Introduction to Genetics

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Introduction to Genetics

• Chapter 11

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What is inheritance?

• Genetics the scientific study of heredity.
• Gregor Mendel an Austrian monk who observed
  patterns of inheritance in pea plants.
• Why peas? Breeding could be easily controlled
  (pollen and eggs), they grew fast, and had
  variable genetic traits (characteristics).

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• Peas had varying traits
• Round, wrinkled, green, yellow, tall, short,
  axial flowers, terminal flowers, etc

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Mendels observations of peas

• Some plants were true-breeding they always
  produced the same offspring. Always tall, green
• Pea plants could be selectively bred by brushing
  on or inhibiting pollen.
• Certain traits would disappear for a
  generation, then return again.
• Repeating patterns emerged.

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Mendels observations

• P generation parent generation.
• F1 generation first filial, offspring of P
  generation.
• Hybrid offspring produced by parents of
  different traits.

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Patterns emerging

• When Mendel bred the purple and the white, all
  the F1 generation were purple. The white
  disappeared.
• White flower trait is recessive.
• All offspring were purple. Purple flower trait
  was dominant.
• Different versions of a gene are called alleles.

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Would the recessive trait return?

• Mendel bred the F1 generation to produce an F2
  generation flower color, pod color, shapeThe
  recessive returned.

What ratio was observed?
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Mendels conclusions

• Each trait has two alleles (versions). Plant
  height Tall (T) or Short (t)..
• These alleles segregate, or separate when gametes
  (egg and sperm) are formed. TT makes T gametes,
  Tt makes T gametes and t gametes..
• The allele from a sperm and the allele from the
  egg is the organisms genotype. TT, Tt, or tt..
• What the organism looks like is its phenotype.
  Tall plant, short plant..

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Genotype, phenotype, homozygous (same) and
heterozygous (different) traits
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Probability and Punnett Squares

• When gametes are formed, there is a 50/50 chance
  that it will get one version of a trait. Like a
  coin flip!
• The different possible offspring, and ratios of
  genotypes produced in a cross can be shown using
  a Punnett square.
• Did the 31 ration for Mendels F1 cross make
  sense?

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A Monohybrid (one trait) cross
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Dihybrid (two traits) cross, Trihybrid

• In reality, all traits are passed on to the
  gametes.
• Most traits go into the gametes independent of
  each other.
• Example A homozygous tall, yellow plant (TTYY)
  produces only the following gamete TY.
• Another example a heterozygous tall, yellow
  plant TtYy can produce the following gametes
  TY, Ty, tY, ty.

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Try a Dihybrid cross
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Other modes of inheritance.

• Incomplete dominance the dominant gene does not
  completely show
• The heterozygous is in-between
• Japanese four-o-clocks, snapdragons

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Other modes

• Codominance both alleles show up in the
  phenotype.
• Examples coat color in cows, speckled hens,
  sickle-cell trait, blood type
• Multiple alleles more than two alleles for a
  trait.

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Meiosis Gamete production!

• In order to make an individual with the full
  component of chromosomes (diploid), gametes must
  be created (haploid).
• Gametes are sex cells with half the amount of
  chromosomes.
• We have 23 homologous (same) pairs of
  chromosomes, total 46.
• Gametes (egg or sperm) have 23.

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Meiosis

• Meiosis consists of 2 divisions of a diploid or
  2N cell, to create 4 haploid cells (1N each).
• Before the first division, the chromosomes
  replicate to form the tetrad (X X figure). The
  homologous pairs and their copies go through
  crossover.
• Crossover produces new combinations of alleles,
  It makes you unique!

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Crossover

• Crossover happens in prophase I, and sections of
  one homologue are traded with sections of
  another.
• Alleles are shuffled from one to another. This
  is a random event, with an infinite number of
  combinations!

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Crossover

• If genes are close together on an arm of a
  chromosome, they are more likely to transfer
  together.
• Because of this, they are said to exhibit
  linkage.
• This helps scientists map the genes on a
  chromosome!

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Gene maps
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Meiosis continued

• Meiosis I chromosomes duplicate, crossover
  occurs, IPMAT, results in 2 diploid cells.
• Meiosis II PMAT, results in haploid cells.
• Note that these 4 gametes are each genetically
  unique!
• Non-disjunction - if the chromosomes fail to
  separate properly, disorders occur.

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Meiosis