Chapter 15: Chromosomal Genetics

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Chapter 15 Chromosomal Genetics

• AP Biology Ms. Rader

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Chromosomal Basis of Inheritance

• Thomas Morgan was the first to positively
  associate a specific gene to a specific
  chromosome.
• Morgan used Fruit Flies, Drosophila melanogaster,
  to conduct his experiments.
• Fruit flies are ideal experimental organisms for
  genetics studies because
• They are prolific breeders (hundreds of offspring
  can be obtained from a single mating)
• New generations can be bred every two weeks
• They only have four pairs of chromosomes (3 pairs
  of autosomes and one pair of sex chromosomes)

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Drosophila Melanogaster Genetics

• Fruit flies can show several variations in the
  normal phenotype
• The normal phenotype is called wild type.
• Alternative types are
• referred to as
• mutant phenotypes.

Wild type red eyes
Mutant white eyes
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Drosophila Melanogaster Genetics

• Morgans geneticists developed a naming system
  for the mutations they discovered. The
  abbreviations for the alleles came from the
  mutated trait and a sign was added to signify
  the wild type allele.
• Ex. Trait vg vestigal wings (alleles code
  for wing shape)
• Alleles vg wild type, normal wings
• vg mutant, small, nonfunctional wings
• Trait w white eyes (alleles code for eye
  color)
• Alleles w wild type, red eyes
• w mutant, white eyes

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Drosophila Melanogaster Genetics

• In the course of Morgans crosses he discovered a
  male fly with white eyes. In further crosses with
  that fly he noted that the white eye trait
  occurred much more frequently in male flies
  versus female flies.
• He determined that the
• eye color character was
• linked to sex in flies and
• must therefore be located
• on a sex chromosome.

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Sex-linked Genes

• Genes that are located on the sex chromosomes are
  called sex-linked genes.
• Sex-linked genes are inherited with the passage
  of the sex chromosomes.

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Sex-linked Genes

• Most sex-linked traits are found on the X
  chromosome and are more prevalent in males
  because they only have one X chromosome.
• A recessive trait on the X chromosome of the male
  is seen in their phenotype as their no good
  copy to override it.

Female expresses Dominant (A) phenotype
Male expresses Recessive (a) phenotype
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Sex-linked Traits

• When the fly inherits a recessive (mutant) eye
  color allele (W) on the X chromosome the presence
  of a dominant (wild type) eye color allele (W)
  on the other X chromosome in a female fly will
  result in the fly exhibiting the wild type red
  eyes.
• In a male fly that inherits the recessive allele
  on its one X chromosome does not have another X
  chromosome to cover the mutation and the fly
  exhibits the mutant white eyes.

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Sex-linked Traits

• Females can inherit sex-linked traits by
  inheriting two mutant alleles (X)

Carrier female
Affected female
Affected male
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Sex linked Traits

• Sex-linked traits are almost always associated
  with the X chromosome as the Y
• chromosome does not carry
• much genetic information

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Sex linked Traits in Humans

• Hemophilia, color blindness, and Duchenne
  Muscular Dystrophy are examples of sex-linked
  traits in humans. All of these disorders are
  carried on the X chromosome.

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X Chromosome Inactivation

• Female mammals inherit two X chromosomes. Each of
  their somatic cells carries two X chromosomes,
  but one of those X chromosomes in each cell
  becomes an inactive Barr body.
• It has been shown that which one of the Xs that
  becomes a Barr body is random for each cell.
• This random selection of which X becomes inactive
  (and therefore not expressed) accounts for the
  fur color expression in calico or tortoiseshell
  cats.

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Tortoiseshell Coloration

• The female cat then
• expresses both the
• black fur and the
• orange fur traits in
• a mosaic pattern.

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Katrina, the tortoiseshell cat and notoriously
cranky old lady!
17 ½ years young!
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Sex Chromosome Systems

• Different organisms have different chromosomal
  systems for determining sex.

