GENETICS
Genetics is a branch of biology concerned with the F1 generation: The generation produced by crossing
study of genes, genetic variation, and heredity in homozygous parental stocks.
organisms. Gregor Mendel, a scientist discovered F2 generation: The generation produced by crossing
genetics in the late 19th-century. Mendel studied trait two F1 organisms.
inheritance, patterns in the way traits are handed down
from parents to offspring. He observed that organisms Monohybrid inheritance and the principle
(pea plants) inherit traits by way of discrete units of of segregation
inheritance.
Monohybrid inheritance is the inheritance of a
Mendel's success was due, in part, to his careful choice
single gene.
of experimental organism, the garden pea, Pisum
sativum.
Inheritance of pod colour in peas
If pea plants with green pods are bred repeatedly with
There were several varieties available with distinct
each other so that they consistently give rise to plants
characteristics.
with green pods, they are said to be pure breeding
The plants were easy to cultivate.
for the character of green pods. Pure breeding strains
The reproductive structures were completely
can be bred for almost any character. Organisms are
enclosed by the petals so that the plant was
homozygous (i.e. they have two alleles that are the
normally self- pollinating.
same) for that particular gene.
Artificial cross-breeding between varieties was
If these pure breeding green pod plants are then
possible and resulting hybrids were completely
crossed with pure breeding yellow pod plants, all the
fertile
offspring, known as the first filial or F1 generation,
Important terms used in genetics turn out to produce green pods. This means that the
Gene: The basic unit of inheritance for a given allele for green pods is dominant to the allele for
characteristic yellow pods, which is therefore recessive. When the
Allele: One of the alternative forms of the same gene heterozygous plants (Gg) of the F1 generation are
responsible for determining contrasting characteristics. crossed with one another (F1 intercross), the
Locus: Position of allele within a DNA molecule offspring (known as the second filial or F2
Homozygous: The diploid condition in which the allele generation) are always in an approximate ratio of
at a given locus are identical. 3plants with green pods to each 1 plant with yellow
Heterozygous: The diploid condition in which the pods
allele at a given locus are different
Phenotype: The observable characteristics of an Let: G represent allele for green colour (dominant)
individual usually resulting from the interaction y represent allele for yellow colour (recessive)
between the genotype and the environment in which Parental phenotypes: Green plants X yellow plants
development occurs. GG × yy
Parental genotype (2n)
Genotype: The genetic constitution of an organism
with respect to the alleles under consideration. Meiosis
Dominant allele: The allele which influences the Gametes (n) y y
G G
appearance of the phenotype even in the presence of an
alternative allele. Random fertilisation
Recessive allele: The allele which influences the F1 genotypes (2n) Gy Gy Gy Gy
appearance of the phenotype only in the presence of
another identical allele. F1 phenotypes all heterozygous green
,The F1 generation were self-pollinated 3. These factors do not blend in the F1 generation
F1 phenotypes green plants X green plants but retain their individuality.
F1 genotypes (2n)
Gy × Gy 4. The green factor is dominant to the yellow
factor, which is recessive.
Meiosis
Gametes (n) y G y The separation of the pair of parental factors, so that
G
one factor is present in each gamete, became known as
Random fertilisation Mendel's first law, or the principle of segregation.
F2 genotypes (2n) GG Gy Gy yy This states that: In diploid organisms, characteristics
are determined by factors that occur in pairs, only one
F2 Phenotypes: 3 green: 1 yellow of each pair of factors can be present in a single
The ratio of dominant phenotypes to recessive gamete.
phenotypes of 3:1 is called the monohybrid ratio These factors determining characteristics, such as
Mendel’s conclusions flower position, are regions of the chromosome known
as genes. By convention, the initial letter of the
1. Since the original parental stocks were pure dominant characteristic is used as the symbol for the
breeding, the character (colour) must have gene and its capital form (e.g. A) represents the
possessed two factors responsible for colour. dominant form of the gene (the dominant allele) while
the lower case (e.g. a) represents the recessive allele.
2. The F1 generation possessed one factor from
each parent which were carried by the gametes.
Test cross: This is a genetic cross between a homozygous recessive individual and a corresponding suspected
heterozygote to determine the genotype of the latter.
