Download Inheritance genetics

Inheriting Genes
Gregor Mendel worked out the basic laws of inheritance of genes. He studied the inheritance of a
number of discrete characteristics in the pea plant: plant height (tall vs dwarf); flower position (axial vs
terminal); flower colour (purple vs white); pea colour (yellow vs green); pea shape (round vs wrinkled);
pod shape (smooth vs ridged); pod colour (green vs yellow). The results of his experiments led to the
development of two laws (these are loosely paraphrased, using modern terminology).
First Law of Inheritance (Law of Segregation)
Of a pair of contrasting characters only one can be represented in the gametes.
There are two versions of each gene. Only one can be passed on to the offspring. This is because
meiosis separates homologous chromosomes so that only one of each pair is passed onto the gamete.
Second Law of Inheritance (Law of Independent Assortment)
Each of a pair of contrasting characteristics segregates (separates) independently of those of
any other pair.
Any one of a pair of chromosomes (genes) can be passed into a gamete. The behaviour of different
pairs of chromosomes is independent of the behaviour of others – look back at Metaphase I and
Anaphase I of meiosis.
Monohybrid Inheritance
This is inheritance involving a single gene. Mendel crossed a pure breeding variety of garden pea with
coloured (red) flowers and a pure breeding variety with white flowers. All offspring had red flowers. He
then selfed the offspring of the first generation and these produced a second generation with 705 redflowered plants and 224 white-flowered plants.
red-flowered
white-flowered
Pure breeding parents
all red-flowered
F1 generation
705 red-flowered
224 white-flowered
3
1
F2 generation
Monohybrid ratio
Q. What can be deduced from the fact that all of the F1 generation were red-flowered?
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It is usual to use a genetic diagram to explain the results of Mendel’s experiments. Two alternatives are
given below in outline. Complete them to illustrate the inheritance of flower colour in the garden pea. By
convention the symbol for the dominant allele is in upper case (capital letter) and the recessive is in
lower case. Hence R represents the allele for red flower whilst r represents the allele for white flower.
pure breeding parents
red
white
RR
rr
phenotype
genotype
R
R
r
r
F1 progeny
R
r
F1 progeny
All offspring are red-flowered
All offspring are red-flowered
F1 are selfed
F1 are selfed
phenotype
genotype
gamete
gamete
red
Rr
R
red
Rr
r
R
X
r
Male gametes
gamete
pure breeding parents
red
white
RR
rr
phenotype
genotype
Female gametes
R
r
R
r
F2 progeny
Offspring are red or white
flowered in the ratio:
Offspring are red or white
flowered in the ratio:
Note that Mendel found similar results in all his reported experiments – when pure breeding individuals
for contrasting characteristics were crossed, the dominant characteristic masked the recessive
characteristic in the first generation. When the heterozygous genotype was ‘selfed’ (i.e. self-fertilised or
bred with other members of the F1 generation) the offspring in the second generation exhibited the
dominant trait in the ratio of 3 : 1.
Similar sorts of results are evident in animals and humans. However it is not ethical to ‘self’
heterozygotes and generation times are long in humans. Despite this, it is possible to deduce the
inheritance of traits from the examination of family pedigrees.
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Dihybrid Inheritance
This is inheritance involving two sets of genes. In one experiment Mendel crossed a pure breeding
variety of garden pea that had round yellow seeds with a pure breeding variety that had wrinkled green
seeds. All offspring had round yellow seeds. He then ‘selfed’ the offspring of the first generation. These
produced a second generation totalling 556 plants. Of these 315 had round yellow seeds, 108 had round
green seeds, 101 had wrinkled yellow seeds and 32 had wrinkled green seeds.
round yellow seeds
wrinkled green seeds
Pure breeding
parents
all round yellow seeds
F1 generation
F2 generation
Dihybrid ratio
315
round yellow
seeds
108
round green seeds
101
wrinkled yellow
seed
32
wrinkled green
seeds
9
3
3
1
Q. What can be deduced from the fact that all of the F1 generation had round yellow seeds?
Again, it is usual to use a genetic diagram to explain the results of such experiments. Two alternatives
are given below in outline. Complete them to illustrate the inheritance of seed texture and colour in the
garden pea. By convention the symbol for the dominant allele is in upper case (capital letter) and the
recessive allele is in lower case (small letter). Hence R represents the allele for round seed and r the
allele for wrinkled seed; Y represents the allele for yellow seed whilst y represents the allele for white
flower.
