Inheritance
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Phenotype and genotype
The overall appearance of an organism depends on
two things:
1. its genes (inherited characteristics)
2. the effects of the environment in which it lives.
All the observable characteristics of an organism are called
its phenotype.
The full set of genes of an organism is called its genotype.
An organism’s phenotype therefore depends on its
genotype plus environmental effects.
phenotype = genotype + environmental effects
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What do you know about chromosomes?
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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 allele
For example, the version of brown for blue
a gene on one chromosome eyes eyes
may code for brown eyes,
whereas the version of the gene
on the other chromosome may
code 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 allele for allele for allele for
brown brown blue blue
eyes eyes eyes 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 allele for The characteristic expressed
brown blue by heterozygous alleles will
eyes eyes 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 there is no dominant allele present.
If a dominant allele is present, the
effect of the recessive allele
is ‘masked’. Recessive alleles
(e.g. blue eyes) are represented
by a lower case letter (e.g. ‘b’).
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What eye colour?
The genotype for someone with an allele for blue eyes and
an allele for brown eyes would be written Bb.
The dominant allele (brown eyes) has a capital letter and
is written before the recessive allele (blue eyes).
So, what colour will the eyes be of an individual who is
heterozygous for eye colour?
allele for allele for The individual will have
brown blue brown eyes, because the
eyes eyes allele for brown eyes masks
the allele for blue eyes.
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Inheritance terms
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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 are sex chromosomes?
Humans cells contain one pair of sex chromosomes, which
control gender.
Y chromosome
Males have one X and
one Y chromosome (XY).
Females have two X
chromosomes (XX).
Y chromosomes are very small
and contain 78 genes, whereas
X chromosomes are larger and
contain 900–1,200 genes. X chromosome
Because females can only produce X gametes, it is the
sperm that determine the sex of the offspring at fertilization.
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Boy or girl?
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The life and work of Gregor Mendel
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Monohybrid crosses
The type of experiment that Mendel carried out, investigating
just a single characteristic, is called a monohybrid cross.
There are two alleles controlling pea shape. This means
there are three possible genotypes that the plants could
inherit, leading to two possible phenotypes.
genotype phenotype
homozygous dominant SS smooth
homozygous recessive ww wrinkly
heterozygous Sw smooth
The likelihood of a trait being produced during a monohybrid
cross can be mapped out using a Punnett Square.
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What are Punnett Squares?
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Expressing outcomes
There are a number of ways of expressing the outcomes of
genetic crosses.
As a ratio: “The chickens are white or
brown in a ratio of 3:1.”
As a percentage: “75% of the chickens are
white and 25% are brown.”
As a probability: “The probability of a chicken
being white is 0.75, or 3 in 4.”
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Monohybrid cross questions
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What is co-dominance?
Sometimes two alleles are equally dominant to each other.
In genetics, this is called co-dominance and means that
neither allele is recessive to the other allele.
How does co-dominance affect the offspring of a cross?
For example, let’s assume that the red allele (R) and
the white allele (W) are co-dominant:
RR WW
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Co-dominance cross – offspring
parental genotype:RR x WW
gametes: R R W W
W W
?
offspring R RW RW
genotype:
R RW RW
The alleles are co-dominant so both are expressed.
What will the offspring flowers look like?
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Co-dominance cross – phenotypes
parental genotype:RR x WW
offspring
RW RW RW RW
genotypes:
offspring
phenotypes:
(Click twice on the buds
to reveal the flowers;
click again to close them.)
All the offspring flowers are pink because both the red
and white alleles are expressed.
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Co-dominance activity
FLASH 5 – Blood groups
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Inheritance multiple-choice quiz
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Anagrams
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Multiple-choice quiz
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