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Chromosomes, Mapping, and the Meiosis-Inheritance Connection

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Title: Chromosomes, Mapping, and the Meiosis-Inheritance Connection


1
Chromosomes, Mapping, and the Meiosis-Inheritance
Connection
  • Chapter 13

2
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

3
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

4
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 male (typo mistake in original)
  • Sex determination in humans is based on the
    presence of a Y chromosome
  • 2 X chromosomes female
  • having a Y chromosome (XY) male

8
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

13
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

14
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

18
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

35
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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