Question

The synthesis of flower pigments is known to be dependent on enzymatically controlled biosynthetic pathways. For the crosses shown here, postulate the role of mutant genes and their products in producing the observed phenotypes: (a) \(P_{1}:\) white strain \(A \times\) white strain \(B\) \(\mathrm{F}_{1}:\) all purple \(\mathrm{F}_{2}: 9 / 16\) purple: \(7 / 16\) white (b) \(P_{1}:\) white \(\times\) pink \(\mathrm{F}_{1}:\) all purple \(\mathrm{F}_{2}: 9 / 16\) purple: \(3 / 16\) pink: \(4 / 16\) white

Short answer

Answer: The key difference between the two crossbreeding scenarios is the parental genotypes being crossed. In scenario (a), we have white strain A (aaBB) and white strain B (AAbb) being crossed. In scenario (b), we have a white strain (aabb) being crossed with a pink strain (AaBb). This difference in parental genotypes leads to different phenotypic ratios in the F2 generation - 9:7 purple:white in scenario (a) and 9:3:4 purple:pink:white in scenario (b). The two different mutant genes contribute to color synthesis, resulting in varied combinations of genotypes and phenotypes in each scenario.

Step-by-step solution

Scenario (a) - White Strain A x White Strain B

First, let's analyze this scenario: \(P_{1}:\) white strain \(A \times\) white strain \(B\) \(\mathrm{F}_{1}:\) all purple \(\mathrm{F}_{2}: 9 / 16\) purple: \(7 / 16\) white. Step 1: Analyze the given ratios Given that the offspring in F2 are ratio 9:7 (purple:white), we can infer that there are two genes at play, with each having two different alleles. We will represent them by A (dominant) / a (recessive) and B (dominant) / b (recessive). Step 2: Infer parental genotypes Since both the white strain A and white strain B are completely white, we can infer that they are both homozygous recessive for one of the genes: aaBB (for strain A) and AAbb (for strain B). Step 3: Determine F1 genotypes All F1 offspring are purple. When strain A and strain B are crossbred, we get AaBb for all F1 offspring, as they inherit the dominant allele A from the white strain A and the dominant allele B from white strain B. Step 4: Conduct a dihybrid cross Now we need to find the ratio of phenotypes in F2. Perform a dihybrid cross using the genotypes from Step 3 (AaBb x AaBb) A Punnett square shows the following offspring genotypes ratio: 9 AABB, AABb, AaBB, AaBb (purple) 3 AAbb, aaBB (non-purple, but non-white as well) 4 aabb (white) This gives us the observed phenotypic ratio - 9:7 purple:white in F2. The presence of both dominant alleles, A and B, is required to produce the purple pigment in flowers. If both A and B are missing in the genotype, the flowers will be white. The presence of only one of the dominant alleles results in hidden intermediates that do not show in color (3/16).

Scenario (b) - White × Pink

Now let's examine the second scenario: \(P_{1}:\) white \(\times\) pink \(\mathrm{F}_{1}:\) all purple $\mathrm{F}_{2}: 9 / 16\( purple: \)3 / 16\( pink: \)4 / 16$ white. Step 1: Analyze the given ratios This time, the observed F2 phenotypes in the ratio 9:3:4 (purple:pink:white). We can still use two genes with the same alleles (A/a and B/b) as in scenario (a). Step 2: Infer parental genotypes In this case, we have a white strain with a genotype of aabb and a unknown genotype for the pink strain. Since we do not know the pink strain's genotype, we need to use the ratios to deduce it: - Pink strains must have the ability to produce purple offspring when crossed with white, so it must have at least one dominant allele (A or B). - Also, they can produce both white and pink offspring in F2, so they must be heterozygous in either A or B. Based on this, we can infer the genotype of the pink strain is AaBb. Step 3: Determine F1 genotypes All F1 offspring are purple. When white and pink strains are crossed, we get AaBb for all F1 offspring, as they inherit the dominant allele A and the dominant allele B from the pink strain. Step 4: Conduct a dihybrid cross Now perform a dihybrid cross using the genotypes from Step 3 (AaBb x AaBb) to get the F2 phenotype ratio. A Punnett square shows the following offspring genotype ratios: 9 AABB, AABb, AaBB, AaBb (purple) 3 AAbb, aaBB (pink) 4 aabb (white) These observed F2 phenotypic ratios are 9:3:4 (purple:pink:white). Two different mutant genes contribute to color synthesis: the presence of both dominant alleles, A and B, produces the purple phenotype, the presence of either A or B produces the pink phenotype, and the absence of both A and B results in the white phenotype.

Key concepts

Dihybrid Cross

Understanding the dihybrid cross is essential when studying the inheritance of two different traits. This genetic cross involves organisms that are heterozygous for two traits. In our exercise cases, the flower coloration depends on two genes, one from each parent.

A classic dihybrid cross follows Mendel's Second Law, also known as the Law of Independent Assortment, which states that the alleles for a pair of traits sort independently from one another during gamete formation. When we construct a Punnett square to illustrate a dihybrid cross, we are looking at the possible genetic combinations for the offspring.

The genotypes AaBb and AaBb in scenario (a) and scenario (b) are perfect examples. Each parent has one dominant and one recessive allele for both genes. By crossing these two heterozygous plants, we come up with a ratio that predicts the probability of their offspring exhibiting certain phenotypes. It’s the combination of alleles for the two genes that dictates flower pigmentation, leading to the observed 9:7 and 9:3:4 ratios respectively for scenarios (a) and (b).

Phenotypic Ratios

Phenotypic ratios are an interpretation of Mendelian inheritance that allow us to predict the outcome of genetic crosses. These ratios represent the relative number of offspring manifesting different traits. The textbook exercise provided examples with distinct phenotypic ratios: a 9:7 purple to white in the first, and a 9:3:4 purple to pink to white in the second.

The phenotypic ratio of 9:7 suggests that the two genes interact in a complementary way: both genes need to contribute a dominant allele to result in the purple phenotype. On the other hand, the 9:3:4 ratio in the second case indicates a different form of gene interaction, where one dominant allele of either gene is enough to produce the pink phenotype.

Understanding these ratios provides crucial insights into gene interactions and genetic dominance. Generally, a 9:3:3:1 ratio is expected from a dihybrid cross involving independent, non-interacting genes; deviations from this ratio hint at more complex inheritance patterns, such as those seen in these flower pigmentation phenotypes.

Biosynthetic Pathways

Biosynthetic pathways are sequences of chemical reactions within cells that lead to the synthesis of a complex molecule from simpler components. In the context of flower pigmentation, these pathways involve multiple steps that convert precursor molecules into pigments, such as anthocyanins, that determine flower color.

The presence or absence of certain enzymes, which are coded by specific genes, influences the progression through these biosynthetic pathways. For pigmentation to occur, all required enzymes must be present and functional. If an enzyme in the pathway is missing or non-functional due to a recessive allele, the pathway does not complete, and pigmentation does not occur, leading to the white phenotype seen in these exercises.

Ultimately, the interaction between genes and the enzymes they produce dictates whether a flower pigment biosynthesis pathway can be successfully executed. When we trace these pathways and know which genes are responsible for each step, we can predict the resulting flower color based on the genotypes of the parent strains.