Question

In four o'clock plants, many flower colors are observed. In a cross involving two true-breeding strains, one crimson and the other white, all of the \(\mathrm{F}_{1}\) generation were rose color. In the \(\mathrm{F}_{2}\), four new phenotypes appeared along with the \(P_{1}\) and \(F_{1}\) parental colors. The following ratio was obtained: Propose an explanation for the inheritance of these flower colors.

Short answer

Answer: The inheritance pattern of flower color in four o'clock plants can be explained by incomplete dominance or codominance. This is evident by the appearance of rose color flowers in the F1 generation when crossing true-breeding crimson (CC) and white (WW) strains. In the F2 generation, the observed color ratios are 1 crimson (CC): 2 rose (CW): 1 white (WW), which aligns with the calculated ratio from the Punnett square. This demonstrates that the flower color inheritance in four o'clock plants follows a simple Mendelian inheritance pattern with incomplete dominance or codominance of the alleles.

Step-by-step solution

Observe the inheritance pattern

Since the \(\mathrm{F}_{1}\) generation resulted in all rose-colored flowers, we can infer that the inheritance of flower color in these plants is likely controlled by an intermediate inheritance, such as incomplete dominance or codominance.

Determine the genotypes

Let the alleles for flower color be represented by the letters C and W. True-breeding crimson plants have the genotype CC, and true-breeding white plants have the genotype WW. In this case, the codominant or incompletely dominant alleles result in the rose color in the \(\mathrm{F}_{1}\) generation, represented by the genotype CW.

Calculate the \(\mathrm{F}_{2}\) generation genotypes

The \(\mathrm{F}_{2}\) generation results from crossing the \(\mathrm{F}_{1}\) generation plants (CW x CW). A Punnett square will help us visualize the inheritance patterns: C | W --|-- C | CC | CW W | CW | WW From the Punnett square, we can determine the genotypic ratios for the \(\mathrm{F}_{2}\) generation: 1 CC (crimson): 2 CW (rose): 1 WW (white), which match the parental colors observed; 1 crimson: 2 rose: 1 white.

Propose an explanation for the observed inheritance pattern

The inheritance of flower color in four o'clock plants can be explained by incomplete dominance or codominance. This is evident by the appearance of rose color flowers in the \(\mathrm{F}_{1}\) generation when crossing true-breeding crimson (CC) and white (WW) strains. In the \(\mathrm{F}_{2}\) generation, we observe 1 crimson (CC): 2 rose (CW): 1 white (WW) color pattern, which aligns with the calculated ratio from the Punnett square. This demonstrates that the flower color inheritance in four o'clock plants follows a simple Mendelian inheritance pattern with incomplete dominance or codominance of the alleles. Moreover, the four new phenotypes observed in the \(\mathrm{F}_{2}\) generation could imply the presence of additional genes that modify the color expression, or they could also be the result of environmental factors. Further genetic analysis would be needed to clarify the specific mechanisms behind the inheritance of flower colors in these plants.

Key concepts

Incomplete Dominance

Incomplete dominance is a fascinating pattern of genetic inheritance where the resulting offspring's phenotype is a blend of the parents' phenotypes. Unlike dominant-recessive inheritance, where one trait completely masks the other, incomplete dominance results in a new, intermediate phenotype.

Let's take our four o'clock plants as an example. When we cross a crimson flower (CC) with a white flower (WW), the offspring doesn't show either color exclusively but presents a rose color (CW). It's like mixing paints: crimson and white blend to create rose.

In incomplete dominance, each parent contributes one allele to the offsprings, and since neither allele is dominant, the resulting phenotype is a mix, a compromise, if you will, that falls squarely in the middle of the two parent phenotypes.

Codominance

Codominance presents a slightly different inheritance pattern, one where both alleles for a trait are expressed equally in the phenotype of the heterozygotes. In codominance, there's no blending; instead, each allele's traits are fully and simultaneously expressed.

Continuing with the example of the four o'clock plants, if the inheritance was codominant, you would expect the F1 generation to have flowers with patches of both crimson and white - a sort of floral checkerboard. Each allele maintains its identity and is observed clearly in the offspring. Codominance is therefore characterized by a phenotypic ratio where both parental traits are visible and distinguishable.

Punnett Square

The Punnett square is an indispensable tool for geneticists, helping predict the probability of offspring inheriting certain traits. It's a simple grid that allows us to map out all possible combinations of parental alleles, providing a visual representation of genetic crosses.

In the case of the four o'clock plants, the Punnett square clearly shows how the alleles from the crimson and white plants combine. We can then analyze the genotypic and phenotypic ratios expected in the F2 generation, as the square depicts the various allele pairings that lead to the next generation's flower colors.

This tool is immensely helpful for both understanding and teaching the principles of heredity, as it visually breaks down concepts that might otherwise be abstract or overwhelming when only described in text.

Mendelian Inheritance

Mendelian inheritance is named after Gregor Mendel, who is known as the father of genetics. His work laid the foundation for our understanding of how traits are passed from parents to offspring. Mendelian inheritance involves principles that describe the behavior of genetic factors that segregate and assort independently during gamete formation, and these factors can come together in different combinations during fertilization.

The four o'clock plant exercise hints at Mendelian patterns as we witness the predictable ratios in the F2 generation, which align closely with Mendel's laws of inheritance. This suggests that the traits in question are controlled by a single gene with two alleles and that these alleles have simple dominant-recessive or incomplete dominance relationships, a hallmark trait of Mendelian genetics.

Phenotypic Ratios

Phenotypic ratios refer to the proportion of offspring with particular phenotypes that arise in a genetic cross. These ratios are a result of the underlying genotypic ratios and are derived from Mendelian laws of segregation and independent assortment.

In our explanation of the four o'clock plants' flower color inheritance, we noted a phenotypic ratio in the F2 generation of 1 crimson: 2 rose: 1 white. This ratio reflects the mix of underlying genotypes and the respective phenotypes they produce. This ratio is incredibly useful as it provides a predictive tool for the appearance of specific traits in offspring and can offer insights into the modes of inheritance operating within a particular organism.