Mendelian
Inheritance I
17 October, 2005
Text Chapter 14
Gregor Mendel’s 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
Mendel followed heritable characters for three generations.
Mendel’s 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).
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.
Mendel’s 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 3:1 ratio of offspring.
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 organism’s appearance.
•genotype: an organism’s genetic makeup, its collection of alleles.
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 1:1 ratio of offspring
(dominant to recessive phenotype).
Independent Assortment