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FYI

• Duck-billed platypuses have 10 sex chromosomes.
• XXXXXXXXXX creates a female, while XYXYXYXYXY
  creates a male. In other words, rather than a
  single chromosome pair, platypuses have a set of
  ten-chromosomes that determine their sex

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Autosomal Linked Genes

• Genes that are located on the same chromosome
  are considered to be linked, as they tend to be
  inherited together.

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Genetic Recombination

• Offspring with a combination of traits from each
  parent are a result of genetic recombination.
• Recombination is often the result of independent
  assortment
• Genes are on the same chromosome are not excluded
  from recombination. The recombination occurs as a
  result of crossing over during prophase I of
  meiosis.

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Recombination in Linked Genes
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Genetic Maps

• Genetics maps are an ordered list of loci of
  genes on a specific chromosome
• The probability of recombination due to crossing
  over can be estimated by the distance between the
  two genes loci. The farther apart the two genes
  are the higher the likelihood they will be
  separated by crossing over.

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Recombination Frequency

• The distance between gene loci are measured in
  map units. One map unit corresponds to a 1
  recombination frequency
• The maximum recombination frequency is 50 which
  is the same as the frequency of recombination
  between different (nonhomologous) chromosomes.

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When it all goes wrong.

• Nondisjunction is when the meiotic spindle does
  not evenly distribute the chromosomes to the
  daughter cells.
• Nondisjunction can
• occur during meiosis I,
• where the homologous
• pairs do not separate,
• or during meiosis II
• where the sister
• chromatids do not
• properly separate

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Chromosome Errors

• Aneuploidy a condition where the offspring has
  an abnormal chromosome number
• Trisomy a condition where there are three
  homologs instead of two for a particular
  chromosome
• Monosomy a condition where there is only one
  homolog instead of two for a particular
  chromosome
• Polyploidy a condition where an organism has
  more than two sets of chromosomes. Polyploidy is
  common in plants (Ex. Triploidy (3n), tetraploidy
  (4n), etc.)

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Human Chromosome Alterations

• Most cases of aneuploidy cause defects so severe
  that the zygote is spontaneously aborted long
  before birth.
• 60 of spontaneous abortions are due to
  abnormalities in chromosome number.
• Maternal age plays a big role in frequency of
  aneuploidy At 25 years, 17 of secondary oocytes
  (eggs) may have chromosomal abnormalities. At 40
  years, up to 74 may contain abnormalities.

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Down Syndrome

• Down Syndrome Also called trisomy 21. It is
  characterized by mental retardation, an abnormal
  pattern of palm creases, a flat face, sparse,
  straight hair, and short stature. People with
  Down syndrome have a high risk of having cardiac
  anomalies, leukemia, cataracts, and digestive
  blockages.
• Life expectancy of Down syndrome individuals is
  in the middle teens but some live much longer.
• The gene responsible for Alzheimers is on
  chromosome 21. Downs are at increased risk for
  developing Alzheimers.

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Incidence of Down Syndrome as it Relates to
Maternal Age
Down Syndrome Traits
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Aneuploidy in Sex Chromosomes

• A male that inherits two X chromosomes, genotype
  XXY, has a disorder called Klinefelters
  Syndrome.
• Affected males are usually normal,
• though they may be tall and
• have small testes. Infertility
• results from absent sperm.
• About half have gynecomastia (breast
• development). About 10 of cases
• are mosaics and are less affected.

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Aneuploidy in Sex Chromosomes

• A female that inherits only one X chromosome,
  genotype XO, has a disorder called Turner
  Syndrome. This is the only viable monosomy in
  humans.
• Symptoms include short
• stature and lack of ovarian
• development. Other features,
• such as webbed neck, arms
• that turn out slightly at the
• elbow.
• Hormone therapy can trigger
• secondary sex characteristics
• and most girls exhibit normal
• intelligence.