Example in the fruit fly, Drosophila, long wing is dominant to vestigial wing. The genotype of a long wing
Drosophila may be homozygous (LL) or heterozygous (LI). In order to establish which is the correct genotype the
fly is test crossed with a double recessive (II) vestigial wing fly. If the test cross offspring are all long wing the
unknown genotype is homozygous dominant. A ratio of 1 long w i n g : l vestigial wing indicates that the unknown
is heterozygous.
Let: L represent allele for long wing
l represent allele for vestigial wing
Homozygous long wing parent Heterozygous long wing parent
Testcross phenotypes: Long wing x vestigial wing Testcross phenotypes Long wing x Vestigial wing
Testcross genotypes (2n): LL x ll Testcross genotypes (2n): Ll x ll
Meiosis Meiosis
Gametes (n) L L x l l Gametes (n) L l x l l
Random fertilisation
Offspring genotypes (2n) Ll Ll Ll Ll Offspring genotypes(2n) Ll Ll ll ll
Offspring phenotypes all long wing (heterozygous) Offspring phenotypes
long wing vestigial
(heterozygous) (homozygous)
1 : 1
A full genetic explanation of how to determine the genotype of an organism showing a dominant characteristic
Mendel's breeding experiment with tall and dwarf plant is an example of monohybrid inheritance. Monohybrid
inheritance is inheritance of a single characteristics determined by one gene. Examples of monohybrid inheritance
in humans include the following,
, ► Albinism. ► Cystic fibrosis. ► Haemophilia.
► Huntington's disease. ► Lactose intolerance. ► Phenylketonuria.
► Rhesus blood group.
Dihybrid inheritance and the principle of independent assortment
This is the simultaneous inheritance of two characters. Mendel using pea shape and pea cotyledon colour as the
characteristics, crossed pure- breeding (homozygous) plants having round and yellow peas with pure-breeding
plants having wrinkled and green peas. The F1 generation seeds were round and yellow. Self-pollination of the F1
plants produced variety of characteristics. He collected a total of 556 F2 seeds from the F2 generation which showed
the following characteristics:
315 round and yellow, 101 wrinkled and yellow, 108 round and green, 32 wrinkled and gree n.
The proportions of each phenotype approximated to a ratio of 9 : 3 : 3 : 1 . This is known as the dihybrid
ratio. Two deductions were made from the above observations.
(a) Let
R represent round seed (dominant) Y represent yellow seed (dominant)
r represent wrinkled seed (recessive) y represent green seed (recessive)
Parental phenotypes: Round seed and yellow seed (homozygous) x wrinkled seed and green seed
(homozygous)
Parental genotypes (2n): RRYY X rryy
Meiosis
Gametes(n) all RY X ry
Random fertilization:
F1 genotypes (2n) all RrYy
F1 phenotypes: All heterozygous round and yellow seeds
Intercrossing F1 offspring
(b) F1 phenotypes round and yellow seed X round and yellow seed
F1 genotypes (2n) RrYy X RrYy
Meiosis
Gametes RY Ry rY ry
Random fertilization RY RRYY RRYy rRYY
Ry RRyY RRyy rRYy rRrr
F2 genotypes(2n) rY RrYY RrYy rrYY rryY
ry RrYy Rryy rrYy rryy
F2 genotypes: 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green seeds
1. Two new combinations of characteristics The two pairs of characteristics (seed shape and
appeared in the F2 generation: wrinkled and colour), whilst combining in the F1 generation, separate
yellow, and round and green. and behave independently from one another in
2. The ratios of each pair of allelomorphic subsequent generations. This forms the basis of
characteristics (phenotypes determined by Mendel's second law or the principle of independent
different alleles) appeared in the monohybrid assortment which states that: any one of a pair of
ratio of 3:1, that is 423 round to 133 wrinkled, characteristics may combine with either one of
and 416 yellow to 140 green. another pair.
, Summary of Mendel's hypotheses one of another pair (the principle of
independent assortment).
1. Each characteristic of an organism is controlled 5. Each allele is transmitted from generation to
by a pair of alleles. generation as a discrete unchanging unit.
