Chromosomes, Mapping, and the Meiosis-Inheritance Connection

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Chromosomes, Mapping, and the Meiosis-Inheritance
Connection

• Chapter 13

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Chromosome Theory

• Chromosomal theory of inheritance
• - developed in 1902 by Walter Sutton
• - proposed that genes are present on chromosomes
• - based on observations that homologous
  chromosomes pair with each other during meiosis
• - supporting evidence was provided by work with
  fruit flies

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Chromosome Theory

• T.H. Morgan isolated a mutant white-eyed
  Drosophila
• red-eyed female X white-eyed male gave a F1
  generation of all red eyes
• Morgan concluded that red eyes are dominant

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Chromosome Theory

• Morgan crossed F1 females X F1 males
• F2 generation contained red and white- eyed flies
  but all white-eyed flies were male
• testcross of a F1 female with a white-eyed male
  showed the viability of white-eyed females
• Morgan concluded that the eye color gene is
  linked to the X chromosome

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Sex Chromosomes

• Sex determination in Drosophila is based on the
  number of X chromosomes
• 2 X chromosomes female
• 1 X and 1 Y chromosome male
• Sex determination in humans is based on the
  presence of a Y chromosome
• 2 X chromosomes female
• having a Y chromosome (XY) male

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Sex Chromosomes

• In many organisms, the Y chromosome is greatly
  reduced or inactive.
• genes on the X chromosome are present in only 1
  copy in males
• sex-linked traits controlled by genes present on
  the X chromosome
• Sex-linked traits show inheritance patterns
  different than those of genes on autosomes.

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Sex Chromosomes

• Dosage compensation ensures an equal expression
  of genes from the sex chromosomes even though
  females have 2 X chromosomes and males have only
  1.
• In each female cell, 1 X chromosome is
  inactivated and is highly condensed into a Barr
  body.
• Females heterozygous for genes on the X
  chromosome are genetic mosaics.

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Chromosome Theory Exceptions

• Mitochondria and chloroplasts contain genes.
• traits controlled by these genes do not follow
  the chromosomal theory of inheritance
• genes from mitochondria and chloroplasts are
  often passed to the offspring by only one parent

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Chromosome Theory Exceptions

• Maternal inheritance uniparental (one-parent)
  inheritance from the mother
• the mitochondria in a zygote are from the egg
  cell no mitochondria come from the sperm during
  fertilization
• in plants, the chloroplasts are often inherited
  from the mother, although this is species
  dependent

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Genetic Mapping

• Early geneticists realized that they could obtain
  information about the distance between genes on a
  chromosome.
• - this is genetic mapping
• This type of mapping is based on genetic
  recombination (crossing over) between genes.

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Genetic Mapping

• To determine the distance between genes
• - dihybrid organisms are testcrossed
• - offspring resembling the dihybrid parent result
  from homologues that were not involved in the
  crossover
• - offspring resulting from a crossover are called
  recombinant progeny

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Genetic Mapping

• The distance between genes is proportional to the
  frequency of recombination events.
• recombination recombinant progeny
• frequency total progeny
• 1 recombination 1 map unit (m.u.)
• 1 map unit 1 centimorgan (cM)

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Genetic Mapping

• Multiple crossovers between 2 genes can reduce
  the perceived genetic distance
• progeny resulting from an even number of
  crossovers look like parental offspring

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Genetic Mapping

• Determining the order of genes can be done with a
  three-point testcross
• the frequency of double crossovers is the product
  of the probabilities of each individual crossover
• therefore, the classes of offspring with the
  lowest numbers represent the double crossovers
  and allow the gene order to be determined

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Genetic Mapping

• Mapping genes in humans involves determining the
  recombination frequency between a gene and an
  anonymous marker
• Anonymous markers such as single nucleotide
  polymorphisms (SNPs) can be detected by molecular
  techniques.

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Human Genetic Disorders

• Some human genetic disorders are caused by
  altered proteins.
• the altered protein is encoded by a mutated DNA
  sequence
• the altered protein does not function correctly,
  causing a change to the phenotype
• the protein can be altered at only a single amino
  acid (e.g. sickle cell anemia)

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Human Genetic Disorders

• Some genetic disorders are caused by a change in
  the number of chromosomes.
• nondisjunction during meiosis can create gametes
  having one too many or one too few chromosomes
• fertilization of these gametes creates trisomic
  or monosomic individuals
• Down syndrome is trisomy of chromosome 21

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Human Genetic Disorders

• Nondisjunction of sex chromosomes can result in
• XXX triple-X females
• XXY males (Klinefelter syndrome)
• XO females (Turner syndrome)
• OY nonviable zygotes
• XYY males (Jacob syndrome)

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Human Genetic Disorders

• genomic imprinting occurs when the phenotype
  exhibited by a particular allele depends on which
  parent contributed the allele to the offspring
• a specific partial deletion of chromosome 15
  results in
• Prader-Willi syndrome if the chromosome is
  from the father
• Angelman syndrome if its from the mother

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Human Genetic Disorders

• Genetic counseling can use pedigree analysis to
  determine the probability of genetic disorders in
  the offspring.
• Some genetic disorders can be diagnosed during
  pregnancy.
• amniocentesis collects fetal cells from the
  amniotic fluid for examination
• chorionic villi sampling collects cells from the
  placenta for examination

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