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ZOOLOGY GENETICS Topic: Back Cross & Test Cross ZOOLOGY GENETICS Topic: Back Cross & Test Cross Introduction The characteristics of the offspring of Mendel’s crosses can be predicted from the genotype of the parents through knowledge of dominant and recessive genes i.e., TT for tall and tt for dwarf traits. Mendel wanted to know whether the genotype of the individual be determined just from its phenotype. For recessive phenotype, it is possible, because it has only one genotype for example, tt for dwarf trait. But, the phenotypically dominant individuals show two types of genotypes – homozygous dominant (TT) and heterozygous dominant (Tt). In order to determine the genotypes of such phenotypes, Mendel employed two types of crosses viz., back cross and test cross. Without the knowledge of the genotype, just by crossing experiment, one can determine the genotype of the given phenotype with the help of these crosses. BACK CROSS Backcross is a cross of a hybrid (F1) with any one of its parents i.e. homozygous dominant or homozygous recessive parent. It is used in horticulture, animal breeding and in production of gene knockout organisms. These are performed in Monohybrid and Dihybrid crosses. 1. Monohybrid Back Cross:- In a monohybrid cross of homozygous tall (TT) and homozygous dwarf (tt) pea plants, the F1 progeny are heterozygous tall (Tt). When the F1 heterozygote (hybrid) is crossed either with its dominant parent or with its recessive parent, it is known as back cross and the results obtained from such a cross is as follows:- a. Back Cross with Homozygous Dominant Parent:- The F1 heterozygote (Tt) gives rise to two kinds of gametes viz., gametes with dominant factor for tall character (T) and gametes with recessive factor for dwarf character (t). Whereas the parent with homozygous dominant genotype (TT) produces all gametes with only dominant factor for tall character (T). In a random mating of the gametes, the following phenotypes and genotypes are obtained: i. Homozygous Tall plants with the genotype TT ii. Heterozygous Tall plants with the genotype Tt Thus, all progeny will be dominant in phenotype, when F1 hybrid is crossed to a homozygous dominant parent. b. Back Cross with Homozygous Recessive Parent:- The F1 heterozygote (Tt) gives rise to two kinds of gametes viz., gametes with dominant factor for tall character (T) and gametes with recessive factor for dwarf character (t). Whereas the parent with homozygous recessive genotype (tt) produces all gametes with only recessive factor for dwarf character (t). In a random mating of the gametes, the following phenotypes and genotypes are obtained: iii. Heterozygous Tall plants with the genotype Tt – 50% iv. Homozygous dwarf plants with the genotype tt – 50% In this cross, 50% of progeny will be dominant and 50% will be recessive when F1 hybrid is crossed to a homozygous recessive parent. Thus, dominant (tall) and recessive (dwarf) phenotypes appear in 1:1 ratio. 2. Dihybrid Back Cross:- In a diybrid cross of homozygous round and yellow-seeded (RRYY) and homozygous wrinkled and green- seeded (rryy) pea plants, the F1 progeny are heterozygous round and yellow seeded (RrYy). When the F1 heterozygote (dihybrid) is crossed either with its dominant parent or with its recessive parent it is known as a Dihybrid back cross and the following results are obtained:- a. Back Cross with Homozygous Dominant Dihybrid Parent: The F1 heterozygote (RrYy) gives rise to four kinds of gametes:- a. RY - gametes with dominant factors for seed shape and seed coat colour b. Ry – gametes with dominant factor for seed shape and recessive factor for seed coat colour c. rY - gametes with recessive factor for seed shape and dominant factor for seed coat colour d. ry - gametes with recessive factors for seed shape and seed coat colour Whereas the parent with homozygous dominant genotype (RRYY) produces all gametes of a single type i.e., dominant factors for both seed shape and seed coat colour (RY). In a random mating of the above said gametes, all the progeny show round and yellow seeds with the following genotypes: a. RRYY : Homozygous Dominant (Round and Yellow) b. RRYy : Heterozygous (Round and Yellow) c. RrYY : Heterozygous (Round and Yellow) d. RrYy : Heterozygous (Round and Yellow) Conclusion: All the progeny (100%) are plants that produce dominant phenotype i.e. round and yellow seeds. b. Back Cross with Homozygous Recessive Dihybrid Parent: The F1 heterozygote (RrYy) gives rise to four kinds of gametes:- a. RY - gametes with dominant factors for seed shape and seed coat colour b. Ry – gametes with dominant factor for seed shape and recessive factor for seed coat colour c. rY - gametes with recessive factor for seed shape and dominant factor for seed coat colour d. ry - gametes with recessive factors for seed shape and seed coat colour Whereas the parent with homozygous recessive genotype (rryy) produces all gametes with only