Genetics Vocabulary

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Genetics Vocabulary
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Allele

• Different versions of one gene.
• An allele is usually represented by a capital or
  lower case letter.
• Examples Pea plants have two alleles for
  height. One allele is for tall plants (T) the
  other allele is for short plants (t).

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• Remember, half your chromosomes came from your
  mom and half from dad. This is true for any
  organism that reproduces sexually. This is why
  one can have more than one allele for a gene.
• Also there can be many alleles in a population
  for one gene. Blood type for example. A B O
• Called Multiple Alleles. Having more than two
  alleles for a genetic trait.

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Dominant

• The allele that is expressed if different alleles
  occur together. By expressed I mean seen or used
  or better yet it is the gene that is transcribed
  into protein.
• For example In humans freckles is the dominant
  allele, so if one has the dominant allele (F) and
  the recessive allele (f), they will have freckles.

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Recessive

• The allele that is not expressed if the dominant
  allele is present. In order to be expressed,
  both alleles need to be recessive. For example
  in height of peas the recessive allele is for
  height is a short plant (t). In order for a
  plant to be short it needs to have both alleles
  for shortness. (t t)

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Homozygous and Heterozygous

• Homozygous the two alleles are the same. TT or
  tt
• Heterozygous the two alleles are different.
• Tt

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Lets Put Some of these Terms to Use

• A pea plant is homozygous recessive for height.
  Its two alleles would be _____
• A pea plant is heterozygous for height. Its two
  alleles would be ____.
• A pea plant is homozygous dominant for height.
  Its two alleles would be _____.

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Genotype and Phenotype

• Genotype Actual alleles or genes present in the
  individual. (Tt) is the genotype. (the actual
  genes / letters)
• Phenotype The genotype above would be a tall
  plant. Tall is the phenotype.
• Phenotype is the physical result of the genes.
  Another way of explaining phenotype, it is the
  protein that results from the genotype and its
  (the proteins) resulting effect on the body to
  produce a trait.

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P generation

• P stands for parents. The p generation are the
  first two individuals crossed in a breeding
  experiment.
• These parents are true-breeding or pure breeding,
  thus they are homozygous.
• Ex A tall pea plant crossed with itself always
  produces tall pea plants.
• A dachshund crossed with a dachshund always has
  dachshund.

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F1 Generation and F2 Generation

• F1 Generation is the offspring (the first
  generation) from the p generation.
• F2 Generation comes from the F1 Generation being
  self pollinated with itself.
• See Figs. 11-4 and 11-5 pp. 311-312

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Mendels Law of Segregation

• The first law states that the two alleles for a
  trait segregate when gametes are formed. Fig.
  11-5 and 11-7
• What does this mean? During meiosis when the
  chromosome number is reduced in half, only 1
  allele goes into each sex cell. Fig. 11-15

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Mendels Law of Independent Assortment

• The alleles of different genes separate
  independently of one another during gamete
  formation. P. 317

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• What does this mean? A gene for height separates
  independently from one for color. During
  metaphase in meiosis 1 the chromosomes line up
  randomly.
• We now know this applies to genes on different
  chromosomes or genes that are far apart on the
  same chromosome.
• Figs. 11-9 and 11-10 on p. 317

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Incomplete Dominance

• Occurs when an individual displays a trait that
  is intermediate between two allele types.
• Tall x short intermediate/medium height

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Example of Incomplete Dominance

• In snapdragons, a pure-bred red flower is crossed
  with a white flower, and the offspring are pink
  flowers. That is because neither red or white
  allele is completely dominant. Due to this, less
  red pigment is produced and the flowers appear
  pink.

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Codominance

• Two dominant alleles are expressed at the same
  time. This is different from incomplete
  dominance. The reason is in codominance both
  dominant traits are displayed.

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Example of Codominance

• Roan coat in horses.
• A cross between homozygous red and homozygous
  white coats produces a roan coat, a heterozygous
  offspring that produces both red and white hair.
  The mixed color is roan.

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Another example

• Blood Type There is type A and type B if you
  have both alleles you will produce both type of
  blood groups.

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Autosomal and Sex Chromosomes

• Autosomal refers to any chromosome besides the
  sex chromosomes. In human these are chromosomes
  1-22.
• Sex chromosomes chromosomes that determine
  whether an organism is female or male.
• In humans XX females XY males

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Sex-Linked Traits

• A trait whose allele is located on the
  X chromosome.
• Sex-linked traits are recessive.

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Males and Sex Linked

• Since the alleles are found on the X chromosome,
  males usually show sex linked traits much more
  frequently than females.
• Males only have one X chromosome, so if they
  receive an X chromosome with the trait, they will
  exhibit the trait.

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Females are carriers

• Females have two X chromosomes, thus to show the
  sex linked trait they need to be homozygous
  recessive. Meaning both of their X chromosomes
  carry the gene.
• Females are carriers because they can be
  heterozygous dominant and still have the
  recessive allele, which they can pass on to their
  children.

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• Sex linked traits happen in other organisms also,
  not just humans.
• Sex linked traits are not related just to male or
  female (sex) characteristics. Example hemophilia
  is a sex-linked trait, but involves clotting
  factors in blood.

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