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F 6 Biology - Ch 9: Heredity and Genetics
Name:____________________________( )
1
INTRODUCTION
Heredity - the tendency of an individual to resemble his parents
Variation - the differences between individuals of the same species,
e.g. height, intelligence, eye colour, hair colour, etc
Genetics - the study of the method of inheritance of 'characteristics' of living organisms
In sexual reproduction a new offspring is derived only from the gametes of its parents. Gametes
contain all the necessary
hereditary information.
9.1.1
THE WORK OF MENDEL
MENDEL'S BLEEDING EXPERIMENT
Mendel carried out cross-
pollination of two varieties of peaflowers. He collected the seeds
and sowed them the following
year. The characteristics of the
daughter plants were then
carefully examined.
He carried out a mono-hybrid cross of flower colour:
pure red and pure white
The F2 generation
gave rise to a ratio of approximately
3 reds: 1 white - monohybrid ratio
Mendel's 1st law:
The characteristics of an organism
are determined by internal factors
which occur in pairs. Only one of a
pair of such factors can be represented in a single gamete.
9.1.2
Interpretation of the results of
Mendel's breeding experiment
Alleles control the expression of flower colour: Red - R; White - r
R gene suppresses the effect of r gene: R gene is dominant;
r gene is recessive
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F 6 Biology - Ch 9: Heredity and Genetics
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9.1.3 Genetic Representation of the Monohybrid Cross
(see p 152 - 153 of text-book)
9.1.4 Genes and Alleles
Phenotype - external appearance of the organism,
e.g. red flower, white flower
Genotype - genetic composition of the organism,
e.g. RR, Rr or rr
A gene is the basic unit of inheritance.
It is a region of the chromosome or a length of the
DNA molecule which has a particular
function. Each gene may have two, or
occasionally more, alternative forms.
Each form of the gene is called anallele
.
96-I-7(a): Distinguish between gene and allele
(2 marks)
Homozygous (pure-breeding) -
individual with its genotype of a particular characteristic consists of two identical alleles,
e.g. RR or rr
It could be homozygous dominant (RR) or homozygous ___________ ( )
Heterozygous (hybrid) - genotype consists of two different alleles, e.g. Rr
A dominant gene can suppress the expression of its recessive allele,
e.g. Rr is red because the red allele R is ___________ over the white allele r
SOME OTHER DOMINANT & RECESSIVE CHARACTERISTICS IN PLANTS AND
ANIMALS
1. In man: tongue rolling - RR, Rr
eye colour - BB, Bb
skin pigment (albinism)- AA, Aa
2. In maize: colour of seeds in a cob - CC, Cc
9.1.5 Dihybrid inheritance (Mendel's Law of Independent Assortment)
Dihybrid inheritance refers to the simultaneous inheritance of two characters:
Each of a pair of contrasted characters may be combined with either of another pair. OR
Each member of an allelic pair may combine randomly with either of another pair.
TABLE 9.2 RESULTS OF MENDEL'S DIHYBRID CROSS
Seed shape
Round Wrinkled Total Approx. ratio
Seed Yellow 315 101 416 3 yellow
colour Green 108 32 140 1 green
Total 423 133
App. ratio 3 round 1 wrinkled
Approximate ratio - round yellow : round green : wrinkled yellow : wrinkled green
___ : ___ : ___ : ___
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F 6 Biology - Ch 9: Heredity and Genetics
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Genetic representation of the dihybrid cross
In the following list, “-“ represents either the dominant or the recessive allele.
R-G- = round, yellow seed 9 (315)
R-gg = round, green seed 3 (108)
rrG- = wrinkled, yellow seed 3 (101)
rrgg = wrinkled, green seed 1 (32)
Allowing for statistical error, Mendel's results were a reasonable approximation to the
expected 9 : 3 : 3 : 1 ratio.
9.2. The Test Cross
FINDING OUT THE GENOTYPE OF AN ORGANISM
- The genotype of an organism with recessive characteristic must be __________
- An organism having dominant characteristic can either be ________________ or
________________
For example: white flower -
red flower -
- If the organism is a self-pollinating plant, we can find out its genotype by self-pollination:
Case 1 - if all of the offspring are red flowered, the parent plant must be __
Case 2 - if a mixture of red-flowered & white flowered offspring, the parent plant must be ___
TEST CROSS for animals which are unisexual & cannot carry out self-fertilization.
