Solving Genetics Problems :
Monohybrid Crosses
Two organisms
One characteristic
Complete dominance
Solving Genetics Problems :
Monohybrid Crosses
• Complete dominance
• the dominant allele completely masks the
effect of the recessive allele in heterozygous
condition.
• Bb
Solving Genetics Problems :
Monohybrid Crosses
• Incomplete dominance
• dominance occurring in heterozygotes in which
the dominant allele is only partially expressed
– Pp
– Sweet peas
Solving Genetics Problems :
Monohybrid Crosses
• The problem involved two gerbils named
Honey and Ritz.
Solving Genetics Problems :
Monohybrid Crosses
• The gene in question is a fur color gene which
has two alleles
• dominant brown (B) and recessive black (b)
An allele is one of two or more forms of a gene
Solving Genetics Problems :
Monohybrid Crosses
• Step One: Figure out the
genotypes of the parents.
– address the question of all of
the possible kinds of babies
they could produce
Solving Genetics Problems :
Monohybrid Crosses
• Step Two: Figure out what kinds of gametes
the parents can produce.
– Now you need to determine all the possible ways
that his sperm can combine with her eggs.
– Punnett Square
Solving Genetics Problems :
Monohybrid Crosses
• Step Three: Set up a
Punnett Square .
– You need to create a chart
with one column for each of
the female's egg types, and
one row for each of the
male's sperm types
Solving Genetics Problems :
Monohybrid Crosses
• Step Four:
Fill in the babies inside the
table by matching the egg
allele at the top of the
column with the sperm allele
at the head of the row.
Solving Genetics Problems :
Monohybrid Crosses
Figure out the genotypic ratio for
your predicted babies.
• So we have now figured out that,
if Honey and Ritz have a lot of
babies, we can predict that:
• 25% should be BB homozygous
• 50% should be Bb homozygous recessive
• 25% should be bb heterozygous
Solving Genetics Problems :
Monohybrid Crosses
Step Five: Figure out the Phenotype
ratio for your predicted babies.
• This is where dominance
really enters the picture
• If B is completely dominant
to b, all gerbils with at least
one B will look pretty much
alike, no matter what their
second allele is
Solving Genetics Problems :
Monohybrid Crosses
Phenotype will be:
• 75% brown fur
• 25% black fur
Solving Genetics Problems :
Monohybrid Crosses
• Where do we come from,
as a species, and how are
we all related
• See how our species
shares its genetic history
with all other species on
the planet
• Genetic engineering
• DNA fingerprinting in areas
such as drug discovery and
forensics