Transcript Ch 9: Heredity and Genetics

Ch 9: Heredity and Genetics
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 THE WORK OF MENDEL
MENDEL'S BLEEDING EXPERIMENT
• Mendel carried out cross-pollination of two varieties
of pea flowers. 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.
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
red
parent
white
RR
x
rr
meiosis
gametes
R
F1
r
Rr (red) x
gametes
R
r
R
F2
RR
Rr
Rr
ratio
red (3)
:
fertilization
Rr (red)
r
rr
white (1)
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 an allele.
Double helix
cell
Chromosome
DNA
strand
a gene
nucleus
chromatid
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)
A gene is the basic unit of heredity. A number of genes form a
chromosome.
An allele is one of a pair of genes occupying the same locus on
homologous chromosomes which separate during meiosis.
A chromosome is a thread like structure found in the nucleus of
eukaryotic cells. A number of chromosomes constitutes the
genetic material. They consist of DNA, RNA and proteins.
A locus is a position where a gene is located on a chromosome.
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
recessive ( rr )
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 dominant over
the white allele r
7. Distinguish between gene pool and genotype (2 marks) –I-1
SOME OTHER DOMINANT & RECESSIVE
CHARACTERISTICS IN PLANTS AND ANIMALS
1. In man: tongue rolling - RR, Rr
eye colour - BB, Bb
skin pigment - AA, Aa
SOME OTHER DOMINANT & RECESSIVE
CHARACTERISTICS IN PLANTS AND ANIMALS
1. In man: tongue rolling - RR, Rr
eye colour - BB, Bb
skin pigment - AA, Aa
SOME OTHER DOMINANT & RECESSIVE
CHARACTERISTICS IN PLANTS AND ANIMALS
1. In man: tongue rolling - RR, Rr
eye colour - BB, Bb
skin pigment - AA, Aa
2. In maize: colour of seeds in a cob - CC, Cc
Independent Assortment
• Different pairs of homologous chromosomes
come together, i.e. assorted but arranged
themselves at the middle of the cell
independently of any pairs of chromosomes.
Independent Assortment
Homologous chromosomes
line up randomly in 2 ways:
Members of a pair separate
Chromatids separate
in 2nd meiotic division
4 combinations
in each case
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
Approximate ratio –
round yellow : round green : wrinkled yellow : wrinkled green
9
:
3
:
3
:
1
9.1.6 Genetic representation of the dihybrid cross
Let R = allele for round seed,
G = allele for yellow seed,
Parents:
phenotypes
genotype
r = allele for wrinkled seed
g = allele for green seed
round, yellow seed
RRGG
x
wrinkled, green seed
rrgg
meiosis
gametes
F1 generation:
RG
genotype: RrGg
rg
phenotype: round, yellow seed
9.1.6 Genetic representation of the dihybrid cross
F1 generation:
gametes:
genotype: RrGg
RG
Rg
phenotype: round, yellow seed
rG
rg
Male gametes
Female gametes
RG
Rg
rG
rg
RG
RRGG
RRGg
RrGG
RrGg
Rg
RRGg
RRgg
RrGg
Rrgg
rG
RrGG
RrGg
rrGG
rrGg
rg
RrGg
Rrgg
rrGg
rrgg
9.1.6 Genetic representation of the dihybrid cross
Male gametes
Female gametes
RG
Rg
rG
rg
RG
RRGG
RRGg
RrGG
RrGg
Rg
RRGg
RRgg
RrGg
Rrgg
rG
RrGG
RrGg
rrGG
rrGg
rg
RrGg
Rrgg
rrGg
rrgg
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 homozygous
- An organism having dominant characteristic can either
be homozygous or heterozygous
For example: white flower - rr
red flower - RR or Rr
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 RR
Case 2 - if a mixture of red-flowered & white flowered offspring,
the parent plant must be Rr
TEST CROSS for animals which are unisexual &
cannot carry out self-fertilization.
A homozygous recessive brown mouse is used.
