Question

Suppose pure line lima bean plants having green pods were crossed with pure line plants having yellow pods. If all the \(F_{1}\) plants had green pods and were allowed to interbreed, 580 \(\mathrm{F}_{2}\) plants, 435 with green pods and 145 with yellow pods would be obtained. Which characteristic is dominant and which is recessive? Of the \(\mathrm{F}_{2}\) plants, how many are homozygous recessive, homozygous dominant and heterozygous? Using \(\mathrm{G}\) to represent the dominant gene and \(g\) to represent the recessive gene, write out a plan showing the segregation of genes from the parents to the \(\mathrm{F}_{2}\) plants.

Short answer

The green pod characteristic (G) is dominant, and the yellow pod characteristic (g) is recessive. In the \(F_2\) generation, there are 145 homozygous recessive (gg) plants, 145 homozygous dominant (GG) plants, and 290 heterozygous (Gg) plants. The segregation of genes from the parent (P) generation to the \(F_2\) generation is as follows: P generation: GG (green pods) x gg (yellow pods) F1 generation: All Gg (green pods, heterozygous) F1 generation cross: Gg x Gg F2 generation: - 145 GG (green pods, homozygous dominant) - 290 Gg (green pods, heterozygous) - 145 gg (yellow pods, homozygous recessive)

Step-by-step solution

Identify the dominant and recessive traits

Since all the \(F_1\) plants have green pods, we can conclude that the green pod characteristic is dominant (represented by G) and the yellow pod characteristic is recessive (represented by g).

Analyze the \(F_2\) generation

In the \(F_2\) generation, there are 435 plants with green pods and 145 with yellow pods. The plants with yellow pods must be homozygous recessive (gg) since they exhibit the recessive trait.

Determine the homozygous dominant and heterozygous plants

The 435 green pod plants consist of homozygous dominant (GG) and heterozygous (Gg) plants. Since the yellow pod plants (gg) make up 145 out of 580 \(F_2\) plants, the expected ratio of different genotypes among these plants is 1:2:1 (GG:Gg:gg). Therefore, we can calculate the number of homozygous dominant (GG) and heterozygous (Gg) plants as follows: GG (homozygous dominant): (1/3) * 435 = 145 Gg (heterozygous): (2/3) * 435 = 290

Write out the segregation plan

The segregation of genes from the parent (P) generation to the \(F_2\) generation can be described as follows: P generation: GG (green pods) x gg (yellow pods) F1 generation: All Gg (green pods, heterozygous) F1 generation cross: Gg x Gg F2 generation: - 145 GG (green pods, homozygous dominant) - 290 Gg (green pods, heterozygous) - 145 gg (yellow pods, homozygous recessive) In conclusion, the green pod characteristic (G) is dominant, while the yellow pod characteristic (g) is recessive. Among the \(F_2\) plants, there are 145 homozygous recessive (gg) plants, 145 homozygous dominant (GG) plants, and 290 heterozygous (Gg) plants.

Key concepts

Dominant and Recessive Traits

In Mendelian genetics, traits are determined by pairs of genes inherited from each parent. These traits are categorized into dominant and recessive types. A dominant trait is expressed in the offspring even if only one parent contributes the gene. In our lima bean plant example, the green pod trait is dominant, represented by the letter 'G'. It appears in the offspring (F1 generation), even when a plant with yellow pods is part of the cross.

Recessive traits, on the other hand, are only expressed when both gene copies are recessive. Here, the yellow pod color is recessive, represented by 'g'. To see yellow pods in the offspring, both parents must contribute a recessive gene, resulting in a homozygous recessive combination (gg) in the F2 generation where those traits become visible again. Identifying these traits helps geneticists predict future generations' characteristics based on genetic cross outcomes.

Punnett Square Analysis

A Punnett square is a tool genetics researchers use to predict the outcomes of genetic crosses. This tool helps visualize how genes from parents might combine in their offspring. For our lima bean plant cross, parents have genotypes GG (green pods) and gg (yellow pods).



In the first generation (F1), each offspring receives one allele from each parent, resulting in all offspring being heterozygous (Gg). This means they all exhibit the green pod trait, as 'G' is the dominant allele. To predict the F2 generation, we cross two heterozygous individuals (Gg x Gg). A Punnett square for this cross shows a 3:1 phenotype ratio (3 green: 1 yellow). Consequently, out of 4 potential offspring combinations: 1 is homozygous dominant (GG), 2 are heterozygous (Gg), and 1 is homozygous recessive (gg). Utilizing Punnett squares provides a visual representation to better understand inheritance patterns.

Genotype and Phenotype Ratios

Genotype refers to the genetic makeup of an organism. It indicates the specific allele combination an organism carries. In our lima bean plants, three genotypes emerge in the F2 generation: GG, Gg, and gg.

Phenotype, on the other hand, is the observable trait or characteristic resulting from the genotype. For instance, plants with green pods exhibit the green pod phenotype regardless of whether they are GG or Gg genetically. Yet, the yellow phenotypic trait only appears in the gg genotype.

The genotype ratio in the F2 generation is 1:2:1. This means:

• 1 part GG (homozygous dominant) green pods
• 2 parts Gg (heterozygous) green pods
• 1 part gg (homozygous recessive) yellow pods

The phenotype ratio is 3:1, indicating that for every 4 offspring from the F1 generation cross, 3 will display green pods and 1 will display yellow pods. By dissecting these ratios, students grasp how inheritance works, making complex genetic principles more digestible.