Question

A true-breeding purple-leafed plant isolated from one side of El Yunque, the rain forest in Puerto Rico, was crossed to a truebreeding white variety found on the other side. The \(\mathrm{F}_{1}\) offspring were all purple. A large number of \(\mathrm{F}_{1} \times \mathrm{F}_{1}\) crosses produced the following results: \\[ \text { purple: } 4219 \quad \text { white: } 5781 \quad(\text { Total }=10,000) \\] Propose an explanation for the inheritance of leaf color. As a geneticist, how might you go about testing your hypothesis? Describe the genetic experiments that you would conduct.

Short answer

Answer: The proposed explanation for the inheritance of leaf color is that the purple color is dominant over the white color, following a simple Mendelian genetics model. A test cross experiment can help confirm this hypothesis by crossing the F1 offspring (heterozygous purple plants) with true-breeding white-leafed plants. If the hypothesis is correct, the resulting offspring should show a 1:1 ratio of purple to white leaf colors.

Step-by-step solution

Based on the data provided, it is likely that the purple leaf color is dominant over the white leaf color. Since the F1 offspring were all purple, it suggests that the purple allele masks the effect of the white allele. In other words, when a plant has both alleles for purple and white color, the plant will express the purple color. #Step 2: Formulate a hypothesis for the inheritance pattern#

We can hypothesize that the inheritance pattern of leaf color follows a simple Mendelian genetics model. The total number of F1 x F1 offspring is 10,000, with purple being 4219 and white being 5781. The observed ratio of purple to white is close to a typical 3:1 ratio, which is supported by a single gene with two alleles, where one is dominant (purple) over the other (white). #Step 3: Design a test cross experiment#

To test the hypothesis, we can perform a test cross experiment. This involves crossing the F1 offspring (heterozygous purple plants) with a true-breeding white-leafed plant, which has two recessive alleles for the white color. The expected outcome is that half of the offspring will be purple, and the other half will be white if the hypothesis is correct. #Step 4: Collect data and analyze the results#

After conducting the test cross experiment, collect data on the resulting offspring's leaf colors. Count the number of purple and white offspring and calculate the ratio between them. #Step 5: Compare the observed data to the expected outcome#

Compare the observed ratio of purple to white offspring from the test cross with the expected ratio of 1:1. If our hypothesis is correct, the observed data should be close to the expected outcome. If the data does not support our hypothesis, consider alternative explanations for the inheritance pattern, such as incomplete dominance or multiple genes affecting the trait. #Step 6: Perform additional genetic experiments#

Further genetic experiments can be performed to support our findings or explore alternative explanations. For example, we could perform crosses between true-breeding purple plants and true-breeding white plants and analyze the resulting offspring's leaf color ratios. Additionally, we could investigate the role of environmental factors in influencing leaf color expression.

Key concepts

Mendelian Genetics

Understanding Mendelian genetics is fundamental to comprehending how traits are transferred from parents to offspring. Gregor Mendel, a 19th-century monk, laid the groundwork for this field by experimenting with pea plants. He observed that certain traits seemed to vanish in one generation, only to reappear in the next, leading to the concept of dominant and recessive alleles. Mendelian genetics revolves around three primary laws: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance.

Mendel's experiments showed that traits are controlled by discrete 'factors', known nowadays as genes, which come in pairs. During reproduction, these pairs separate, or segregate, ensuring that each parent contributes one allele for each trait to their offspring. This is the essence of the Law of Segregation. The Law of Independent Assortment states that the segregation of alleles for one trait does not affect the segregation of alleles for another trait. Lastly, the Law of Dominance explains how dominant alleles can mask the expression of recessive alleles in heterozygotes.

Mendel's principles explain the genetic inheritance observed in the purple and white leafed plants. The consistent purple color in the first generation (F1) suggests the purple allele's dominance over the white allele, perfectly fitting into Mendelian patterns of inheritance.

Test Cross Experiment

A test cross is a method used to determine the genotype of an organism expressing a dominant trait, as its phenotype doesn’t reveal whether it's homozygous dominant or heterozygous. The organism in question is bred with another organism that has a homozygous recessive genotype for the trait being considered.

The essence of a test cross lies in its ability to reveal the unknown genotype through the offspring phenotypes it produces. If any of the offspring display the recessive phenotype, then the test organism must be heterozygous. Conversely, if all offspring show the dominant phenotype, the test organism is likely homozygous dominant. This is precisely the plan for the F1 purple-leafed plants from our problem. By crossing them with the true-breeding white plants, geneticists can infer the unknown genotypes of the F1 individuals. It’s a beautifully simple yet powerful experiment, crucial for examining Mendelian inheritance patterns.

Dominant and Recessive Alleles

Alleles are different versions of the same gene. In genetic terms, dominant alleles are those that express their effect even when they are present in just one copy (heterozygous condition), while recessive alleles require two copies (homozygous condition) to manifest a particular trait.

For the leaf color of plants in our scenario, it was observed that the purple allele is dominant over the white allele—an individual plant only needs one purple allele to express the purple leaf color. This is in contrast to the white allele, which is recessive and only expressed phenotypically when a plant is homozygous for the white allele. The heterozygous condition will still result in a purple-leafed plant, as the dominant allele 'overpowers' the recessive one. This distinction between allele dominance is critical in predicting the genotypic and phenotypic outcomes of genetic crosses, such as the hypothetical crosses discussed in the problem.

Phenotypic Ratios

The phenotypic ratio is a crucial element in genetics, providing a numeric expression that summarizes the visible traits, or phenotypes, of offspring from a particular cross. This ratio allows geneticists to predict the probability of traits being passed on to the next generation.

For example, a phenotypic ratio of 3:1 in a monohybrid cross of two heterozygous individuals (both carrying one dominant and one recessive allele) suggests that 75% of the offspring will exhibit the dominant phenotype and 25% will exhibit the recessive phenotype. In the problem we’re considering, the ratio of purple to white offspring from the F1 crosses was close to 3:1, fitting the expected Mendelian single-factor cross outcome but with slight deviations that could be attributed to chance or experimental error. Understanding phenotypic ratios is integral for interpreting genetic data and can provide insights into inheritance patterns within a breeding population.