Download Inheritance and monhybrid

Inheritance terms
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Objectives
Outcomes
Define key inheritance terms.
3: Explain how the inheritance of
characteristics is controlled by
dominant and recessive alleles.
Determine genotypes and
phenotypes of the offspring of a
monohybrid cross using a punnet
grid.
5: Construct genetic cross diagrams.
7: Predict and/or explain the
outcomes of crosses between
individuals for each possible
combination of dominant and
recessive alleles of the same gene.
Key terms: monohybrid, heterozygous, homozygous, dominant, recessive
Family resemblance
Members of the same
family often look similar.
Which parent do these
children look more like?
If the son and daughter
have children of their own
one day, will they also
look like their parents?
Why do members of the same family look similar?
Humans, like all organisms, inherit characteristics from
their parents. How are characteristics passed on?
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Different versions of genes
Chromosomes in a homologous pair contain the same type of
genes that code for the same characteristics, such as eye
colour.
Each chromosome in the pair,
however, may have a different
version of the gene.
allele for
For example, the version of brown
a gene on one chromosome eyes
may code for brown eyes,
whereas the version of the gene on
the other chromosome may code for
blue eyes.
allele
for blue
eyes
Each different version of a gene is called an allele.
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Homozygous alleles
If the alleles for a characteristic in a homologous pair are
the same, the organism is said to be homozygous for that
characteristic.
What colour eyes will these homozygous pairs of alleles
produce?
allele for
brown
eyes
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allele for
brown
eyes
allele for
blue
eyes
allele for
blue
eyes
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Heterozygous alleles
If the alleles for a characteristic in a homologous pair are
different, the organism is said to be heterozygous for that
characteristic.
What colour eyes will this heterozygous pair of alleles
produce?
?
allele for
brown
eyes
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allele for
blue
eyes
The characteristic expressed
by heterozygous alleles will
depend on which allele is
dominant and which allele
is recessive.
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Dominant or recessive?
The phenotype for a particular characteristic depends on
which allele is dominant and which allele is recessive.
 Dominant alleles are always expressed in a cell’s
phenotype. Only one copy of the dominant allele needs
to be inherited in order for it to be expressed. Dominant
alleles (e.g. brown eyes) are represented by an upper
case letter (e.g. ‘B’).
 Recessive alleles are only expressed in a cell’s
phenotype if two copies of it are present. If only one
copy is present, its effect is ‘masked’ by the dominant
allele. Recessive alleles (e.g. blue eyes) are
represented by a lower case letter (e.g. ‘b’).
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What eye colour?
The allele for brown eyes is dominant over the allele for
blue eyes.
So, what colour will the eyes be of an individual who is
heterozygous for eye colour?
allele for
brown
eyes
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allele for
blue
eyes
The individual will have
brown eyes, because the
allele for brown eyes masks
the allele for blue eyes.
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Homozygous cross
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Heterozygous cross
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Finding the genotype
For some characteristics, the genotype of a homozygous
recessive individual can be determined from their phenotype.
For example, the allele for brown fur (B) in mice is dominant
over the allele for white fur (w). This means that all white
mice must therefore have the genotype
.
But what about individuals that
have brown fur? Is their genotype
BB or Bw ?
A test cross can be used to determine
whether an individual is homozygous
or heterozygous for a dominant trait.
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What is a test cross?
During a test cross, an individual with an unknown genotype
is crossed with a homozygous recessive individual. The
phenotype of the offspring will reveal the unknown genotype.
 If all the offspring display the dominant phenotype, then
the parent of unknown genotype must be homozygous
for the characteristic.
 If half the offspring show the
dominant phenotype, and
half show the recessive
phenotype, then the parent
must be heterozygous for
the characteristic.
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Using test crosses to find genotype
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What is incomplete dominance?
Sometimes two different alleles are neither fully dominant
or recessive to each other.
In heterozygous individuals, this creates a phenotype that
is an intermediate mix of the other two. This is called
incomplete dominance.
For example, when a red Mirabilis
jalapa plant (also called the
snapdragon or ‘Four o'clock flower’)
is crossed with a white Mirabilis
jalapa plant, all the offspring flowers
are pink because both the red and
white alleles are expressed.
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Work through the questions on
the sheets.
Outcomes
3: Explain how the inheritance of
characteristics is controlled by
dominant and recessive alleles.
5: Construct genetic cross diagrams.
7: Predict and/or explain the
outcomes of crosses between
individuals for each possible
combination of dominant and
recessive alleles of the same gene.
Key terms: monohybrid, heterozygous, homozygous, dominant, recessive
BLOOD TYPE IS GENETIC
• The A and B antigen molecules on the
surface of red blood cells are produced by
two different enzymes. These two enzymes
are encoded by different alleles, of the same
gene: A and B.
• The A and B alleles code for enzymes that
produce the type A and B antigens
respectively.
• A third version of this gene, the O allele,
codes for a protein that is not functional and
does not produce surface molecules.
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Construct a diagram to show the
possible blood types that may be
inherited.
Calculate the genotype and
phenotype
Outcomes
3: Explain how the inheritance of
characteristics is controlled by
dominant and recessive alleles.
5: Construct genetic cross diagrams.
7: Predict and/or explain the
outcomes of crosses between
individuals for each possible
combination of dominant and
recessive alleles of the same gene.
Key terms: monohybrid, heterozygous, homozygous, dominant, recessive
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• Blood plasma is packed with proteins
called antibodies. The body produces a
wide variety of antibodies that will
recognize and attack foreign molecules
that may enter from the outside world.
• A person's plasma does not contain any
antibodies that will bind to molecules that
are part of his or her own body.
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WHEN BLOOD TYPES MIX
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• When conducting a blood transfusion, it is important to carefully
match the donor and recipient blood types.
• If the donor blood cells have surface molecules that are different
from those of the recipient, antibodies in the recipient's blood
recognize the donor blood as foreign.
• This triggers an immune response resulting in blood clotting.
• If the donor blood cells have surface molecules that are the same as
those of the recipient, the recipient's body will not see them as
foreign and will not mount an immune response.
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SPECIAL BLOOD TYPES
• People with type O blood are universal
donors because there are no molecules on
the surface of the red blood cells that can
trigger an immune response. People with
type AB blood are universal recipients
because they do not have any antibodies
that will recognize type A or B surface
molecules.
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