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F 6 Biology - Ch 9: Heredity and Genetics
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Test Cross: For animals which are unisexual and cannot carry out self-fertilization.
A homozygous recessive brown mouse is used.
It is possible to perform a dihybrid test cross:
A round and yellow seed has 4 possible genotypes -
To determine its genotype, cross it with one with wrinkled green seed -
Possible genotypes of
round yellow seeds
Possible
gametes
Genotypes of offspring
crossed with wrinkled green
seeds(gamete=rg)
Phenotype
(type of seeds
produced)
½
9.3 Sex Determination
autosomes:
sex chromosomes (heterosomes):
homogametic sex:
heterogametic sex:
* Sex determination differ in other
organisms:
Male being XX, female being XY,
e.g.
Male being XO, female being XX,
e.g.
In fruit flies, the female is XX, the
male is XY. But the Y chromosome has a completely different shape:
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F 6 Biology - Ch 9: Heredity and Genetics
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9.4 Linkage
Linked genes are
All the genes on a single chromosome form a linkage group.
Under normal circumstances, all the linked genes remain together during cell division and so pass
into the gamete, and hence the offspring, together. They do not therefore segregate in accordance
with Mendel's Second Law of Independent Assortment.
9.4.1 Crossing Over and Recombination
It is known that genes for flower colour and fruit colour in tomatoes are on the same
chromosomes. Plants with yellow flowers bear red fruit while white flowers bear yellow fruit.Let R = allele for red fruit (dominant) and r = allele for yellow fruit (recessive)
W = allele for yellow flowers (dominant) and w = allele for white flowers (recessive)
Yellow flowers, red fruit x White flowers, yellow fruit
F1 generation: All yellow flowers and red fruit (WwRr)
If the F1 generation is self-pollinated, the following results should be expected:
Genotypes: WWRR : WwRr : WwRr : wwrr
Phenotypes: 3 yellow flowers, red fruit : 1 white flowers, yellow fruit
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F 6 Biology - Ch 9: Heredity and Genetics
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However, the following results were obtained in F2 if the F1 generation is self-pollinated:
Yellow flowers and red fruit - 68
Yellow flowers and yellow fruit - 7
White flowers and red fruit - 7
White flowers and yellow fruit - 18
** The new recombinants are the result of crossing over in prophase I of meiosis.
9.4.2 Sex Linkage
Sex linkage refers to the carrying of genes on the sex chromosomes. These genes determine body
characters and have nothing to do with sex. The X chromosome carries many such genes. The Y
chromosome has very few.
Crosses in fruitflies:
*Wild type - a term to describe and organism as it normally occurs in nature
*Reciprocal cross - a cross of the male and female with the same genetic features
but sexes are reversed
R = allele for the red eye (dominant gene)
r = allele for the white eye (recessive gene)
Red-eye female x white-eye male
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F 6 Biology - Ch 9: Heredity and Genetics
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Red-eyed female x red-eyed male:
Reciprocal cross: white-eye female x red-eye male
Reciprocal cross: red-eye female x white-eyed male:
Inheritance of red green colour-blindness:
* Carriers: The heterozygous females are not affected by the defect but are capable of passing the
recessive gene to their offspring.
Let X
B
represent the allele fornormal sight and
Xb represent the allele for
colour-blindness
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F 6 Biology - Ch 9: Heredity and Genetics
Name:____________________________( )
8
The inheritance of haemophilia:
The inability of the blood to clot leading to slow and persistent bleeding, especially in the joints ---
a potentially fatal disease.
Haemophiliac females are highly improbable, and are unlikely to have children
because
Haemophilia is the result of an individual being unable to produce one of the many clotting factors,
namely factor 8 or antihaemophiliac globulin (AHG).
Any mutant recessive gene, such as that causing haemophilia, is normally rapidly diluted among
the many normal genes in a population. Its expression is rare but it occurrence is high in many
European royal families, e.g. England’s Queen Victoria. Why ?
