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Chapter 11-3: Exploring
Mendelian Genetics
Independent Assortment
To determine if the segregation of
one pair of alleles affects the
segregation of another pair of
alleles, Mendel performed a twofactor cross (dihybrid cross).
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The Two-Factor Cross: F1
•Mendel crossed true-breeding
plants that produced round yellow
peas (genotype RRYY) with truebreeding plants that produced
wrinkled green peas (genotype
rryy).
•All of the F1 offspring produced
round yellow peas (RrYy).
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The alleles for round (R) and yellow
(Y) are dominant over the alleles for
wrinkled (r) and green (y).
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The Two-Factor Cross: F2
•Mendel crossed the heterozygous
F1 plants (RrYy) with each other to
determine if the alleles would
segregate (separate) from each
other in the F2 generation.
RrYy × RrYy
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The Punnett square predicts a
9 : 3 : 3 :1 phenotype ratio in the F2
generation.
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dihybrid
Phenotype ratio
from dihybrid cross is
9
3 3 1
:
:
:
16 16 16 16
or
9:3:3:1
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Dihybrid phenotype ratio:
-9 of 16
traits
-3 of 16
recessive
-3 of 16
dom.
-1 of 16
recessive
show both dominant
show one dom., one
show one rec., one
show both
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Law of independent assortment
• During meiosis, genes for
different traits are separated
and distributed to gametes
independently of one another
(if the genes are on different
chromosomes).
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The alleles for seed shape
segregated independently of
those for seed color. This
principle is known as independent
assortment.
Genes that segregate
independently do not influence
each other's inheritance.
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Trihybrid!
A Summary of Mendel's Principles
• Genes are passed from parents
to their offspring.
• If two or more forms (alleles)
of the gene for a single trait
exist, some forms of the gene
may be dominant and others may
be recessive.
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• In most sexually reproducing
organisms, each adult has two
copies of each gene. These genes
are segregated from each other
when gametes are formed.
• The alleles for different genes
usually segregate independently
of one another.
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Incomplete Dominance
• When both alleles contribute
to the phenotype of a
heterozygous individual to
produce a trait not exactly
like either parent.
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A cross
between
red (RR)
and white
(WW) four
o’clock
plants
produces
pink-colored
flowers
(RW).
RR
WW
1:
2:
1
Think of pink as an
“incomplete”
red color
Codominance
• 2 dominant alleles are
expressed at the same time.
They both contribute to the
phenotype.
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Codominance: roan fur coat
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Codominance:
Human blood types
3 Alleles:
A
I,
B
I,
i
6 genotypes:
A
A
I I ,
A
B
I I ,
A
I i,
B
B
I I ,
B
I i,
and ii
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Having more
than 2 alleles is
called
Multiple alleles
Multiple alleles:
ABO Blood Group System
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
Blood Type
A
B
AB
O
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Polygenic Traits
•Traits controlled by two or more
genes are said to be polygenic
traits.
•Skin color in humans is a polygenic
trait controlled by more than four
different genes.
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Polygenic Traits
Human
Skin
Color
One more thing:
X Inactivation in female mammals
http://arbl.cvmbs.colostate.edu/hbooks/gene
tics/medgen/chromo/mosaics.html
In a cross involving two pea plant traits,
observation of a 9 : 3 : 3 : 1 ratio in the F2
generation is evidence for
• the two traits being inherited together.
• an outcome that depends on the sex of
the parent plants.
• the two traits being inherited
independently of each other.
• multiple genes being responsible for
each trait.
Traits controlled by two or more genes
are called
• multiple-allele traits.
• polygenic traits.
• codominant traits.
• hybrid traits.
In four o'clock flowers, the alleles for
red flowers and white flowers show
incomplete dominance. Heterozygous
four o'clock plants have
• pink flowers.
• white flowers.
• half white flowers and half red
flowers.
• red flowers.
A white male horse and a tan female
horse produce an offspring that has
large areas of white coat and large
areas of tan coat. This is an example
of
• incomplete dominance.
• multiple alleles.
• codominance.
• a polygenic trait.
Mendel's principles apply to
• pea plants only.
• fruit flies only.
• all organisms.
• only plants and animals.
Chimeras: combination of two very
early embryos such that their cells
intermix and the resulting
“conceptus” has cells from both
original embryos
http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html
Chimeric mice and sheep-goat chimeras have
been most useful in answering fundamental
questions about developmental biology and
pathology. There is also some potential that
this technique can be applied to problems such
as rescue of endangered species. It is possible,
for example to construct a goat-sheep chimera
such that a goat fetus is "encased" in a sheep
placenta. This enables a sheep to carry a goat
to term, which will not occur if you simply
transfer goat embryos into sheep (the sheep
will immunologically reject the goat placenta
and fetus). It may be possible to extend this
procedure to allow embryos from severely
endangered species to be carried by recipient
mothers from another species.