Genetics and Mendel
Mendel’s Work
• Mendel’s pea experiments led him to
conclude that inheritance is determined by
factors (genes) passed from one
generation to the next.
• These factors control all the various traits
of an organism and may come in different
forms (alleles).
Principle of Dominance
• For a specific gene
one allele for a trait is
dominant over the
others (recessive).
– If the dominant allele
is present, this is the
characteristic the
organism displays.
– The only time the
recessive allele is
expressed is if there is
NO dominant allele.
Dominance
The F1 generation
from a cross of
true-breeding
plants are all
Hybrids.
The F2 generation
shows the return
of the recessive
allele.
Segregation
• Mendel showed that the
recessive allele does not
disappear. Crossing 2 Hybrid
plants (F1) produced offspring
that exhibited the recessive
allele once again.
• For all the F2 offspring
produced about ¼ of them
exhibited the recessive allele.
• 3:1 ratio of
Dominant:Recessive traits
displayed.
Segregation
• How did this occur?
• Mendel suggested that the 2 alleles for a specific trait
that an organism contains segregate during sex cell
formation (meiosis).
• Effectively, an organism will make two types of gametes:
1 type has one allele for a gene and the 2nd type holds
the other allele.
• This way, of the 2 possible alleles a parent can
contribute to offspring only one allele from each parent
and becomes somewhat a game of chance.
Punnett Squares
• We can use Punnett Squares to
predict the outcome of a genetic
cross for a particular trait.
– Homozygous: Contains 2 of the same
allele for a trait.
– Heterozygous: Contains 2 different
alleles for a trait (hybrids!)
– Genotype: The genetic makeup of an
organism for a trait.
– Phenotype: The physical characteristic
displayed for a specific trait.
Independent Assortment
• Mendel then looked at the bigger picture.
What happens to all of the genes of an
organism during gamete formation.
• Does segregation of one gene influence
another? In other words as one gene
segregates does it impact the segregation
of another?
• Mendel examined a cross of plants for two
specific traits
Independent Assortment
• He crossed a plant that was Homozygous
for yellow round peas (YYRR) with a plant
that was Homozygous wrinkled green
peas (yyrr).
• The F1 generation was all yellow round as
expected.
– What’s their genotype?__________
Independent Assortment
• Mendel then let the plants self-pollinate
(just as before) to produce the F2
generation.
• Of the resulting F2 generation every
possible combination of traits were
shown.
– 556 seeds produced 315 were
round/yellow.
– 32 were wrinkled/green
– 209 were a combination of phenotypes
round/green and wrinkled/yellow
– The results supports the idea that traits
segregate separately.
Independent Assortment
• Genes for different traits segregate
independently during the formation of
gametes.
• One gene’s segregation does not
influence another’s.
Exceptions to Mendel
• Mendel’s findings and rules hold true for
many cases in genetics. But there are
some exceptions. (Figures, right?)
• Some genes do not have a dominant or
recessive form, and some have more than
just 2 alleles.
Incomplete Dominance
• One allele is not
dominant over another.
• The result of these
crosses produce
heterozygous offspring
showing traits between
both homozygous
parents.
• Example: snapdragon
Codominance
• Similar to incomplete
dominance. In
codominant cases both
alleles contribute to the
organism’s phenotype.
• What you see is both
phenotypes being
expressed!
• Example: Roan cattle;
color of chicken feathers
Multiple Alleles
• Many genes have more
than 2 possible forms
(alleles) in a population.
• Makes for greater
diversity for a trait.
• Example: Blood type in
humans
Polygenetic Traits
• Not every trait is controlled
by one gene. Often,
multiple genes influence
how certain traits are
displayed.
• Traits controlled by two or
more genes are said to be
polygenetic.
• Example: Skin color in
humans, fruit fly eye color.