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F 6 Biology - Ch 9: Heredity and Genetics
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9.5 Allelic Interaction
Codominance: when neither allele completely dominates the other
Multiple al leles : when more than two alleles exist for a given gene
9.5.1 Incomplete dominance/Co-dominance
In snapdragon: red flower x white flower → F1 with pink flowers
9.5.2 Partial Dominance
Sometimes both alleles express themselves in the phenotype, but one more so than another. This
an intermediate stage between complete dominance and codominance. There are many blends of
partial dominance which lead to a wide range of intermediate varieties between two extremes, e.g.
a cross between a black dog and a white dog give patches of predominantly black fur in their
offspring.
9.5.3 Multiple Alleles
In humans the inheritance of the ABO blood group is determined by a gene I which
has 3 different alleles. Any 2 of these can occur at a single locus at any one time - codominance
Allele IA
causes production of antigen A on RBCs
Allele IB causes production of antigen B on RBCs
Allele IO
causes no production of antigens on RBCs
Alleles IA
and IB
are codominant and allele IO
is recessive to both
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F 6 Biology - Ch 9: Heredity and Genetics
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Table 9.4 Possible genotypes of blood groups in the ABO system
The transmission of these alleles occurs in normal Mendelian fashion.
A cross between individuals of blood group A and certain individuals of blood group B may
produce offspring with any one of the 4 blood groups:
Paternity suits
- to show an individual could not possibly be the father
Dominance series
Coat colour in rabbits is determined by a gene C which has 4 possible alleles:
Allele CF determines full coat colour and is dominant to
Allele CCH which determines chinchilla coat and is in turn dominant to
Allele CH which determines Himalayan coat and is in turn dominant to
Allele CA which determines albino coat colour
There is a dominance series, and each type has a range of possible genotypes.
Inheritance follows the Mendelian fashion.
Table 9.5 Possible genotypes of rabbits with different coat colour
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F 6 Biology - Ch 9: Heredity and Genetics
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9.5.4 Lethal Genes
A lethal gene will cause death of the offspring when present in homozygous condition,
e.g. in mice: homozygous yellow embryo always dies
Let Y represent the allele for yellow fur (dominant)
and y represent the allele for grey/agouti fur (recessive)
9.6 Gene Interaction - the alleles of more than one gene (at more than one locus) interact
9.6.1 Simple Interaction
This occurs when a group of genes or a gene complex act together to determine a single character.
Inheritance with an intermediate form arises.
Example in humans - skin pigmentation controlled by two genes A and B with
AABB being darkly pigmented,
AaBB or AABb being dark brown,
AAbb or aaBB or AaBb being half-coloured,
Aabb or aaBb being light brown,
aabb being white.
Example in poultry (epistasis)- blending leads to new features:
comb shape in chicken with entirely new features
rose comb x pea comb → F1 all walnut type → interbreed to give 4 types of combs in F2
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F 6 Biology - Ch 9: Heredity and Genetics
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9.6.2 Polygenes
Many genes acting together are referred to as polygenes. For example, a character determined by
5 genes, each gene having a dominant or recessive allele. An
organism inheriting 5 dominant alleles will lie at one end of
the spectrum and one with 5 recessive alleles will lie at the
other. Between these extremes will a continuum of types
depending on the relative proportions of dominant and
recessive alleles. Polygenes give rise to continuous
variation.
A normal distribution of continuous variation characteristics
Choice of species for genetic crosses
Organisms favouring for genetic research are fruit fly ( Drosophila melanogaster ), mice and maize
plants combine most, if not all, the following factors:1. Easy to breed
2. Readily grown
3. Cheap and easy to feed
4. Small size
5. Short life cycle
6. Production of many offspring
7. Early sexual maturity
8. Obviously recognizable features
9. Sexual dimorphism
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F 6 Biology - Ch 9: Heredity and Genetics
Name:____________________________( )
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Exercise:
1. 2000-I-10
2. Distinguish between gene and allele. 96-I-7 (a))
3.Briefly explain the following terms: gene, allele, chromosome, locus. (8 marks) (80-II-5)
4. 81-I-4
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F 6 Biology - Ch 9: Heredity and Genetics
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5. 82-I-1
6.
7. 84-I-1
D istinguish betw een a) test cross and back cross b) gene pool and genotype (4 m arks)
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