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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Eye Color Is a Sex-Linked Trait in Drosophila
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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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New Genes Identified on the Human Y Chromosome
Testis Determining Factor (SRY)
Channel Flipping (FLP)
Catching and Throwing (BLZ-1)
Self Confidence (BLZ-2) - (note unlinked to
ability)
Preadolescent fascination with Arachinida and
Reptilia (MOM-4U)
Addiction to Death and Destruction Films (T2)
Sitting on John Reading (SIT)
Selective Hearing Loss (HUH?)
Lack of Recall for Important Dates (OOPS)
Inability to Express Affection Over the Phone
(ME-2)
Spitting (P2E)
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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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What happened here?
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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