2. If an organism has two unlike alleles for a 6. Each organism inherits one allele (for each
given characteristic, one may be expressed (the characteristic) from each parent.
dominant allele) to the total exclusion of the
other (the recessive allele) NB The mechanism of dihybrid inheritance
3. During meiosis each pair of alleles separates and the typical dihybrid ratio of 9:3:3:1 only
(segregates) and each gamete receives one of apply to characteristics controlled by genes on
each pair of alleles (the principle of different chromosomes. Genes situated on the
segregation). same chromosome may not show this pattern of
4. During gamete formation in each sex, either independent assortment.
one of a pair of alleles may enter the same
gamete cell (combine randomly) with either
Meiosis and fertilization Mendel’s hypotheses
Diploid cells contain pairs of Characteristics controlled by pairs of factors
chromosomes(homologous chromosomes)
Homologous chromj0bmosomes separate during Pairs of factors separate during gamete
meiosis formation
One homologous chromosomes passes into each Each gamete receives one factor
gamete
Only the nucleus of the male gamete with the egg Factors are transmitted from generation to
cell nucleus generation as discrete units
Homologous pairs of chromosomes are restored at Each organism inherits one factor from each
fertilisation, each gamete contributing one parent
homologous chromosome.
Linkage breeding grey-bodied long-winged Drosophila are
crossed with black-bodied vestigial-winged
Genes situated on the same chromosome are said to be Drosophila, 3:1 F2 phenotypic ratio was produced not
linked. All genes on a single chromosome form a the 9:3:3:1as e x p e c t e d . This is because the genes for
linkage group and usually pass into the same gamete body colour and wing length are found on the same
and are inherited together. As a result of this, genes chromosome that is they are linked.
belonging to the same linkage group usually do not
show independent assortment. Since these genes do not
conform to Mendel's principle of independent
assortment they fail to produce the expected 9 : 3 : 3 : 1
ratio in a breeding situation involving the inheritance of
two pairs of contrasted characteristics (dihybrid
inheritance).
In Drosophila the genes for body colour and wing
length have the following allelomorphs (phenotypic
characteristics determined by different alleles): grey
and black body, and long and vestigial (short) wings.
Grey body and long wing are dominant. When pure-
Genetics is a branch of biology concerned with the F1 generation: The generation produced by crossing
study of genes, genetic variation, and heredity in homozygous parental stocks.
organisms. Gregor Mendel, a scientist discovered F2 generation: The generation produced by crossing
genetics in the late 19th-century. Mendel studied trait two F1 organisms.
inheritance, patterns in the way traits are handed down
from parents to offspring. He observed that organisms Monohybrid inheritance and the principle
(pea plants) inherit traits by way of discrete units of of segregation
inheritance.
Monohybrid inheritance is the inheritance of a
Mendel's success was due, in part, to his careful choice
single gene.
of experimental organism, the garden pea, Pisum
sativum.
Inheritance of pod colour in peas
If pea plants with green pods are bred repeatedly with
There were several varieties available with distinct
each other so that they consistently give rise to plants
characteristics.
with green pods, they are said to be pure breeding
The plants were easy to cultivate.
for the character of green pods. Pure breeding strains
The reproductive structures were completely
can be bred for almost any character. Organisms are
enclosed by the petals so that the plant was
homozygous (i.e. they have two alleles that are the
normally self- pollinating.
same) for that particular gene.
Artificial cross-breeding between varieties was
If these pure breeding green pod plants are then
possible and resulting hybrids were completely
crossed with pure breeding yellow pod plants, all the
fertile
offspring, known as the first filial or F1 generation,
Important terms used in genetics turn out to produce green pods. This means that the
Gene: The basic unit of inheritance for a given allele for green pods is dominant to the allele for
characteristic yellow pods, which is therefore recessive. When the
Allele: One of the alternative forms of the same gene heterozygous plants (Gg) of the F1 generation are
responsible for determining contrasting characteristics. crossed with one another (F1 intercross), the
Locus: Position of allele within a DNA molecule offspring (known as the second filial or F2
Homozygous: The diploid condition in which the allele generation) are always in an approximate ratio of
at a given locus are identical. 3plants with green pods to each 1 plant with yellow
Heterozygous: The diploid condition in which the pods
allele at a given locus are different
Phenotype: The observable characteristics of an Let: G represent allele for green colour (dominant)
individual usually resulting from the interaction y represent allele for yellow colour (recessive)
between the genotype and the environment in which Parental phenotypes: Green plants X yellow plants
development occurs. GG × yy
Parental genotype (2n)
Genotype: The genetic constitution of an organism
with respect to the alleles under consideration. Meiosis
Dominant allele: The allele which influences the Gametes (n) y y
G G
appearance of the phenotype even in the presence of an
alternative allele. Random fertilisation
Recessive allele: The allele which influences the F1 genotypes (2n) Gy Gy Gy Gy
appearance of the phenotype only in the presence of
another identical allele. F1 phenotypes all heterozygous green
,The F1 generation were self-pollinated 3. These factors do not blend in the F1 generation
F1 phenotypes green plants X green plants but retain their individuality.