recessive factors for seed shape and seed coat colour (ry). In a random mating of the gametes, the progeny show seeds with the following genotypes: a. RrYy : Heterozygous Dominant (Round and Yellow) b. Rryy : Heterozygous (Round and Green) c. rrYy : Heterozygous (Wrinkled and Yellow) d. rryy : Homozygous Recessive (Wrinkled and Green) Conclusion: In this cross, all four phenotypes appear in 1:1:1:1: ratio. Applications of Back Cross:- In horticulture, it is employed to retain all desirable characters of a popular adapted variety and replace undesirable allele at particular locus. It is used for the transfer of disease resistance and incorporation of quality traits into a variety Backcrossing is generally employed in animals to transfer a desirable trait of an animal from inferior genetic background to an animal of superior genetic background. ************ TEST CROSS Definition: Test cross is a cross between an organism with a dominant phenotype (Eg: Tall) whose genotype is unknown and an organism that is homozygous recessive (Eg: tt, Dwarf) for that phenotype. Reason for performing Test Cross: • This cross is used to determine whether the individual in question is heterozygous (Tt) or homozygous (TT) for a certain allele. • In the example given below, there are three plants- 2 tall pea plants and 1 dwarf pea plant. The genotype of a dwarf pea plant is tt because it is recessive whereas the genotype of a tall pea plant can be either homozygous (TT) or heterozygous dominant (Tt) genotype. In order to determine the genotype of such organism with dominant phenotype, test cross is performed. 1. Monohybrid Test Cross: A tall pea plant can be either homozygous dominant tall (TT) or heterozygous dominant tall (Tt). To determine its genotype, both tall pea plants (homozygous and heterozygous) are crossed with a recessive dwarf pea plant (tt). a. In the first cross, when one tall pea plant is crossed with a dwarf pea plant, all the progeny are found to be tall. b. In the second cross, when the second tall pea plant is crossed with a dwarf pea plant, both tall and dwarf pea plants appeared in 1:1 ratio. Explanation: In the first cross, the pea plant is of homozygous dominant genotype (TT). It produces only one type of gamete with dominant factor (T) for tall character and recessive dwarf plant also produces one type of gamete with recessive factor (t) for dwarf character. In a random mating of the gametes, when gametes with tall factor (T) fertilize the gametes with dwarf factor (t), resulting progeny will be phenotypically tall with the genotype, Tt. In the second cross, the pea plant is of heterozygous dominant genotype (Tt). It produces 50% of gametes with dominant factor for tall character (T) and 50% of gametes with recessive factor for dwarf character (t) whereas recessive dwarf plant produces all gametes having recessive factor (t) for dwarf character. In a random mating of gametes, the plants with the genotype Tt will be tall and the plants with the genotype tt will be dwarf. Thus, both dominant and recessive phenotypes appear in 1:1 ratio. Conclusion: The test cross ratio for a monohybrid cross involving heterozygous dominant individual is 1:1. Thus, it can be said that when both dominant and recessive individuals appear in 1:1 ratio in a test cross, the individual in question is always a heterozygote. 2. Dihybrid Test Cross: A pea plant with dominant traits for seed shape (Round) and seed coat colour (Yellow) can be either homozygous round and yellow (RRYY) or heterozygous round and yellow (RrYy). To determine its genotype, both pea plants with dominant traits (homozygous and heterozygous) are crossed with a double recessive pea plant (rryy). a. In the first cross, when one round and yellow-seeded pea plant is crossed with a wrinkled and green-seeded pea plant, all the progeny are found to be plants that produce round and yellow seeds. b. In the second cross, when the second round and yellow-seeded pea plant is crossed with a wrinkled and green-seeded pea plant, all the four phenotypes appeared in 1:1:1:1 ratio. Explanation: In the first cross, the pea plant is of homozygous dominant genotype (Round and Yellow, RRYY) which produced gametes of only one type, having dominant factors for seed shape and seed coat colour (RY) and similarly, double recessive dwarf plant (Wrinkled and Green, rryy) produces gametes of only one type, having recessive factors for the same traits (ry). In a random mating of the gametes, when gametes with dominant factors (RY) fertilize the gametes with recessive factors (ry), resulting progeny will be phenotypically round and yellow-seeded plants with the genotype (RrYy). In the second cross, the pea plant is of heterozygous dominant genotype (RrYy). It produces four different types of gametes (RY, Ry, rY and ry) whereas double recessive dwarf plant produces only one kind of gametes having recessive factors for the same traits (ry).
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