7. Distinguish between a) test cross and back cross (2 marks)
84-I-1
It is possible to perform a dihybrid test cross:
round & yellow x wrinkled & green (rrgg)
A round and yellow seed has 4 possible genotypes –
RRGG, RrGG, RRGg, RrGg
To determine its genotype, cross it with one with
wrinkled & green seed (rrgg)-
Possible genotypes Possible
of round yellow
gametes
seeds
Genotypes of offspring
crossed with wrinkled
green seeds(gamete=rg)
Phenotype (type of seeds
produced)
RRGG
RG
RrGg
All round & yellow
RrGG
RG
RrGg
rG
rrGg
½ round & yellow
½ wrinkled & yellow
RG
RrGg
½ round & yellow
Rg
Rrgg
½ round & green
RG
RrGg
¼ round & yellow
Rg
Rrgg
¼ round & green
rG
rrGg
¼ wrinkled & yellow
rg
rrgg
¼ wrinkled & green
RRGg
RrGg
9.3 Sex Determination
Autosomes: 22 pairs of chromosomes
which are identical in both sexes/ordinary
paired chromosomes which are not sex
chromosomes
Sex chromosomes (heterosomes):
the pair (23rd)which determine sex of
the organism
Homogametic sex: XX (female)
Heterogametic sex: XY (male)
4.
81-I-4
+ a genetic diagram
* Sex determination differ in other organisms:
Male being XX, female being XY,
e.g. birds, most reptiles, some fish, butterflies
Male being XO, female being XX,
e.g. some insects (grasshopper)
In fruit flies, the female is XX, male is XY
But the Y chromosome has a completely different shape:
XY
9.4 Linkage
Linked genes are genes which are located on the same
Chromosome.
9.4 Linkage
Linked genes are genes which are located on the same
Chromosome.
• 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
Yellow flowers, red fruit
x
White flowers, yellow fruit
F1 generation: All yellow flowers and red fruit
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
• 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.
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.
• Features linked on the Y chromosome will only arise in
heterogametic sex (XY)
• Features linked on the X chromosome may arise in
either sex
e.g. white eye colour is a sex-linked gene in fruitfly,
haemophilia and red-green colour-blindness in man
5.
82-I-1
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
Red-eyed female x red-eyed male:
Reciprocal cross:
white eye female x red eye male
Reciprocal cross:
red eye female x white eye 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 XB represent the allele for normal sight and
Xb represent the allele for colour-blindness
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
the onset of menstruation in puberty is often fatal
• 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 ?
It's because the royal families tended to marry within a relatively
small circle.
9.5 Allelic Interaction
• Codominance: when neither allele completely
dominates the other
• Multiple alleles: when more than two alleles exist for
a given gene
9.5.1 Incomplete dominance
In snapdragon:
red flower x white flower  F1 with pink flowers
9.5.1 Incomplete 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.
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
Table 9.4 Possible genotypes of blood groups in the ABO system
Blood group
A
B
AB
O
Possible genotypes
IAIA or IAIO
IBIB or IBIO
IAIB
IOIO
The transmission of these alleles occurs in normal
Mendelian fashion.
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
6.
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
-----------------------------------------------------------------------------
Coat colour
Possible genotypes
----------------------------------------------------------------------------Full
CFCF or CFCCH or CFCH or CFCA
Chinchilla
CCHCCH or CCHCH or CCHCA
Himalayan
CHCH or CHCA
Albino
CACA
-----------------------------------------------------------------------------
9.5.4 Pleiotropy and Lethal Genes
• An allele may determine or affect more than one
character. Such an allele is termed pleiotropic,
e.g. allele for cystic fibrosis (viscous mucus):
- blocking pancreatic duct with enzymes digesting
Islets of Langerhans, thus diabetes
- blocking alveoli & bronchioles,
thus respiratory problems
• A lethal gene will cause death of the offspring
when present in homozygous condition,
e.g. in mice:
homozygous yellow embryo always dies
Lethal genes
Let Y represent the allele for yellow fur (dominant) and
y represent the allele for grey 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, (4 dominant alleles)
AaBB or AABb being dark brown, (3 dominant alleles)
AAbb or aaBB or AaBb being half-coloured, (2..)
Aabb or aaBb being light brown, (1..)
aabb being white. (no dominant alleles)
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
9.6.3 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.
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.
2.
3.
4.
5.
6.
7.
8.
9.
Easy to breed
Readily grown
Cheap and easy to feed
Small size
Short life cycle
Production of many offspring
Early sexual maturity
Obviously recognizable features
Sexual dimorphism