Download 3) Monohybrid

Monohybrid Crosses
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Punnett Squares
Test Cross
When two organisms with known genotypes are mated the possible
offspring can be determined using a punnet square. For instance when
two heterozygotes are mated a punnet square is drawn as follows:
Individuals who possess the dominant trait could be either homozygous dominant (pure
breeding) or Heterozygous (carrier). To determine their genotype a test cross can be
performed. This involves mating the individual with an unknown genotype with a homozygous
recessive individual (these are easy to identify because they have the recessive trait).
X
Genotype & Phenotype
Pp
Upper and lower case letters are often used to represent two different versions of a gene
(alleles). E.g. P = allele for purple flower colour and p = allele for white flower colour.
Organisms that reproduce sexually inherit two alleles, one from each parent. An
organism’s genotype is the combination of alleles it possesses. Its phenotype is the
resulting characteristic or trait. One allele is usually dominant - is always expressed even
in the presence of another allele. The other allele is usually recessive which means it is
only expressed if there are no other alleles present. Therefore, both the genotypes PP
and Pp produce a purple flower phenotype.
Genotype
P?
Pp
X
pp
During meiosis the two alleles
segregate to form gametes
that only carry only one
allele. Thus, half of the egg /
sperm from the above parents
will contain the P allele and
the other half will contain the
p allele. The grid itself
contains the genotypes of the
possible offspring.
Phenotype
Homozygous Recessive (pp)
In this case we would expect the following ratio of offspring:
Heterozygous (Pp)
Genotype Ratio: 1 : 2 : 1
PP Pp
pp
Homozygous Dominant (PP)
Phenotype Ratio:
Heterozygous Cross
3 : 1
3
:
1
Purple : White
If the unknown individual were homozygous dominant (PP) we would expect all the resulting
offspring to be purple (Pp). If the unknown individual were heterozygous (Pp) then we would
expect roughly half the resulting offspring to be purple (Pp) and half to be white (pp). A large
number of offspring need to be produced in order for a test cross to be reliable. If only a few
purple offspring are produced there is still a chance that parent is actually heterozygous (Aa)
and that by chance the dominant allele (P) had been passed on each time.
Incomplete Dominance
Lethal Alleles
Multiple Alleles
When neither allele is dominant over the other heterozygotes will show an intermediate
or mixed phenotype.
Some alleles will cause an organism to die when two copies are inherited (homozygous).
These are called lethal alleles.
Many examples examine genes for which there are only two possible versions (alleles),
but in reality there are usually many different alleles for each gene in the population.
For instance the CR allele codes for red
flowers and the CW allele codes for white
flowers. Note that because neither allele is
dominant upper and lower case letters are
often avoided. Heterozygous individuals who
inherit both alleles (CRCW) are pink.
For instance, in mice a dominant allele (A) codes for the darker Agouti coat colour. The
recessive allele (y) codes for yellow coat colour. Individuals who inherit two copies of
the dominant allele are miscarried and do not survive to birth.
For instance there are three different alleles that code for a particular carbohydrate
groups on the surface of our blood cells. This is what determines you blood group. There
are three different versions of the gene that code for this surface carbohydrate.
CRCR
CRCW
CWCW
When two of these heterozygotes are crossed the following
CR CW ratios are observed:
C
R
C
IA codes for the A type carbohydrate (blood group A)
IB codes for the B type carbohydrate (blood group B)
iO codes for no type (blood group O)
For a heterozygous cross (Aa x Aa) we would normally expect a 3:1 phenotype ratio to be
observed amongst the offspring. However, because the homozygous dominant (AA)
individuals are miscarried they are not included in the ratio of offspring.
W
Aa
X
The iO allele is recessive. The IA and IB alleles are codominant – if both are present they
are expressed simultaneously and the blood cells will have both A and B type surface
carbohydrates (Blood type AB).
Aa
Phenotype Ratio:
CR
Red
R R
CC
A
White
a
IAIA or IAiO
CC
CW
1
CRCW
Pink
R W
:
2
:
1
A
CW CW
When two different alleles are both be
expressed simultaneously. Essentially both
alleles behave like they are dominant.
AA
Aa
Aa
aa
Agouti
1 : 2 : 1
iOiO
2
Yellow
:
1
IA
Did you know?
For instance the heterozygote fish shown here has both red and blue patches. The pattern
of inheritance and expected ratios of offspring are exactly the same as with incomplete
dominance. They key difference in this case is that the
characteristics do not mix; rather heterozygotes will
display both characteristics simultaneously.
Heterozygous Cross
IA IB
Phenotype Ratio:
a
Codominance
IBIB or IBiO
Achondroplasia is a type of
dwarfism in humans that is
caused by the presence of a
dominant allele (A). The
presence of two dominant
alleles is lethal.
iO
IB
Heterozygous Cross
2 : 1
I AI B
I Bi O
I Ai O
iOiO
iO
Multiple alleles means there are
more than two different possible
phenotypes. The cross shown (left)
between an individual with blood
group A and blood group B can
produce offspring with all four
different blood groups.