Problem 11 - Biological Science

Question

In garden peas, yellow seeds ( \(Y\) ) are dominant to green seeds \((y),\) and inflated pods (I) are dominant to constricted pods (i). Suppose you have crossed \(Y Y I I\) oarents with vvii parent .Draw the \(\mathrm{F}_{1}\) Punnett square and predict the expected \(\mathrm{F}_{1}\) phenotype(s). .List the genotype(s) of gametes produced by \(\mathrm{F}_{1}\) individuals. .Draw the \(\mathrm{F}_{2}\) Punnett square. Based on this Punnett square, predict the expected phenotype(s) in the \(\mathrm{F}_{2}\) generation and the expected frequency of each phenotype.

Step-by-Step Solution

Answer

The expected F1 phenotype will be yellow seeds and inflated pods, with a genotype of YyIi. In the F2 generation, there are four possible phenotypes: Yellow seeds and Inflated pods (9/16), Yellow seeds and Constricted pods (3/16), Green seeds and Inflated pods (3/16), and Green seeds and Constricted pods (1/16).

Step 1: Understand the given information

We are given the information that yellow seeds (Y) are dominant to green seeds (y), and inflated pods (I) are dominant to constricted pods (i). We have to cross two parents with genotypes YYII (homozygous dominant for both traits) and yyii (homozygous recessive for both traits).

Step 2: Perform the F1 cross and draw the Punnett square

When we cross the two parents YYII and yyii, the resulting offspring of F1 generation will have only one possible genotype: YyIi (heterozygous for both traits). Since the Punnett square for this cross involves only one combination, there is no need to draw the Punnett square for the F1 generation. The expected F1 phenotype will be yellow seeds and inflated pods due to the dominance of Y and I alleles.

Step 3: List the genotype(s) of gametes produced by F1 individuals

To produce gametes, F1 individuals (YyIi) undergo the process of meiosis, which results in four possible combinations of genotype for their gametes. These are: YI, Yi, yI, and yi.

Step 4: Draw the F2 Punnett square

Now, we have to perform a cross between F1 individuals (YyIi x YyIi) to obtain the F2 generation. The four possible gametes (YI, Yi, yI, yi) will be combined using a 4x4 Punnett square. The Punnett square for the F2 generation is as follows: | | YI | Yi | yI | yi | |---------|------------|-------------|------------|-------------| | **YI** | YYII | YYIi | YyII | YyIi | | **Yi** | YYIi | YYii | YyIi | Yyii | | **yI** | YyII | YyIi | yyII | yyIi | | **yi** | YyIi | Yyii | yyIi | yyii |

Step 5: Predict the F2 phenotypes and their frequencies

Now, we can determine the expected F2 phenotypes and their frequencies based on the genotypes from the Punnett square. There are four possible phenotypes: 1. Yellow seeds, Inflated pods (YYII, YYIi, YyII, YyIi): 9/16 (Reason: 1 YYII, 2 YYIi, 2 YyII, and 4 YyIi) 2. Yellow seeds, Constricted pods (YYii, Yyii): 3/16 (Reason: 2 YYii and 1 Yyii) 3. Green seeds, Inflated pods (yyII, yyIi): 3/16 (Reason: 1 yyII and 2 yyIi) 4. Green seeds, Constricted pods (yyii): 1/16 (Reason: 1 yyii) The expected F2 phenotypes and their frequencies are as follow: - Yellow seeds and Inflated pods: 9/16 - Yellow seeds and Constricted pods: 3/16 - Green seeds and Inflated pods: 3/16 - Green seeds and Constricted pods: 1/16

Key Concepts

Genotype

When studying genetics, the term genotype refers to the specific combination of alleles an organism possesses for a particular trait. Alleles are the different versions of the same gene, residing at the same locus on matching chromosomes. In the case of our garden pea exercise, genotype can be represented by letters such as 'YY' for yellow seeds or 'yy' for green seeds, where 'Y' is the dominant allele and 'y' is the recessive allele.

For the inflated and constricted pods, the genotype 'II' represents inflated pods, while 'ii' indicates constricted pods, with 'I' being dominant. It's essential to understand that genotype is the genetic makeup hidden within an organism, and it dictates the potential physical characteristics, or phenotype. However, because of the dominance and recessiveness of alleles, different genotypes can produce the same phenotype, as seen within the Punnett square predictions for the F2 generation in our exercise.

Phenotype

Opposite to genotype, phenotype is the physical expression or characteristics of that genotype. It's what we can see or measure in an organism, such as the color of the seeds or the shape of the pods in peas. For the original exercise, the F1 phenotype for the garden peas was uniformly yellow seeds with inflated pods regardless of the organism’s heterozygous genotype (YyIi), thanks to the presence of dominant alleles.

In the F2 generation, the phenotype frequencies revealed the various combinations of seed color and pod shape, showcasing how different genotypes result in distinct phenotypes. Students often struggle with the concept that multiple genotypes can result in the same phenotype due to the effects of dominant alleles, highlighting the importance of understanding the principle that phenotype does not always directly reveal genotype.

Dominant and Recessive Alleles

Alleles are classified as dominant or recessive, and this distinction is paramount when predicting an organism's phenotype. A dominant allele masks the expression of a recessive allele in a heterozygous combination. In the context of our exercise, the dominant alleles were 'Y' for yellow seed color and 'I' for inflated pods. A single copy of these dominant alleles is sufficient to express the associated trait.

Recessive alleles, on the other hand, such as 'y' for green seeds and 'i' for constricted pods, only express their phenotype when both alleles are recessive (homozygous recessive genotype). Therefore, a plant with a 'Yy' genotype will have yellow seeds, not revealing the presence of the green-seed allele. It's important for students to understand that an individual with a dominant phenotype could be either homozygous dominant or heterozygous, but if the phenotype is recessive, the individual must be homozygous recessive.

Mendelian Inheritance

The principles of Mendelian inheritance serve as the foundation for understanding how traits are passed from parents to offspring through dominant and recessive alleles. Gregor Mendel's experiments with pea plants led to the discovery of these principles, which are well demonstrated in the considered exercise. The first principle, segregation, suggests that an individual inherits two alleles, one from each parent, and these alleles separate during gamete formation.

The second principle, independent assortment, is illustrated in the crossing of traits for seed color and pod shape; these traits are inherited independently of each other when located on separate chromosomes. The Punnett square for the F2 generation combines these principles to predict the frequency of phenotypes, showcasing a classic 9:3:3:1 ratio resulting from a dihybrid cross, assuming no linkage between the gene loci. Understanding Mendelian inheritance allows students to accurately predict the statistical outcome of breeding experiments based on genotype and allele dominance.