Genetics

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Genetics
How did this happen?
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Genes and Chromosomes

• A gene is a location on DNA that codes for a
  TRAIT, such as hair color, earlobe shape, blood
  type.
• An allele is a variant of a gene. So, for hair
  color, it could be red or not red. For earlobes
  it is attached or free. Blood type is A, B, AB
  or O.
• An individual gets only 2 alleles for each gene.

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Chromosomes and Genetics

• Here is a homologous pair of single chromosomes.
• The marked area is the location of a gene.
• The two colors indicate that there are different
  alleles.

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Karyotype Shows the chromosomes in an
individual.

• See p. 168

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Sex and all that

• In sexually reproducing organisms, gametes (egg
  and sperm) are made.
• Each gamete has one of the two sets of
  chromosomes of the parent. So, in humans, 2n
  46. Gametes have 23 csomes, one of each kind.
  N 23 for us.
• Meiosis is the process by which diploid (2n)
  cells become haploid (n)

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General Life Cycle of Sexually Reproducing
Organisms
Meiosis
Sperm (N) and Egg (N)
Mitosis
Fertilization
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Human Life Cycle
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Review of mitosis. Remember, mitosis is NOT for
making gametes

Before mitosis starts, gap 1
See p. 177 for comparison, also, 174-175.
At start, csomes are doubled
Csomes line up, not matched pairs
Centromeres split
Daughter cells identical to starting cell
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Compare Mitosis and Meiosis
Meta - I
Meta - II
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Meiosis
Before meiosis gap 1
At start of meiosis
Synapsis (prophase I), tetrads align in metaphase
I
Separation of tetrads in Anaphase I. Double
csome to each daughter cell. 2n -gt n
Meta II - like mitosis, but haploid. Ana II
centromere splits
n gametes, single csomes
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Compare Mitosis and Meiosis
Meta - I
Metaphase
Csomes are single
Csomes are double
Meta - II
Csomes are single
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How did this happen?
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Independent Assortment Hair color. Alleles
red, not red Earlobe shape Alleles attached
earlobe, free earlobe
Not red
red hair
attached
Free earlobe
What alleles are carried on each of these gametes
on the left? What alleles are carried on each of
these gametes on the right? Which gametes did
the parents give to their offspring?
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Meiosis is how this happened
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Mistakes in Meiosis often have huge effects on
offspring, or can be lethal
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Nondisjunction the failure to separate

• Can happen in men or women, chance increases with
  age.
• Shown is nondisjunction in Meiosis I

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Nondisjunction

• See section 11.8 in your text
• Diagrams pp. 11.23 11.26

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Disorders due to Nondisjunction

• Down Syndrome (trisomy 21)
• 1/800-1000 live births
• Other survivable trisomys 13 and 18
• Turners syndrome (monosomy OX)
• Klinefelters syndrome (XXY)
• Trisomy XXX
• XYY
• OY

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Nondisjunction Disorders in Sex Chromosomes

• Monosomy X
• Only survivable monosomy
• Female appearance
• No onset of puberty
• Normal intelligence
• Short stature
• Not fertile
• OY not viable, no development of fetus

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• XXY Klinefelters syndrome
• Male appearance, with some breast development
• May be tall
• Low to normal intelligence
• fertile

• XYY normal male May have learning difficulties
• XXX normal female May have learning difficulties

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Other common trisomies

• Trisomy 18 or Edwards syndrome
• 1/3000 conceptions
• 95 die in utero
• Of live births, most die in first few days
• heart and breathing abnormalities

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Some other mistakes in meiosis

• Having a part of a chromosome left behind
• Cri du chat or LeJuenes syndrome (part of
  chromosome 5 is lost). 1/20,00-50,000 live
  births
• Missing part of chromosome 15 Prader-Willi
  syndrome
• 1/10,000-15,000 live births. Obesity, poor
  muscle tone and strength, speech delays, poor
  coordination, excessive sleeping