F1 genotypes (2n)
Gy × Gy 4. The green factor is dominant to the yellow
factor, which is recessive.
Meiosis
Gametes (n) y G y The separation of the pair of parental factors, so that
G
one factor is present in each gamete, became known as
Random fertilisation Mendel's first law, or the principle of segregation.
F2 genotypes (2n) GG Gy Gy yy This states that: In diploid organisms, characteristics
are determined by factors that occur in pairs, only one
F2 Phenotypes: 3 green: 1 yellow of each pair of factors can be present in a single
The ratio of dominant phenotypes to recessive gamete.
phenotypes of 3:1 is called the monohybrid ratio These factors determining characteristics, such as
Mendel’s conclusions flower position, are regions of the chromosome known
as genes. By convention, the initial letter of the
1. Since the original parental stocks were pure dominant characteristic is used as the symbol for the
breeding, the character (colour) must have gene and its capital form (e.g. A) represents the
possessed two factors responsible for colour. dominant form of the gene (the dominant allele) while
the lower case (e.g. a) represents the recessive allele.
2. The F1 generation possessed one factor from
each parent which were carried by the gametes.
Test cross: This is a genetic cross between a homozygous recessive individual and a corresponding suspected
heterozygote to determine the genotype of the latter.
Example in the fruit fly, Drosophila, long wing is dominant to vestigial wing. The genotype of a long wing
Drosophila may be homozygous (LL) or heterozygous (LI). In order to establish which is the correct genotype the
fly is test crossed with a double recessive (II) vestigial wing fly. If the test cross offspring are all long wing the
unknown genotype is homozygous dominant. A ratio of 1 long w i n g : l vestigial wing indicates that the unknown
is heterozygous.
Let: L represent allele for long wing
l represent allele for vestigial wing
Homozygous long wing parent Heterozygous long wing parent
Testcross phenotypes: Long wing x vestigial wing Testcross phenotypes Long wing x Vestigial wing
Testcross genotypes (2n): LL x ll Testcross genotypes (2n): Ll x ll
Meiosis Meiosis
Gametes (n) L L x l l Gametes (n) L l x l l
Random fertilisation
Offspring genotypes (2n) Ll Ll Ll Ll Offspring genotypes(2n) Ll Ll ll ll
Offspring phenotypes all long wing (heterozygous) Offspring phenotypes
long wing vestigial
(heterozygous) (homozygous)
1 : 1
A full genetic explanation of how to determine the genotype of an organism showing a dominant characteristic
Mendel's breeding experiment with tall and dwarf plant is an example of monohybrid inheritance. Monohybrid
inheritance is inheritance of a single characteristics determined by one gene. Examples of monohybrid inheritance
in humans include the following,
, ► Albinism. ► Cystic fibrosis. ► Haemophilia.
► Huntington's disease. ► Lactose intolerance. ► Phenylketonuria.
► Rhesus blood group.
Dihybrid inheritance and the principle of independent assortment
This is the simultaneous inheritance of two characters. Mendel using pea shape and pea cotyledon colour as the
characteristics, crossed pure- breeding (homozygous) plants having round and yellow peas with pure-breeding
plants having wrinkled and green peas. The F1 generation seeds were round and yellow. Self-pollination of the F1
plants produced variety of characteristics. He collected a total of 556 F2 seeds from the F2 generation which showed
the following characteristics:
315 round and yellow, 101 wrinkled and yellow, 108 round and green, 32 wrinkled and gree n.
