Question

In flies, small wings are recessive to normal wings. If a cross between two flies produces 8 small wing offspring and 28 normal wing offspring, what are the most likely genotypes of the parents? (Use \(S\) to represent the normal wing allele and \(s\) to represent the short wing allele.)

Short answer

The most likely genotypes of the parents are both heterozygous (Ss), as a cross between them would produce a 3:1 phenotypic ratio of normal wings to small wings, fitting the observed offspring distribution.

Step-by-step solution

Understanding the Genetics Terminologies

For this exercise, it's essential to understand some key genetic concepts. The genotype refers to the set of genes in an organism's DNA responsible for a particular trait. When there are two identical alleles for the trait, the organism is homozygous for that trait. If the two alleles are different, the organism is heterozygous. The phenotype is the actual physical appearance or characteristic resulting from the organism's genotype.

Interpreting the Provided Information

From the exercise, we know that small wings are recessive to normal wings. This means that in order for an offspring to exhibit small wings, they must inherit the small wing allele (s) from both parents (ss). On the other hand, normal-winged flies can either be homozygous (SS) or heterozygous (Ss).

Creating a Punnett Square

The Punnett square is a simple tool that geneticists use to predict the possible genotypes of offspring from a genetic cross. In this case, we can create a Punnett square for the cross of two heterozygotes (Ss x Ss), based on the phenotypic ratio given in the problem. 1. Draw a 2 x 2 square and label one side with one parent's alleles (S and s), and the other side with the second parent's alleles (S and s). 2. Fill out the squares by combining the corresponding alleles from each parent.

Interpreting the Results of the Punnett Square

The results of filling out the Punnett square should be SS, Ss, Ss, and ss. This gives the ratio of 1:2:1 for SS: Ss: ss genotypes. In terms of phenotypes, three out of the four possible combinations are normal-winged (SS, Ss, and Ss), and one is small-winged (ss). This results in a phenotypic ratio of 3:1 (normal wings to small wings), which fits the observed ratio in the problem.

Determining the Parents' Genotypes

Based on the analysis, the cross that would produce the given offspring phenotype ratio must occur between two heterozygous flies (Ss) with normal wings. They would be carriers of the recessive allele, but they would express the dominant phenotype. So, the most likely genotypes of the fly parents are both Ss.

Key concepts

Genotype and Phenotype

In the world of genetics, understanding the difference between genotype and phenotype is crucial. The genotype refers to the genetic makeup of an organism, specifically the particular set(s) of alleles it carries. For example, if the allele for normal wings in flies is denoted by 'S', an organism with normal wings could have the genotypes 'SS' or 'Ss', each comprising two alleles.

On the flip side, the phenotype is the outward expression of a genotype, manifested as visible traits such as wing size. In our exercise, the small wings (recessive trait) observed in the phenotype must be rooted in a 'ss' genotype, because the presence of the dominant 'S' allele would result in normal-sized wings. Deciphering the link between genotype and phenotype helps predict trait inheritance patterns.

Punnett Square

The Punnett square is a fundamental tool used in genetic problem-solving. It provides a visual representation of all possible combinations of parental alleles and their resulting offspring genotypes. To perform the analysis, alleles from each parent are listed on the square's axes, and the possible genetic combinations are filled into the squares.

This method, developed by Reginald Punnett, simplifies the calculation of the probability of inheriting specific traits. In the exercise concerning fly wing size, a 2x2 Punnett square predicts the offspring's genotypes from heterozygous parents, shedding light on the genetic patterns that define the flies' phenotypes.

Recessive and Dominant Alleles

Alleles are variants of a gene, and they can be either recessive or dominant. Dominant alleles, such as the 'S' allele for normal wings in flies, will mask the expression of recessive alleles when present. This means that a fly only needs one copy of the 'S' allele to exhibit the dominant trait of normal wings.

Contrastingly, recessive alleles, like the 's' allele for small wings, only express their phenotype when an individual carries two copies - one from each parent. Understanding this concept explains why some traits appear more frequently than others and helps in deducing the likelihood of certain phenotypes emerging from genetic crosses.

Heterozygous and Homozygous

Let's delve into the concepts of heterozygous and homozygous conditions in organisms. An individual is homozygous for a trait if it carries two identical alleles, such as 'SS' for normal wings or 'ss' for small wings in flies. Homozygous individuals, therefore, consistently pass on the same allele to their offspring.

In contrast, a heterozygous organism has two different alleles for a trait, like 'Ss' for normal wings. These individuals can pass on either allele to their offspring, which makes their genetic crosses more variable. The exercise illustrates that when both fly parents are heterozygous ('Ss'), their offspring can exhibit a variety of wing sizes based on the allele combinations received.