Bio1151b
Chapter
14
Mendel and the Gene Idea
Gregor
Mendel
experimented with
peas,
When Mendel mated two true-breeding varieties (the
generation), all of the offspring (the
generation) from this
cross
had
flowers.
Mendel proposed two principles of inheritance to explain his results.
Law of
.
"Heritable factors" (genes) have
alternative
versions called
.
If the two alleles at a locus differ, the organism is
, and the
allele determines the organism's
appearance,
or
.
The two alleles
in meiosis, and rhe distribution of the segregated phenotypes can be calculated from a
model called the
square.
How can we determine the genotype of an organism with the dominant
? A
testcross
with an individual that is homozygous
for the
trait.
Law of Independent
.
Crossing true-breeding parents differing in
two
characters produces
in the F1 generation heterozygous for both characters, and
phenotypes in the F2 generation.
Each pair of alleles
segregates
and randomly during gamete formation, assuming they are on different
,
Mendel's laws are based on probability, and obey rules of
and
.
In a
cross, the probability of allele
frequencies
in the
is the
of the probabilities of allele frequencies in the
.
In a
cross, the probabilities of multiple allele
combinations
is the
of the probabilities of the individual
alleles.
Extending Mendelian Genetics.
In
dominance, the
of the heterozygote and
dominant
homozygote are identical.
In
dominance, the
of
is somewhere
between
the phenotypes of homozygotes. An example is the flower color of
snapdragons.
In
, two dominant alleles affect the phenotype. An example is the human blood group
ABO,
where phenotypes are determined by
alleles.
Some traits exhibit
inheritance which oftens shows
variation.
In
, a gene at one
may alter the phenotypic expression of a gene at a second locus. An example is the mouse coat
color.
Inheritance in human families can be studied by
analyzing
a
showing the inheritance of alleles across generations.