Mendelian Genetics
Terminology related to Mendelian Genetics
1. Character:The term character is used for the phenotypic property of the
individual such as stem height, flower colour, type of hair, etc.
2. Trait: It is an inherited character such as tall or dwarf, purple or white
colour of flower, curly or straight hair.
3. Gene: A gene is the unit of inheritance which is carried from parent by a
gamete. Gene occurs in a chromosome and controls the expression of a
character in cooperation with other genes and environment. Chemically
gene is a segment of DNA called cistron that has a particular function. One
cistron controls the production of one polypeptide.
4. Alleles: The term alleles were given by Bateson in 1905. Alleles are forms of
the same gene which are found at the same place or locus on homologous
chromosome and control particular trait of a character e.g. tallness and
dwarfness.
5. Homozygous individual: An individual having two identical alleles
(isoalleles) of a trait is called homozygous individual. It is also called pure
line. The term pure line was coined by Johannsen in 1910. For example RR
is pure red and rr is pure white trait.
6. Dominant and recessive factor or allele: Out of two contrasting alleles of
heterozygous or hybrid individual, only one allele can express the effect
itself is called dominant factor or allele.
The other allele or factor present in hybrid is unable to express its effect
but it can express its effect in homozygous condition in next generation is
called recessive factor or allele. Dominant allele is denoted by capital letter
but recessive allele is denoted by small letter. For example, in hybrid red
(Rr), R allele expresses red trait but r allele cannot express white trait. So, R
allele and r allele are dominant allele and recessive alleles respectively.
7. Phenotype: It is the physical or observable expression of an individual. It
can be expressed in word like tallness, dwarfness, etc. Phenotype can be
modified by environmental changes.
8. Genotype: It is the gene complement or genetic constitution of an
individual. It is expressed in symbol like TT for pure tall, tt for pure dwarf ,Tt
for hybrid tall, etc. A genotype cannot be observed directly.
, 9. Hybrid: The heterozygous organism produced after crossing two genetically
different individuals is called hybrid e.g. hybrid red(Rr), hybrid tall(Tt).
10.Monohybrid Cross: It is a cross between two organisms of a species which
is made to study the inheritance of single pairs of factors or alleles of a
character.
11.Monohybrid Ratio: It is a ratio which is obtained in F₂ generation when a
monohybrid cross is made and the offspring of F1 generation are self-bred.
Monohybrid ratio is usually 3:1 (phenotypic ratio) or 1:2:1 (genotypic ratio)
in which 25% of the individuals carry the recessive trait, 25% pure dominant
and 50% have hybrid dominant trait.
12.Dihybrid Cross: It is a cross between two organisms of a species which is
made to study the inheritance of two pairs of factors or alleles of two
genes.
13.Dihybrid Ratio: It is a ratio which is obtained in the F₂ generation when a
dihybrid cross is made and the offspring of F1 generation are self-bred.
Dihybrid ratio is 9:3:3:1 (phenotypic ratio).
14.Emasculation: The removal of stamens from bisexual flowers in order to
avoid self-pollination in these flowers during hybridization.
15.Back Cross: The cross of an F 1 hybrid with one of the two parents is called
backcross.
16.Test cross: A cross between an individual of unknown genotype and
recessive parent is called test cross.
17.Reciprocal Cross: A second cross of the same genotypes in which the sexes
of the parental generation are reversed, is called reciprocal cross.
18.Punnett square: It is a graphical representation to calculate the probability
of all possible genotypes of offspring in a genetic cross. The possible
gametes are written on two sides, the top row (horizontal row) and left
column (vertical column). Usually male gametes are writ- ten in top row
and female gametes in left column
Terminology related to Mendelian Genetics
1. Character:The term character is used for the phenotypic property of the
individual such as stem height, flower colour, type of hair, etc.
2. Trait: It is an inherited character such as tall or dwarf, purple or white
colour of flower, curly or straight hair.
3. Gene: A gene is the unit of inheritance which is carried from parent by a
gamete. Gene occurs in a chromosome and controls the expression of a
character in cooperation with other genes and environment. Chemically
gene is a segment of DNA called cistron that has a particular function. One
cistron controls the production of one polypeptide.
4. Alleles: The term alleles were given by Bateson in 1905. Alleles are forms of
the same gene which are found at the same place or locus on homologous
chromosome and control particular trait of a character e.g. tallness and
dwarfness.
5. Homozygous individual: An individual having two identical alleles
(isoalleles) of a trait is called homozygous individual. It is also called pure
line. The term pure line was coined by Johannsen in 1910. For example RR
is pure red and rr is pure white trait.
6. Dominant and recessive factor or allele: Out of two contrasting alleles of
heterozygous or hybrid individual, only one allele can express the effect
itself is called dominant factor or allele.
The other allele or factor present in hybrid is unable to express its effect
but it can express its effect in homozygous condition in next generation is
called recessive factor or allele. Dominant allele is denoted by capital letter
but recessive allele is denoted by small letter. For example, in hybrid red
(Rr), R allele expresses red trait but r allele cannot express white trait. So, R
allele and r allele are dominant allele and recessive alleles respectively.
7. Phenotype: It is the physical or observable expression of an individual. It
can be expressed in word like tallness, dwarfness, etc. Phenotype can be
modified by environmental changes.
8. Genotype: It is the gene complement or genetic constitution of an
individual. It is expressed in symbol like TT for pure tall, tt for pure dwarf ,Tt
for hybrid tall, etc. A genotype cannot be observed directly.
, 9. Hybrid: The heterozygous organism produced after crossing two genetically
different individuals is called hybrid e.g. hybrid red(Rr), hybrid tall(Tt).
10.Monohybrid Cross: It is a cross between two organisms of a species which
is made to study the inheritance of single pairs of factors or alleles of a
character.
11.Monohybrid Ratio: It is a ratio which is obtained in F₂ generation when a
monohybrid cross is made and the offspring of F1 generation are self-bred.
Monohybrid ratio is usually 3:1 (phenotypic ratio) or 1:2:1 (genotypic ratio)
in which 25% of the individuals carry the recessive trait, 25% pure dominant
and 50% have hybrid dominant trait.
12.Dihybrid Cross: It is a cross between two organisms of a species which is
made to study the inheritance of two pairs of factors or alleles of two
genes.
13.Dihybrid Ratio: It is a ratio which is obtained in the F₂ generation when a
dihybrid cross is made and the offspring of F1 generation are self-bred.
Dihybrid ratio is 9:3:3:1 (phenotypic ratio).
14.Emasculation: The removal of stamens from bisexual flowers in order to
avoid self-pollination in these flowers during hybridization.
15.Back Cross: The cross of an F 1 hybrid with one of the two parents is called
backcross.
16.Test cross: A cross between an individual of unknown genotype and
recessive parent is called test cross.
17.Reciprocal Cross: A second cross of the same genotypes in which the sexes
of the parental generation are reversed, is called reciprocal cross.
18.Punnett square: It is a graphical representation to calculate the probability
of all possible genotypes of offspring in a genetic cross. The possible
gametes are written on two sides, the top row (horizontal row) and left
column (vertical column). Usually male gametes are writ- ten in top row
and female gametes in left column