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pure breeding parents
round yellow
wrinkled green
RRYY
rryy
phenotype
genotype
gamete
RY
F1
progeny
ry
All offspring are round and yellow seeded
F1 are selfed
phenotype
genotype
gamete
F2 progeny
round yellow
R r Y y
RY
RRYY
Ry
RRYy
RrYY
rY
RrYy
RRyY
round yellow
R r Y y
ry
RRyy
RY
RryY
Rryy
Ry
rRYY
rRYy
rY
rrYY
rrYy
ry
rRyY
rRyy
rrYy
rryy
Lost the will to live? Too many criss-crossy lines? Time for the alternative Punnett square.
However, in case you persevered, the offspring are round yellow, round green, wrinkled
yellow and wrinkled green in the ratio:
9:3:3:1
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pure breeding parents
round yellow
wrinkled green
RRYY
rryy
phenotype
genotype
gamete
RY
ry
F1 progeny
genotype
All offspring are round and yellow seeded
Male gametes
F1 are selfed
X
RY
RY
RRYY
Female gametes
Ry
rY
ry
Ry
rY
ry
Offspring are round yellow, round green, wrinkled yellow or
wrinkled green in the ratio:
Note that Mendel found similar results in all his reported experiments into the inheritance of pairs of
characteristics in garden peas. Many other species have shown similar inheritance patterns.
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Codominance
Sometimes the alleles of genes always exert their effect – they are neither dominant nor recessive.
Instead they are said to be codominant. Examples include flower colour in snapdragon (Antirrhinum),
coat colour in short horn cattle, AB blood group and sickle cell trait in humans.
We will look at one example – flower colour in snapdragon. A single gene controls flower colour in
snapdragon. There are two alleles – red-flowered and white-flowered. When pure breeding red-flowered
snapdragons are crossed with pure breeding white-flowered snapdragons all the offspring have pink
flowers. When these heterozygous individuals are selfed, they produce a second generation with red,
pink and white flowers in the ratio 1 : 2 : 1. Use your knowledge to complete the following genetic
diagrams to explain the results of such crosses.
NB: As the alleles are codominant, it is usual to have a separate upper case symbol for each allele. R for
red-flowered allele and W for white-flowered allele seem reasonable.
phenotype
genotype
pure breeding parents
red
white
RR
WW
pure breeding parents
red
white
RR
WW
phenotype
genotype
gamete
gamete
F1 progeny
F1 progeny
All offspring are pink-flowered
F1 are selfed
F1 are selfed
pink
RW
pink
RW
X
Male gametes
phenotype
genotype
All offspring are pink-flowered
gamete
Female gametes
R
W
R
W
F2 progeny
genotype
Offspring are red-, pink- or
white-flowered in the ratio:
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Offspring are red-, pink- or
white-flowered in the ratio:
Multiple Alleles
Sometimes there are more than two alleles for a gene, as is the case with the ABO blood grouping
system in humans. Here blood group is controlled by an immunoglobulin (I) that has three possible
alleles:
 IA codes for antigen A on red blood cell membranes
 IB codes for antigen B on red blood cell membranes
 IO results in neither antigen being present in the red cell membrane
A
I and IB are codominant with each other and IO is recessive to both IA and IB. Only two alleles can be
present in any individual (one inherited from the mother, the other inherited from the father).
Use your knowledge of genetics to complete the following genetic diagrams to explain how a mother
who is blood group A and a father who is blood group B can potentially have children who have group A,
B, AB or O.
Parents are heterozygous and so
must contain contrasting alleles.
Must be IAIO and IBIO
gamete
Blood group A
IAIO
IA
IO
Blood group B
IBIO
IB
IO
X
Male gametes
phenotype
genotype
Parents are heterozygous and so
must contain contrasting alleles.
