GENETICS AND INHERITANCE
Introduction:
Genetics is the study heredity. Each individual inherits a variety of characteristics from his parents, for
example, he/she inherits hair colour, eye colour, skin colour, intelligence, etc. In plants, it inherits the
colour and shape of the seed, the height of the plant and the colour of the flowers. The genetic
characteristics are transferred through gametes (sperm cell and ova)
Gregor Mendel is the father of genetics.
Genetic concepts
1 Chromatin is a substance within a chromosome consisting of DNA and protein.
2 Chromosomes are threadlike structures of nucleic acids and found in the nucleus carrying genetic
information.
3 Genes are small portion of DNA coding for a particular characteristic/ protein.
4 Allele is the different form of the same gene.
5 Phenotype is the physical appearance of an organism determined by the genotype, e.g., tall, short.
6 Genotype is the genetic composition (make- Alleles up) of an organism.
7 Homozygous are two identical alleles for a particular characteristic, BB or bb.
8 Heterozygous are two different alleles for a particular characteristic, Bb.
9 Dominant alleles are an allele that is expressed in the phenotype when found in the heterozygous (Tt)
and homozygous (TT) condition.
10 Recessive alleles are an allele that is masked in the phenotype when found in the heterozygous (Tt)
condition and it is only expressed in the homozygous (tt) condition.
11 Monohybrid cross is a cross which involve only one characteristic or trait is being shown in the genetic
cross.
12 Mendel's Law of Segregation states that a diploid organism passes a randomly selected allele for a trait
to its offspring, such that the offspring receives one allele from each parent.
Phenotype Genotype
gg
Green (homozygous) 1
Gg
Green (heterozygous)
3
2
Gg
Green (heterozygous)
GG
1 Black (homozygous) 1
Ration: 3:1 Ration: 1:2:1
, The genetic cross format
P1 PHENOTYPE : _______________ X _____________
GENOTYPE : _______________ X _____________
MEIOSIS : ______ ; _______ X _____ ; ______
FERTILISATION :
F1 GENOTYPE : _______; ________; ________; ________
PHENOTYPE :
RATIO : _____:_____ or _____:_____:_____ or more
NB:
• P1 represent parent generation.
• F1 represent first offspring generation.
Types of dominance
1 COMPLETE DOMINANCE
• genetic cross where the dominant allele masks (blocks) the expression of a recessive allele in the
heterozygous condition.
2 INCOMPLETE DOMINANCE
• genetic cross where none of the two alleles of a gene are dominant over one another, resulting in an
intermediate phenotype in the heterozygous condition
3 CO-DOMINANCE
• A genetic cross in which both alleles are expressed equally in the phenotype.
Complete dominance
P1 PHENOTYPE : TALL X SHORT
GENOTYPE : TT X tt
MEIOSIS : T ; T X t ; t
FERTILISATION :
F1 GENOTYPE : Tt Tt Tt Tt
PHENOTYPE : ALL TALL
RATIO : 4 TALL
NB!
• Alleles for Tallness is dominant over alleles for shortness since they are all masked.
Introduction:
Genetics is the study heredity. Each individual inherits a variety of characteristics from his parents, for
example, he/she inherits hair colour, eye colour, skin colour, intelligence, etc. In plants, it inherits the
colour and shape of the seed, the height of the plant and the colour of the flowers. The genetic
characteristics are transferred through gametes (sperm cell and ova)
Gregor Mendel is the father of genetics.
Genetic concepts
1 Chromatin is a substance within a chromosome consisting of DNA and protein.
2 Chromosomes are threadlike structures of nucleic acids and found in the nucleus carrying genetic
information.
3 Genes are small portion of DNA coding for a particular characteristic/ protein.
4 Allele is the different form of the same gene.
5 Phenotype is the physical appearance of an organism determined by the genotype, e.g., tall, short.
6 Genotype is the genetic composition (make- Alleles up) of an organism.
7 Homozygous are two identical alleles for a particular characteristic, BB or bb.
8 Heterozygous are two different alleles for a particular characteristic, Bb.
9 Dominant alleles are an allele that is expressed in the phenotype when found in the heterozygous (Tt)
and homozygous (TT) condition.
10 Recessive alleles are an allele that is masked in the phenotype when found in the heterozygous (Tt)
condition and it is only expressed in the homozygous (tt) condition.
11 Monohybrid cross is a cross which involve only one characteristic or trait is being shown in the genetic
cross.
12 Mendel's Law of Segregation states that a diploid organism passes a randomly selected allele for a trait
to its offspring, such that the offspring receives one allele from each parent.
Phenotype Genotype
gg
Green (homozygous) 1
Gg
Green (heterozygous)
3
2
Gg
Green (heterozygous)
GG
1 Black (homozygous) 1
Ration: 3:1 Ration: 1:2:1
, The genetic cross format
P1 PHENOTYPE : _______________ X _____________
GENOTYPE : _______________ X _____________
MEIOSIS : ______ ; _______ X _____ ; ______
FERTILISATION :
F1 GENOTYPE : _______; ________; ________; ________
PHENOTYPE :
RATIO : _____:_____ or _____:_____:_____ or more
NB:
• P1 represent parent generation.
• F1 represent first offspring generation.
Types of dominance
1 COMPLETE DOMINANCE
• genetic cross where the dominant allele masks (blocks) the expression of a recessive allele in the
heterozygous condition.
2 INCOMPLETE DOMINANCE
• genetic cross where none of the two alleles of a gene are dominant over one another, resulting in an
intermediate phenotype in the heterozygous condition
3 CO-DOMINANCE
• A genetic cross in which both alleles are expressed equally in the phenotype.
Complete dominance
P1 PHENOTYPE : TALL X SHORT
GENOTYPE : TT X tt
MEIOSIS : T ; T X t ; t
FERTILISATION :
F1 GENOTYPE : Tt Tt Tt Tt
PHENOTYPE : ALL TALL
RATIO : 4 TALL
NB!
• Alleles for Tallness is dominant over alleles for shortness since they are all masked.
