Monohybrid cross: A genetic cross between parents that differ in single pair of contrasting characters, controlled by one particular gene is called as Monohybrid cross. One parent is having two dominant alleles and the other parent is having twp recessive alleles. All the F1 generation progeny have one dominant and one recessive allele for that gene i.e. they are hybrid at that one locus. Crossing between the F1 generation progeny yields a characteristic 3:1 ratio in F2 generation. The genotypic ratio in monohybrid cross is
Dihybrid cross: A genetic cross between parents that differ in two pairs of contrasting characters, controlled by genes at different loci is called as dihybrid cross. Mendel executed a dihybrid cross using the characteristics like seed color and texture of pea plant. The parental plants had either smooth yellow seeds (SSYY)-dominant characters or wrinkled green seeds (ssyy)-recessive characters. All the offsprings had smooth yellow seeds being heterozygous (SsYy) for the two alleles. Crossing between these F1 offsprings produced an F2 generation of plants with smooth yellow, smooth green, wrinkled yellow and wrinkled green seeds in the ration 9:3:3:1. These results formed the basis of Law of Independent Assortment.
Monohybrid and dihybrid crosses proved three principles of genetics,
Each of these crosses yielded approximately 3:1 ratio in the second filial generation. The three basic principles of inheritance were established by Mendel monohybrid crosses.
Mendel’s Law of segregation is also called as law of purity. This law states that the hybrids or heterozygotes of F1 generation have two contrasting characters or alleles of dominant and recessive nature. These alleles remain together for long time but do not mix with each other and segregate only during the time of gametogenesis. This way each gamete receives only one allele of the dominant character or recessive character. Even for the experimentation of this Law of Segregation Mendel used Monohybrid crosses.
The Law of segregation has widespread applications and it is found to occur both in plants and animals. Mendel observed Segregation phenomenon in Pea plant.
For the thorough understanding let us consider a monohybrid cross between long-winged homozygous Drosophila with Vestigial-winged Drosophila. The F1 Heterozygotes are all long-winged. When the F1 hybrids are allowed to self fertilize, they produce both long winged and vestigial winged Drosophila in F2 generation in 3:1 ratio. The reappearance of vestigial wings in F2 generation indicated the process of segregation.
The mechanism of segregation in the above mentioned monohybrid cross between long-winged and vestigial winged Drosophila can be best understood by assuming that the homozygous long winged Drosophila has the alleles V+V+ and Vestigial winged Drosophila has the alleles vv. The Drosophila with V+V+ alleles produces gametes with single allele V+ and the Drosophila with vv alleles produces gametes with single allele v. The gametes of both united to form a hybrid or heterozygote having the alleles V+ v both for long and vestigial winged. But due to incomplete dominance the allele V+ for long winged partially expresses itself in F1 hybrids, while the allele v for vestigial wing remains recessive.
Both the alleles V+ and v remain together for long time but they do not affect each other. Neither they mix not they contaminate each other. Because a gamete can contain only one chromosome of a homologous pair, each gamete can carry single allele V+ or v. At the time of gametogenesis two types of gametes are produced by F1 hybrids in equal amounts. Half of the gametes carry the allele V+ and other half carry the allele v. these gametes during the process of fertilization can unite in three possible ways like V+ V+, V+ v and v v, to produce three types of individuals in F2 generation. Thus in F2 75% individuals have long wings and 25% have vestigial wings. The appearance of vestigial wings in F2 generation indicates that in the hybrid the allele (v) for vestigial wings remains along with the allele (V+) for long wings but they do not mix with each other and they segregate during the gametogenesis.
The genotypic ratio in this cross is 1:2:1 and the phenotypic ratio for this cross is 3:1
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