Must be IAIO and IBIO
Female gametes
IA
IO
IB
IO
progeny
genotype
Offspring are group AB, A, B or O
in the ratio:
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Offspring are group AB, A, B or O
in the ratio:
Sex Linkage
As you will be aware, a pair of sex chromosomes determines which sex you are. In humans the female
is the homogametic sex, having two X-chromosomes whilst the male is the heterogametic sex having a
single X-chromosome and a Y-chromosome. The chromosomes are different shapes and sizes – the Xchromosome is large and carries many genes, not just ones controlling the development of sex. In
contrast the Y-chromosome is much smaller and carries fewer genes, most of which are concerned with
controlling the development of sex.
Genes which are carried on the non-homologous portion of the X-chromosome are said to be sex linked.
In the male such genes are always expressed as there is only one copy. In the female the situation
might be different as she has two copies of the X-chromosome, so different alleles may be carried on
each. Sex linked conditions include haemophilia (lack of Factor VIII protein), colour blindness and
Duchenne muscular dystrophy.
Complete the following genetic diagrams to explain how parents with normal clotting can have a
haemophiliac son.
X and Y refer to the sex chromosomes.
H is the gene for normal blood clotting.
h is the allele for abnormal clotting (lack of Factor VIII and haemophilia).
The female parent must be
heterozygous – a carrier for the
recessive haemophilia allele
The female parent must be
heterozygous – a carrier for the
recessive haemophilia allele
gamete
Female normal
XHXh
XH
Xh
Male normal
XHY
XH
Y
X
Male gametes
phenotype
genotype
Female gametes
XH
Xh
XH
Y
progeny
Offspring are normal female, carrier
female, normal male, haemophiliac
male in the ratio:
Offspring are normal female,
carrier female, normal male,
haemophiliac male in the ratio:
Q. Explain why sex linked recessive conditions are more common in males than females.
Q. Explain why sex linked recessive conditions are never passed from father to son.
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Linkage and Crossing Over
Mendelian ratios are produced when the genes are inherited independently of each other. This happens
when the genes are carried on different chromosomes, but not if they are on the same chromosome.
Genes which are present on the same chromosome are physically linked together, and so they are
inherited as a unit. Chiasma formation (crossing over) during prophase I of meiosis can result in
swapping of genes from one homologous chromosome to another. Consider the example of inheritance
of body colour and wing length in the fruit fly, Drosophila melanogaster.
Pure breeding grey-bodied and long-winged fruit fly were crossed with pure breeding, black-bodied and
vestigial (short) winged fruit fly. All the offspring were grey-bodied and long-winged. These were then
crossed with black-bodied and vestigial-winged flies.
Q. What do the results of the first cross suggest?
Q. What results might you expect from the second cross? Completing the outline genetic diagram might
help you.
phenotype
genotype
pure breeding parents
grey body
black body
long wing
vestigial wing
gamete
F1
progeny
genotype
All offspring are grey-bodied and long-winged
F1 females are crossed with black-bodied, vestigial-winged males
Female gametes
X
Male
gametes
Offspring:
In the ratio:
Such a cross actually produced: 192 grey-bodied, long-winged flies; 170 black-bodied, vestigial-winged
flies; 43 black-bodied, long-winged flies; 39 grey-bodied, vestigial-winged flies. This was instead of the
expected 111 of each (as there were 444 flies produced in total).
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If the genes for body colour and wing length are on separate chromosomes, they would produce typical
Mendelian ratios. If they are present on the same chromosome, genes are linked and inherited together.
In this case you might expect to get typical monohybrid ratios. However, crossing over can occur, which
causes recombination and the ratio is distorted. When genes are further apart on the chromosome, the
chances of cross-over and recombination are greater and the closer the observed ratios will be to the
Mendelian ratios. When genes are close together on the chromosome, cross-over is less common and so
the observed ratio is very different from expected.
Epistasis
Sometimes genes interact with each other, one gene influencing the expression of another gene at a
separate locus. Such interaction can produce unusual phenotypic ratios.
