Question

Based on Mendel's experiments, what is the expected phenotype ratio in the \(F_{2}\) generation from a single-factor cross? a. 1: 2: 1 b. 2: 1 d. 9: 3: 3: 1 e. 4: 1

Short answer

The expected phenotypic ratio in the F2 generation from a single-factor cross, per Mendel's first law, is 3:1.

Step-by-step solution

Recognize Mendel's Experiment

This question refers to Mendel's single-factor cross experiment. In these experiments, Mendel focused on one character (e.g., flower color) and crossed two varieties that had two different versions of that character (e.g., purple flowers and white flowers).

Recall Mendel's First Law: The Law of Segregation

Mendel's law of segregation (the first law) states that during the formation of reproductive cells (gametes), pairs of hereditary traits (alleles) for a specific character separate so that offspring receive one factor from each parent.

Apply Mendel's Law to the F2 Generation

The F2 generation is produced by crossing individuals from the F1 generation. For a single-factor cross (where only one trait is being looked at and the individuals being crossed are heterozygous for that trait), the Mendelian phenotypic ratio in the F2 generation is 3:1.

Key concepts

Mendel's First Law

Understanding Mendel's First Law is crucial when studying genetics, as it outlines the basic principles of heredity. This law, also known as the Law of Segregation, relates to how individual alleles for a trait are transmitted from parents to offspring.

The law stipulates that an organism with two alleles for a trait, one from each parent, will segregate these alleles during gamete formation. Each sex cell, or gamete, will receive only one of the two alleles. During fertilization, offspring receive one allele for each trait from each parent, restoring the pair.

This concept explains why, despite organisms having two alleles for each trait, they only pass on one allele to their progeny. It underscores the random nature of allele segregation, which is a crucial aspect of genetic variation and inheritance.

Law of Segregation

The Law of Segregation is a cornerstone of Mendelian genetics and provides a framework for understanding genetic inheritance. The separation of alleles occurs during a process known as meiosis, which is how sex cells are produced. To illustrate this, consider a plant that has one allele for purple flowers and another for white flowers. Each gamete made by the plant will contain only one of these alleles.

When gametes unite during fertilization, the offspring, known as the F1 generation, acquires one allele from each parent. If both alleles are different, the offspring is heterozygous, and the dominant trait will usually express itself. The law is essential for predicting the distribution of traits and allows us to calculate the likelihood of certain phenotypes appearing in the next generation.

F2 Generation

The F2 generation is a key term in the study of Mendelian genetics, referring to the second filial generation of offspring. This generation arises when members of the F1 generation are crossed or self-pollinate.

Since the F1 plants are typically heterozygous, possessing one dominant and one recessive allele for a particular trait, the F2 generation displays new combinations of these alleles. The Mendelian phenotypic ratio observed in the F2 generation, typically 3:1 for a single-factor cross involving dominant and recessive alleles, is attributable to this segregation and subsequent assortment of alleles. This ratio represents three offspring showing the dominant phenotype for every one offspring with the recessive phenotype.

Single-factor Cross

A single-factor cross is a breeding experiment that focuses on the inheritance of one particular trait. In Mendelian genetics, this cross involves parents that are homozygous for a trait but have different alleles, resulting in heterozygous F1 offspring.

For instance, if one parent plant has purple flowers and the other has white flowers, a single-factor cross would examine the inheritance of flower color. The F1 generation would likely all have purple flowers, as purple is often the dominant allele over white. When these F1 plants are crossed to create the F2 generation, we can expect a 3:1 phenotypic ratio of purple to white flowering plants, assuming purple is dominant. This simple cross lays the groundwork for understanding more complex genetic interactions.