Mendelian Inheritance I 17 October, 2005 Text Chapter 14

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Mendelian Inheritance I17 October, 2005Text
Chapter 14
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Mendels Experiments
Gregor Mendels experiments used pea plants as a
model system. He examined the inheritance of
characters like flower color and seed shape by
mating plants and observing the offspring.
character a heritable feature, like flower
color.
trait a variant of a character, like purple or
white flowers.
true-breeding plants that, when self pollinated,
produce offspring of the same variety
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Mendel followed heritable characters for three
generations.
Mendels results refuted the blending hypothesis.
He proposed a particulate theory of inheritance
where characters are determined by genes (recipes
for a character) that come in different versions
(alleles).
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Alleles are different versions of a gene.
Diploid organisms have two copies of each gene.
These copies can be the same or different. One
copy was inherited from each parent. If the two
alleles differ, then one, the dominant allele
determines the appearance of the
organism. During gamete formation, the two
alleles segregate into gametes.
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Mendels Rules of Inheritance

• Each parent has two alleles.

• Gametes contain only one allele.

• Offspring have two alleles - one allele from each
  parent.

• When both alleles are present, the dominant
  allele determines appearance.

• Gametes contain only one allele.

• Offspring have two alleles - one allele from each
  parent.

• When both alleles are present, the dominant
  allele determines appearance.

• This leads to a 31 ratio of offspring.

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Important terms

• homozygous a diploid organism that has two
  copies of the same allele for a given gene.

• heterozygous a diploid organism that has two
  different alleles for a given gene.

• phenotype an organisms appearance.

• genotype an organisms genetic makeup, its
  collection of alleles.

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Testcross
We cannot be sure of the genotype of an
individual with a dominant phenotype. That
individual could be homozygous or heterozygous.
A testcross can reveal the genotype of the
individual in question. A homozygous dominant
individual will produce all dominant phenotype
offspring in a testcross. A heterozygote will
produce a 11 ratio of offspring (dominant to
recessive phenotype).
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Independent Assortment
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