Coat Colour in Mice
Coat colour in mice is controlled by two genes. One gene codes for the production of pigment. The
dominant allele (B) enables pigment to be produced, whilst the double recessive (bb) is unable to
produce pigment and is albino. The other gene determines the nature of the pigment. The dominant
allele (A) codes for alternating bands of pigment on the hair which produces the agouti pattern (greyish
coat colour). The recessive allele (a) codes for a black coat colour. Consequently mice can be black,
albino or agouti.
Use your knowledge of genetics to predict the phenotypic ratio of mice resulting from several crosses of
individuals heterozygous for both genes (AaBb).
Cross involving mice heterozygous for both coat colour genes
Genotype AaBb
gametes
gametes
X
Offspring are:
Ratio:
Note: The coat can only be agouti or black if the gene allowing pigment production is present. Without
it, the mouse will be albino.
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Inheritance of Flower Colour in Sweet Pea
Two different genes can affect flower colour in sweet pea. Gene C codes for an enzyme that converts a
colourless precursor into a colourless intermediate. Gene P codes for an enzyme that converts the
colourless intermediate into a purple pigment.
allele C codes for enzyme
colourless precursor
allele P codes for enzyme
colourless intermediate
coloured pigment (purple)
Q. Explain why both dominant alleles must be present if flowers are purple.
Q. Explain why flowers are colourless (white) if the genotype is homozygous cc.
Q. What would you expect from a cross between ccPP and CCpp (both of which are white-flowered)?
Q. If the offspring from the cross above were self-fertilised, what phenotypic ratio would you expect to
see among the offspring?
Inheritance of Comb Shape in Poultry
Comb shape in poultry is controlled by two gene
loci, each with two alleles:
Rose gene: R or r
Pea gene: P or p
Chicken with rose or pea comb shape are pure
breeding. When they are crossed, all offspring have
walnut comb. If the F1 generation is selfed the F2
generation has 9 walnut : 3 rose : 3 pea : 1 single
combed chicken. The 9 : 3 : 3 : 1 ratio confirms
that it is an example of dihybrid inheritance, but it
is unusual because the F1 chickens have a comb
shape that neither of the parents had and yet
another comb form appears in the F2 generation.
Rose comb:
Pea comb:
Walnut comb:
Single comb:
Note: The rose gene, if present as RR or Rr will
produce a rose type comb – but only if the pea
gene is homozygous recessive (pp). The pea gene,
if present as PP or Pp will produce a pea type comb
– but only if the rose gene is homozygous
recessive (rr). If one dominant allele is present for both the pea and rose genes, a walnut type comb is
produced, i.e. R_P_ gives walnut comb (the dash indicates that the second allele can be either dominant
or recessive). If both alleles are present in double recessive condition (rrpp), the single comb is
produced.
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Complete the genetic diagram to show inheritance of comb shape in chicken.
phenotype
genotype
pure breeding parents
rose comb
pea comb
gamete
F1 progeny
genotype
All offspring are walnut comb
F1 are selfed
gametes
gametes
X
Offspring are:
Ratio:
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Test Cross
There may be occasions when we need to know if an organism showing a dominant trait is homozygous
or heterozygous. Simply looking at it will not do because they will look the same – the presence of the
dominant allele means that the dominant trait will be expressed.
It is however possible to deduce the genotype from the results of a test cross to the homozygous
recessive type. The recessive type will produce gametes which all contain the recessive allele. Similarly a
homozygous dominant will produce gametes that all have the dominant allele. In contrast if the
individual is heterozygous half of its gametes will contain the dominant allele whilst the other half will be
recessive. Complete the following genetic diagrams to show the outcome of test crosses for tall garden
peas.
Test cross of heterozygous
dominant to recessive line
Test cross of homozygous
dominant to recessive line
X
Tall pea gametes
Tall pea gametes
X
Dwarf pea gametes
t
t
T
T
100% if offspring are tall
Dwarf pea gametes
t
t
T
t
50% of offspring are tall and
50% are dwarf
Clearly, if when testing a tall pea of unknown genotype, all the offspring were tall you would deduce
that the unknown was _________________. In contrast if 50% of them were dwarf you would deduce
that the unknown was _________________.
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