Genotype vs Phenotype Expression — NEET Biology
📚 Practice ConceptIf we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the Tall trait and t for the 'dwarf', and T and t are alleles of each other. Hence, in plants the pair of alleles for height would be TT, Tt or tt. Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous. TT and tt, respectively. TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype. What then would be the phenotype of a plant that had a genotype Tt?
Which of the following statements are NOT correct about a heterozygous genotype (e.g., Tt) in the context of Mendelian genetics? S1: A genotype such as Tt, controlling a single character, is accurately referred to as a monohybrid. S2: When a heterozygous Tt plant is crossed with a homozygous dominant TT plant, it yields a 3:1 phenotypic ratio (tall:dwarf). S3: According to the Law of Segregation, a heterozygous individual produces two distinct types of gametes in equal proportions. S4: Self-pollination of a heterozygous Tt plant will invariably produce uniform tall plants in the F2 generation. S5: The phenotype of a heterozygous genotype is typically identical to that of the homozygous dominant genotype in cases of complete dominance.
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Correct answer: B — S2 and S4 are not correct
This question tests the understanding of a heterozygous genotype and its behavior in Mendelian crosses. S1: A heterozygous genotype like Tt, where genes control one character, is indeed called a monohybrid. So, S1 is correct. S2: When a heterozygous Tt plant is self-pollinated (Tt × Tt), the F2 generation yields a phenotypic ratio of 3 tall : 1 dwarf (3:1). However, when a heterozygous Tt plant is crossed with a homozygous dominant TT plant (Tt x TT), the phenotypic ratio will be all tall (TT, Tt), not 3:1 tall:dwarf. The statement is not universally true, making it NOT correct. S3: A heterozygous individual (Tt) produces two types of gametes (T and t) in equal proportions (50% T and 50% t), according to the Law of Segregation. So, S3 is correct. S4: If a heterozygous plant (Tt) is self-pollinated, it produces both tall (TT, Tt) and dwarf (tt) offspring in the F2 generation. It does not produce uniform tall plants; 75% will be tall and 25% dwarf. Therefore, S4 is NOT correct. S5: In cases of complete dominance, the phenotype of a heterozygous genotype (Tt) is identical to that of the homozygous dominant (TT). For instance, both Tt and TT pea plants are tall. So, S5 is correct. Therefore, S2 and S4 are NOT correct.
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