Question

Three gene pairs located on separate autosomes determine flower color and shape as well as plant height. The first pair exhibits incomplete dominance, where the color can be red, pink (the heterozygote), or white. The second pair leads to personate (dominant) or peloric (recessive) flower shape, while the third gene pair produces either the dominant tall trait or the recessive dwarf trait. Homozygous plants that are red, personate, and tall are crossed to those that are white, peloric, and dwarf. Determine the \(F_{1}\) genotype(s) and phenotype(s). If the \(\mathrm{F}_{1}\) plants are interbred, what proportion of the offspring will exhibit the same phenotype as the \(F_{1}\) plants?

Short answer

Answer: The genotype of the F1 offspring is RrPpTt (Pink, Personate, Tall). The proportion of F2 offspring that will exhibit the same phenotype as the F1 plants is 1/8 (12.5%).

Step-by-step solution

Identify given genotypes

For the given gene pairs, let's assign letters to represent the traits, where uppercase letters represent dominant alleles and lowercase letters represent recessive alleles. - Color: R (red, dominant), r (white, recessive) - with an incomplete dominance where heterozygotes (Rr) result in pink color. - Shape: P (personate, dominant), p (peloric, recessive) - Height: T (tall, dominant), t (dwarf, recessive) The given genotypes for the two parents are as follows: Parent 1 (Red, personate, and tall): RRPPTT Parent 2 (White, peloric, and dwarf): rrpptt

Determine F1 genotype and phenotype

Now that we know the genotypes of the parents, we can find the genotypes of the F1 offspring. Since the parents are homozygotes, the F1 offspring will be heterozygotes for all traits. The F1 genotype will be RrPpTt. The F1 offspring will display the following phenotypes due to incomplete dominance and dominant traits: - Color: Pink (Rr) - Shape: Personate (Pp) - Height: Tall (Tt)

Calculate proportions for F2 genotypes and phenotypes

To determine the proportion of offspring that exhibit the same phenotype as the F1 plants, we will interbreed the F1 plants (RrPpTt × RrPpTt). We can use the Punnett squares for each of the traits to find the combinations of gene pairs in the F2 generation. - Color: (Rr × Rr) => 1 RR : 2 Rr : 1 rr - Shape: (Pp × Pp) => 1 PP : 2 Pp : 1 pp - Height: (Tt × Tt) => 1 TT : 2 Tt : 1 tt The proportion that exhibits the same phenotypes as the F1 plants will result from those with genotypes Rr (pink), Pp (personate), and Tt (tall). To find this proportion, we will multiply the probabilities of each of these genotypes appearing: Color: \(\frac{2}{4}\) chance of offspring being Rr Shape: \(\frac{2}{4}\) chance of offspring being Pp Height: \(\frac{2}{4}\) chance of offspring being Tt The proportion of F2 plants with the same phenotype as the F1 plants (Pink, Personate, Tall) is the product of the probabilities for the individual traits: \(\frac{2}{4} \times \frac{2}{4} \times \frac{2}{4} = \frac{8}{64}\) So, the proportion of F2 offspring that will exhibit the same phenotype as the F1 plants is \(\frac{8}{64}\) or \(\frac{1}{8}\).

Key concepts

Incomplete Dominance

Incomplete dominance is a fascinating concept in genetics, which occurs when a dominant allele doesn't completely hide the effects of a recessive allele. Therefore, the offspring's phenotype is a mixture or an intermediate between the two parent phenotypes. Consider the classic example of flower color where one parent has red flowers (RR) and the other has white flowers (rr). In incomplete dominance, the F1 generation will display pink flowers (Rr), neither completely red nor white, showcasing a blending of traits from both parents.

When these pink-flowered plants are crossed (Rr x Rr), the F2 generation will have a variety of colors based on the combination of alleles received: 1/4 red (RR), 1/2 pink (Rr), and 1/4 white (rr). With incomplete dominance, genetic variation is more evident as it gives rise to new phenotypes that are not present in the parents.

Autosomal Traits

Autosomal traits are characteristics determined by genes located on the autosomes, which are the chromosomes that do not determine an organism's sex. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes. Dominant autosomal traits only require one copy of the dominant allele to be expressed, while recessive autosomal traits require two copies of the recessive allele.

In our exercise, flower color, shape, and height are all autosomal traits, which means they follow the classical Mendelian genetics rules. For example, for plant height, a tall plant (TT or Tt) doesn't necessarily come from two tall parents because if it carries the Tt genotype, one parent could have been dwarf (tt).

Punnett Square

A Punnett square is a tool that scientists and students use to predict the genotypic and phenotypic outcomes of genetic crosses. By placing the alleles from each parent along the top and side of a grid, we can fill in the possible genetic combinations for their offspring. You can think of it like a genetic prediction chart.

For instance, when F1 plants with genotype RrPpTt are crossed, we use individual Punnett squares for each trait to calculate the F2 generation's genetic makeup. This simplifies complex predictions by allowing us to address one trait at a time, making the exercise more manageable and the results easier to interpret.

Phenotype and Genotype

Understanding the difference between phenotype and genotype is crucial in genetics. Genotype refers to the specific genetic makeup of an organism, the actual alleles in its DNA. Phenotype, on the other hand, is the outward, physical expression of the genotype that you can observe, such as flower color or plant height.

In our example, all F1 plants have the genotype RrPpTt, which results in the phenotype of pink flowers (from Rr), personate shape (from Pp), and tall stature (from Tt). It's important to note how different genotypes can lead to the same phenotype, particularly when dominant and recessive interactions are involved.

F1 and F2 Generations

In classical genetics, the F1 (first filial) generation is the direct offspring of the parental generation, which in our exercise would be the uniformly heterozygous RrPpTt plants resulting from the RRPPYY x rrppyy cross. The F2 (second filial) generation stems from crossing two F1 individuals with each other, leading to greater genetic diversity.

It's intriguing to observe the genetic patterns that emerge in the F2 generation, where traits can recombine in various ways, resulting in new genotypic ratios and a broader spectrum of traits. For example, the F2 generation's genotypic ratio for each trait follows a 1:2:1 pattern, as we see with the color trait, where we get 1/4 RR (red), 1/2 Rr (pink), and 1/4 rr (white). The phenotypic ratios may vary, however, depending on whether traits exhibit incomplete dominance or codominance.