In a cross homozygous parent (tall plant with purple flower) is crossed with a hetrozygous (tall with white flower). Find out the progenies and write their phenotypic and genotypic ratios.
Answers
By the 1800s, people generally understood that offspring inherited traits from their parents. Initially, however, it was thought that the traits of both parents blended together in their offspring. Called the “blending hypothesis”, this theory explained inheritance like two different liquids mixing together. Gregor Mendel’s careful work with thousands of pea plants in the 1860 proved the blending hypothesis wrong and explained how inheritance really happens.
Mendel's Experiments
Mendel studied several different traits of a pea plant. For example, some pea plants have purple flowers and others have white flowers. Pea plants can either self-fertilize or cross-fertilize. Crossing two plants is called hybridization. To start, however, Mendel needed plants that were true-breeding. This means that after generations of self-breeding, the pea plant expressed only one version of the trait. After many generations a purple flowered plant only produced plant never produced a white flowered plant.
Mendel then crossed a true-breeding purple flower plant and a true-breeding white flower plant. This is called a monohybrid experiment. A test cross is performed between two plants that breed true for one trait, and the resulting trait for each offspring plant is determined
A chart of a monohybrid experiment between a purple and white flower
The first, true-breeding generation, is called the parent, P generation. The first generation of offspring, the first filial generation, is the F1 generation. Mendel found that these plants all had purple flowers. The F1 generation was crossed with itself. The next generation, F2 generation, had a 3:1 ration of purple to white flowers.
If the blending hypothesis was correct, the F1 generation should have all had light purple flowers. Instead, all the F1 plants had dark purple flowers. How did Mendel explain the complete loss of the white flower characteristic in the first generation and its reappearance in the second generation?
The Law of Segregation
Mendel explained what he saw using the law of segregation. Each gene can have different alleles. For example, one gene determines flower color in pea plants. Different versions of that same gene are alleles. The two different alleles here are purple flowers and white flowers. Each plant has two copies of each gene: one copy from each parent plant. The F1 generation inherits a purple allele and a white allele. Even though the plant has one copy of each, we only see the trait from the purple flower gene. When a plant has one copy of each gene, the gene we see is the dominant allele. We indicate this with a capital letter for the allele, “P”. The gene that is masked by the dominant allele is recessive. Here the white allele is recessive, and we represent this using a lower case letter for the gene “p”.
The F1 plants all have one purple allele (P) and one white allele (p). The genotype is the alleles for each gene in the plant. Here the genotype is Pp. A plant with two of the same alleles is homozygous. A plant with two different alleles is heterozygous. The characteristic that we see with our eyes is the phenotype. For Pp, the phenotype is purple flowers.
This inheritance model is explained using a Punnett square.
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