Download Ch. 15 The Chromosomal Basis of Inheritance

Ch. 15
The Chromosomal Basis of Inheritance
Nov 12 ­ 12:58 PM
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Essential Question:
Are chromosomes the basis of inheritance?
Nov 12 ­ 1:00 PM
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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
Nov 12 ­ 1:01 PM
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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 offspring
­only 4 pairs of chromosomes
­3 pair autosomes, 1 pair sex chromosomes
­chromosomes can be seen with light microscope
Nov 12 ­ 1:06 PM
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Fruit flies
­wild type= normal phenotype for a character
­mutant phenotype= alternative trait to wild type
w = allele for white eyes in fruit flies
+
w = allele for wild­type (red eyes)
the gene takes the symbol from the first mutant discovered
Nov 12 ­ 1:20 PM
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Morgan discovered sex linked inheritance mated a white­
eyed 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 sex
supported evidence of chromosome theory:specific genes carried on specific chromosomes
Nov 12 ­ 1:28 PM
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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
Nov 12 ­ 1:36 PM
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Genetic recombination and Linkage
Recombination of unlinked genes
YyRr x yyrr
if offspring match the parent phenotypes= parental types
if offspring do not match parent phenotypes = recombinant types or recombinants
Nov 12 ­ 1:42 PM
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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
Nov 12 ­ 1:47 PM
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Determining Recombination Frequency
Nov 20­7:13 AM
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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
Nov 12 ­ 1:51 PM
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Genetic mapping:discovered by Alfred Sturdevant
genetic map =ordered list of the genetic loci along a particular chromosome
­recombination 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
Nov 12 ­ 1:56 PM
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A partial genetic map of a Drosophila chromosome
Nov 20­7:17 AM
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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 genes
­1 map unit = 1% recombination frequency ­aka centimorgans
­do not correspond to actual distances
Nov 12 ­ 1:59 PM
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cytogenic map = map that locates genes with respect to chromosomal features,like stained bands
Nov 12 ­ 2:09 PM
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Sex­linked genes
XX = female, only produces X gametes
XY = male, produces 50% X and 50% Y gametes
sex of offspring = 50:50 chance
SRY gene on Y chromosome codes for testes development
­if SRY absent = ovaries
sex linked if gene is located on sex chromosomes
Nov 12 ­ 2:10 PM
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Some chromosomal systems of sex determination
Nov 20­7:19 AM
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­if sex­linked trait is recessive, female will have it only if she is homozygous
­males who receive recessive trait from mother, will have the trait
­examples
colorblindness
Duchenne Muscular Dystrophy
­weakening of muscles and coordination loss
­lack dystrophin protein
Hemophilia ­ blood clotting disorder
Nov 12 ­ 2:22 PM
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The transmission of sex linked recessive traits
Nov 20­7:20 AM
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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
Nov 12 ­ 2:20 PM
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Ex. tortoiseshell cats
Ex. in humans
alleles for sweat glands ­ so can have patches of skin with sweat glands and others without sweat glands
Nov 12 ­ 2:35 PM
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Calico cats have white areas that are determined by another gene
What is this called?
Nov 20­7:25 AM
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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 copies
­other gamete gets none
* can also occur during mitosis
aneuploidy= abnormal number of a particular chromosome
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nondisjunction
Nov 20­7:26 AM
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Trisomic = 2n +1 chromosomes
Monosomic= 2n ­1 chromosomes
polyploidy = when have 2 or more complete chromosome sets (common in plants)
triploidy = 3n
tetraploidy = 4n
*more normal in appearance than aneuploids
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Triploidy in humans
http://education.uncc.edu/cmste/papers%20­%20OLD/Triploidy.doc
Nov 20­7:32 AM
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Tetraploid mammal Tympanoctomys barrerae
­sperm head unusually large
­may 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 meiosis
duplication­when deleted fragment attaches as an extra segment of a sister chromatid
­usually during meiosis
inversion­when fragment reattaches to a segment but in reverse order
­can affect phenotype expression
translocation­fragment joins a nonhomologous chromosome
­can affect phenotype expression
Nov 12 ­ 2:47 PM
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Nov 20­7:35 AM
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Human disorders due to chromosomal alterations
Down syndrome ­ aneuploid condition
­1 in 700 in U.S.
­extra chromosome 21
­aka trisomy 21
­characteristics: 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
Nov 12 ­ 2:52 PM
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Down syndrome
Nov 20­7:36 AM
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Aneuploidy of Sex chromosomes
Klinefelter syndrome = XXY
­have male sex organs, but small
­sterile
­may have breast enlargement
­normal intelligence
Males with XYY no different than normal, except taller
Trisomy X (XXX)
­healthy, only tell by karyotype
Nov 12 ­ 2:57 PM
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Nov 20­7:37 AM
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Turner Syndrome­ Monosomy X (XO)
­phenotypically female
­sterile
­sex organs do not mature
­if treated with estrogen, can develop secondary sex characteristics
­normal intelligence
Nov 12 ­ 3:00 PM
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Disorders of Structurally altered chromosomes
Deletions
Cri du Chat (Cry of the cat)­ deletion of chromosome 5
­mentally retarded
­small head, round face, sm. receding chin
­widely spaced eyes, low set ears
­cry that sounds like mewing cat
­die in infancy Nov 12 ­ 3:02 PM
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Translocations
chronic myelogenous leukemia ­ cancer of the blood cells ­reciprocal translocation
­chromosome 22 is replaced by tip of chromosome 9 = shortened chromosome 22 and extra long chromosome 9
=Philadelphia chromosome
Nov 12 ­ 3:04 PM
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Some traits depend on the parent that you receive them from = genomic imprinting
­mostly autosome genes
­occurs during formation of gametes
­one allele is silenced
­zygote 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 allele
­can also work opposite, methylation can activate a gene such as in Igf2
* genomic imprinting seems to be important for development
Nov 12 ­ 3:14 PM
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Inheritance of organelle genes
extranuclear genes ­ genes found in organelles in cytoplasm
found in mitochondria and chloroplasts
­have circular DNA
­reproduce by themselves
­genes go to daughter organelles
­do not display Mendelian inheritance
mitochondrial genes come from mother
­ can cause mitochondrial myopathy ­ weakness, muscle deterioration
­also related to some heart disease or diabetes
Nov 12 ­ 3:16 PM
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So, how are the chromosomes the basis of inheritance?
Nov 24­7:04 AM
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