan early 20th century first to provide evidence that chromosomes contained heritable factors
used fruit flies (Drosphilia melanogaster)quick breeders (offspring in 2 weeks)produce hundreds of offspringonly 4 pairs of chromosomes
3 pair autosomes, 1 pair sex chromosomeschromosomes can be seen with light microscope
5Nov 12 1:20 PM
Fruit flieswild type= normal phenotype for a charactermutant phenotype= alternative trait to wild type
w = allele for white eyes in fruit fliesw = allele for wildtype (red eyes)
the gene takes the symbol from the first mutant discovered
+
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Morgan discovered sex linked inheritance mated a whiteeyed male vs. redeyed female
F1 generation = all redeyed
F2 generation = 3:1 ratio, red:white but white eyes only in males
therefore: eye color is related to its sexsupported evidence of chromosome theory:specific genes carried on specific chromosomes
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Since chromosomes have hundreds of genes on same chromosome, they tend to be inherited together in crosses called linked genes
evidence for linked genes
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Genetic recombination and LinkageRecombination of unlinked genes
YyRr x yyrr
if offspring match the parent phenotypes= parental typesif offspring do not match parent phenotypes = recombinant types or recombinants
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if 50% of offspring are recombinants, then there is a 50% frequency of recombination
this is seen if any two genes are located on different chromosomes
*due to random orientation of homologous chromosomes on metaphase 1 plate of meiosis= independent assortment of alleles
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Determining Recombination Frequency
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Recombination of linked genes
what Morgan saw in his flies was crossing over that caused recombination of linked genes
happens during prophase 1 in meiosis
portions of nonsister chromatids trade places each time crossover occurs bring alleles in new combinations
12Nov 12 1:56 PM
Genetic mapping:discovered by Alfred Sturdevant
genetic map =ordered list of the genetic loci along a particular chromosomerecombination frequencies depend on the distance the genes are apart
*the farther apart two genes are, the higher probability that a crossover will occur between them and therefore the higher the recombination frequency
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A partial genetic map of a Drosophila chromosome
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linkage map = a genetic map based on the recombination frequencies
is map of order of genes on a chromosome, but not precise locations
map units= distance between genes1 map unit = 1% recombination frequency aka centimorgansdo not correspond to actual distances
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cytogenic map = map that locates genes with respect to chromosomal features,like stained bands
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Sexlinked genesXX = female, only produces X gametesXY = male, produces 50% X and 50% Y gametes
sex of offspring = 50:50 chance
SRY gene on Y chromosome codes for testes developmentif SRY absent = ovaries
sex linked if gene is located on sex chromosomes
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Some chromosomal systems of sex determination
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if sexlinked trait is recessive, female will have it only if she is homozygousmales who receive recessive trait from mother, will have the traitexamples
colorblindnessDuchenne Muscular Dystrophy
weakening of muscles and coordination losslack dystrophin protein
Hemophilia blood clotting disorder
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The transmission of sex linked recessive traits
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X inactivation of female mammals
of two X chromosomes, one gets in inactivated = Barr body (condensed form) these genes are not expressed
reactivated in cells that make eggs
Mary Lyonfemales have a "mosaic" of two types of cells: those with active X from father and those with active X from mother
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Ex. tortoiseshell cats
Ex. in humansalleles for sweat glands so can have patches
of skin with sweat glands and others without sweat glands
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Calico cats have white areas that are determined by another gene
What is this called?
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How can chromosome number be altered?
nondisjunctionwhen members of a pair of homologous chromosomes fail to move apart properly during meiosis I or sister chromatids fail to separate in meiosis II
one gamete gets two copiesother gamete gets none* can also occur during mitosis
aneuploidy= abnormal number of a particular chromosome
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nondisjunction
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Trisomic = 2n +1 chromosomesMonosomic= 2n 1 chromosomes
polyploidy = when have 2 or more complete chromosome sets (common in plants)
triploidy = 3ntetraploidy = 4n
*more normal in appearance than aneuploids
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http://education.uncc.edu/cmste/papers%20%20OLD/Triploidy.doc
Triploidy in humans
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Tetraploid mammal Tympanoctomys barreraesperm head unusually largemay have arisen when ancestor doubled chromosome # by errors in mitosis or meiosis within the reproductive organs
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Alterations of Chromosome structure
deletion chromosomal fragment lacking a centromere is lost (missing genes)
usually during meiosisduplicationwhen deleted fragment attaches as an extra segment of a sister chromatid
usually during meiosisinversionwhen fragment reattaches to a segment but in reverse order
can affect phenotype expressiontranslocationfragment joins a nonhomologous chromosome
can affect phenotype expression
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30Nov 12 2:52 PM
Human disorders due to chromosomal alterations
Down syndrome aneuploid condition1 in 700 in U.S.extra chromosome 21aka trisomy 21characteristics: short stature, heart defects,
respiratory infections, mental retardation, prone to leukemia and Alzheimer's, most sexually underdeveloped and sterile
risk increases if mother over 30
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Down syndrome
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Aneuploidy of Sex chromosomesKlinefelter syndrome = XXY
have male sex organs, but smallsterilemay have breast enlargementnormal intelligence
Males with XYY no different than normal, except taller
Trisomy X (XXX)healthy, only tell by karyotype
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Turner Syndrome Monosomy X (XO)phenotypically femalesterilesex organs do not matureif treated with estrogen, can develop secondary sex characteristicsnormal intelligence
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Disorders of Structurally altered chromosomes
DeletionsCri du Chat (Cry of the cat) deletion of
chromosome 5mentally retardedsmall head, round face, sm. receding chinwidely spaced eyes, low set earscry that sounds like mewing catdie in infancy
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Translocationschronic myelogenous leukemia cancer of
the blood cells reciprocal translocationchromosome 22 is replaced by tip of chromosome 9 = shortened chromosome 22 and extra long chromosome 9 =Philadelphia chromosome
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Some traits depend on the parent that you receive them from = genomic imprinting
mostly autosome genesoccurs during formation of gametesone allele is silencedzygote expresses only one allele, transmitted during mitosis
ex. gene imprinted for maternal allele will always be imprinted for maternal expression
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genomic imprinting
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ex. insulin growth factor(Igf2)paternal allele is expressed
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cause of a genome imprint believed to be methylation of cytosine nucleotides of DNA silences the allelecan also work opposite, methylation can activate a gene such as in Igf2
* genomic imprinting seems to be important for development
41Nov 12 3:16 PM
Inheritance of organelle genes
extranuclear genes genes found in organelles in cytoplasmfound in mitochondria and chloroplastshave circular DNAreproduce by themselvesgenes go to daughter organellesdo not display Mendelian inheritance
mitochondrial genes come from mother can cause mitochondrial myopathy weakness, muscle
deteriorationalso related to some heart disease or diabetes
42Nov 247:04 AM
So, how are the chromosomes the basis of inheritance?