Backcross :- Crosses between F1 offsprings with either of the two parents (hybrid) are known as back cross.
When F1 offsprings are crossed with the dominant parents all the F2 offsprings develop dominant character.
Plant and animal breeder use back cross to bring desirable symptoms to the homozygous stage.
Example :- A cross between heterozygous F1 hybrid with homozygous tall pea plant products all Tall offsprings. But only 50% of them are all homozygous and the other 50% are heterozygous tall.
Example :- A cross between tall heterozygous F1 hybrid with dwarf homozygous recessive (P1 ) produces tall and dwarf in equal proportion indicating that F1 hybrids are heterozygous.
Genotype :- The term genotype designates the genetic make up of an organism which determines the characters.
Example :- In F2 generation of Tall and Dwarf pea plants, the tall plants have two types of genetic composition. 1/3rd of them are homozygous TT and 2/3rd heterozygous Tt. TT and Tt is the genotype of the tall plants.
Phenotype :- Phenotype is the appeearance of an individual like form, sex, colour and behaviour etc.
Example :- In F2 generation of Tall and Dwarf pea plants, the tall plants have two types of genetic composition. 1/3rd of them are homozygous TT and 2/3rd heterozygous Tt. The tall and dwarf characters are their phenotypes.
Homozygous :- Every organism possesses two genes for every character, if in an organism the genes for a particular character are identical it is said to be pure or homozygous for that character.
Example :- Tall plants with TT or dwarf plants with tt are homozygous. They produce only one type of gametes.
Heterozygous :- The organism possesses contrasting genes of a pair. It receives two different alleles for the same character from its two parents.
Example :- Tall plants with Tt will be heterozygous. These produce two types of gamets T and t.
Allele :- Allele is a Greek word which means 'belonging to one another'.
Alleles are a pair of genes representing the two alternatives of the same character and located at the same locus in the homologous chromosomes.
Example :- In a gene pair Tt, T is present on one chromosome and t on the other homologue.
Dominant and Recessive :-
A heterozygous possesses two contrasting genes or alleles but only one of the two is able to expess itself, while the other remains hidden.
The gene which gains expression in F1 hybrid is known as dominant gene, while its allele is unable to expess itself in presence of the dominant gene is the recessive gene.
When two homologous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrid are dominant characters and those do not appear in F1 are recessive characters.
When F1 offsprings are crossed with the dominant parents all the F2 offsprings develop dominant character.
Plant and animal breeder use back cross to bring desirable symptoms to the homozygous stage.
Example :- A cross between heterozygous F1 hybrid with homozygous tall pea plant products all Tall offsprings. But only 50% of them are all homozygous and the other 50% are heterozygous tall.
Test cross :- The test cross is a cross between heterozygous F1 hybrid and the double recessive homozygous.
The test cross is used to determine whether the individuals exhibiting dominant character are homozygous or heterozygous.Example :- A cross between tall heterozygous F1 hybrid with dwarf homozygous recessive (P1 ) produces tall and dwarf in equal proportion indicating that F1 hybrids are heterozygous.
Genotype :- The term genotype designates the genetic make up of an organism which determines the characters.
Example :- In F2 generation of Tall and Dwarf pea plants, the tall plants have two types of genetic composition. 1/3rd of them are homozygous TT and 2/3rd heterozygous Tt. TT and Tt is the genotype of the tall plants.
Phenotype :- Phenotype is the appeearance of an individual like form, sex, colour and behaviour etc.
Example :- In F2 generation of Tall and Dwarf pea plants, the tall plants have two types of genetic composition. 1/3rd of them are homozygous TT and 2/3rd heterozygous Tt. The tall and dwarf characters are their phenotypes.
Homozygous :- Every organism possesses two genes for every character, if in an organism the genes for a particular character are identical it is said to be pure or homozygous for that character.
Example :- Tall plants with TT or dwarf plants with tt are homozygous. They produce only one type of gametes.
Heterozygous :- The organism possesses contrasting genes of a pair. It receives two different alleles for the same character from its two parents.
Example :- Tall plants with Tt will be heterozygous. These produce two types of gamets T and t.
Differences between Homozygous and Heterozygous
Eg. Tt
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Produce only one type of gametes.
Eg. T
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Eg. T and t
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Breed true
to the particular character.
Eg. All the offsprings possess the same genotype
and phenotype when such parents are
self-pollinated.
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Allele :- Allele is a Greek word which means 'belonging to one another'.
Alleles are a pair of genes representing the two alternatives of the same character and located at the same locus in the homologous chromosomes.
Example :- In a gene pair Tt, T is present on one chromosome and t on the other homologue.
Dominant and Recessive :-
A heterozygous possesses two contrasting genes or alleles but only one of the two is able to expess itself, while the other remains hidden.
The gene which gains expression in F1 hybrid is known as dominant gene, while its allele is unable to expess itself in presence of the dominant gene is the recessive gene.
or
When two homologous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrid are dominant characters and those do not appear in F1 are recessive characters.
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