The proportions of each phenotype approximated to a ratio of 9 : 3 : 3 : 1 . This is known as the dihybrid
ratio. Two deductions were made from the above observations.
(a) Let
R represent round seed (dominant) Y represent yellow seed (dominant)
r represent wrinkled seed (recessive) y represent green seed (recessive)
Parental phenotypes: Round seed and yellow seed (homozygous) x wrinkled seed and green seed
(homozygous)
Parental genotypes (2n): RRYY X rryy
Meiosis
Gametes(n) all RY X ry
Random fertilization:
F1 genotypes (2n) all RrYy
F1 phenotypes: All heterozygous round and yellow seeds
Intercrossing F1 offspring
(b) F1 phenotypes round and yellow seed X round and yellow seed
F1 genotypes (2n) RrYy X RrYy
Meiosis
Gametes RY Ry rY ry
Random fertilization RY RRYY RRYy rRYY
Ry RRyY RRyy rRYy rRrr
F2 genotypes(2n) rY RrYY RrYy rrYY rryY
ry RrYy Rryy rrYy rryy
F2 genotypes: 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green seeds
1. Two new combinations of characteristics The two pairs of characteristics (seed shape and
appeared in the F2 generation: wrinkled and colour), whilst combining in the F1 generation, separate
yellow, and round and green. and behave independently from one another in
2. The ratios of each pair of allelomorphic subsequent generations. This forms the basis of
characteristics (phenotypes determined by Mendel's second law or the principle of independent
different alleles) appeared in the monohybrid assortment which states that: any one of a pair of
ratio of 3:1, that is 423 round to 133 wrinkled, characteristics may combine with either one of
and 416 yellow to 140 green. another pair.
, Summary of Mendel's hypotheses one of another pair (the principle of
independent assortment).
1. Each characteristic of an organism is controlled 5. Each allele is transmitted from generation to
by a pair of alleles. generation as a discrete unchanging unit.
2. If an organism has two unlike alleles for a 6. Each organism inherits one allele (for each
given characteristic, one may be expressed (the characteristic) from each parent.
dominant allele) to the total exclusion of the
other (the recessive allele) NB The mechanism of dihybrid inheritance
3. During meiosis each pair of alleles separates and the typical dihybrid ratio of 9:3:3:1 only
(segregates) and each gamete receives one of apply to characteristics controlled by genes on
each pair of alleles (the principle of different chromosomes. Genes situated on the
segregation). same chromosome may not show this pattern of
4. During gamete formation in each sex, either independent assortment.
one of a pair of alleles may enter the same
gamete cell (combine randomly) with either
Meiosis and fertilization Mendel’s hypotheses
Diploid cells contain pairs of Characteristics controlled by pairs of factors
chromosomes(homologous chromosomes)
Homologous chromj0bmosomes separate during Pairs of factors separate during gamete
meiosis formation
One homologous chromosomes passes into each Each gamete receives one factor
gamete
Only the nucleus of the male gamete with the egg Factors are transmitted from generation to
cell nucleus generation as discrete units
Homologous pairs of chromosomes are restored at Each organism inherits one factor from each
fertilisation, each gamete contributing one parent
homologous chromosome.
Linkage breeding grey-bodied long-winged Drosophila are
crossed with black-bodied vestigial-winged
Genes situated on the same chromosome are said to be Drosophila, 3:1 F2 phenotypic ratio was produced not
linked. All genes on a single chromosome form a the 9:3:3:1as e x p e c t e d . This is because the genes for
linkage group and usually pass into the same gamete body colour and wing length are found on the same
and are inherited together. As a result of this, genes chromosome that is they are linked.
belonging to the same linkage group usually do not
show independent assortment. Since these genes do not
conform to Mendel's principle of independent
assortment they fail to produce the expected 9 : 3 : 3 : 1
ratio in a breeding situation involving the inheritance of
two pairs of contrasted characteristics (dihybrid
inheritance).
In Drosophila the genes for body colour and wing
length have the following allelomorphs (phenotypic
characteristics determined by different alleles): grey
and black body, and long and vestigial (short) wings.
Grey body and long wing are dominant. When pure-
