Question

An attached-X female fly, \(\overline{X X} Y\) (see the "Insights and Solutions" box \(),\) expresses the recessive X-linked white-eye mutation. It is crossed to a male fly that expresses the X-linked recessive miniature-wing mutation. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

Short answer

The predicted outcome for their offspring is as follows: - 50% of female offspring will have red eyes and normal wings - 50% of female offspring will have white eyes and normal wings - 50% of male offspring will have red eyes and normal wings - 50% of male offspring will have red eyes and miniature wings

Step-by-step solution

Understanding X-linked inheritance and recessive traits

X-linked inheritance refers to the transmission of traits that are carried on the X chromosome. Since males have only one X chromosome (XY) and females have two X chromosomes (XX), the inheritance patterns differ between the sexes. Recessive traits are expressed only when an individual has two copies of the recessive allele. In this case, the attached-X female fly has the white-eye mutation on both X chromosomes (\(\overline{X X}\)) and the male fly has the miniature-wing mutation on its single X chromosome (X).

Setting up the Punnett square

A Punnett square is a tool used to predict the possible genotypes and phenotypes of offspring from a genetic cross. To set up the Punnett square, list the possible gametes (sex cells) of each parent along the top and side. In this case, the female fly can produce gametes with the white-eye mutation on the X chromosome (\(\overline{X}\)) and the male can produce gametes with the mini-wing mutation on the X chromosome (\(X_m\)) or gametes with the Y chromosome (Y).

Filling in the Punnett square

Pair each gamete from the female fly with each gamete from the male fly by filling in the squares: | | \(\overline{X}\) | Y | |---|-------------|------------| | X\(_m\) | \(\overline{X}\)X\(_m\) | X\(_m\)Y | | X | \(\overline{X}\)X | XY |

Analyzing the Punnett square

From the Punnett square, we can determine the phenotypes of the offspring: 1. \(\overline{X}\)X\(_m\): Female flies with one white-eye mutation and one mini-wing mutation. They will have red eyes and normal wings because both mutations are recessive and they do not have two copies of either mutation. 2. X\(_m\)Y: Male flies with the mini-wing mutation. They will have red eyes and miniature wings. Since males only have one X chromosome, they express the mutation recessively. 3. \(\overline{X}\)X: Female flies with the white-eye mutation. They will have white eyes and normal wings. These females have two copies of the white-eye mutation which makes them express the trait. 4. XY: Male flies with no mutations. They will have red eyes and normal wings as they are not carrying either mutation. In conclusion, the outcomes of this cross in terms of sex, eye color, and wing size are: - \(\frac{1}{2}\) of female offspring will have red eyes and normal wings - \(\frac{1}{2}\) of female offspring will have white eyes and normal wings - \(\frac{1}{2}\) of male offspring will have red eyes and normal wings - \(\frac{1}{2}\) of male offspring will have red eyes and miniature wings

Key concepts

Recessive Traits

Imagine that certain traits, like hidden features waiting for their moment, only become visible under certain conditions. This is essentially what recessive traits are in genetics. These traits are hidden when paired with dominant traits and only expressed when an individual inherits two copies, one from each parent.

For example, in some fly populations, having white eyes is a recessive trait. This means that a fly needs to inherit the gene for white eyes from both its mother and father to display white eyes. If it inherits a recessive white-eye gene from one parent and a dominant red-eye gene from the other, the fly will have red eyes, revealing the underlying power of dominance in genetics.

• If a recessive allele for a trait is represented by 'a', and the dominant by 'A', only 'aa' will display the recessive trait.
• In X-linked recessive traits, males (XY) only need one copy of the recessive allele (since they have only one X chromosome) to express the trait, while females (XX) need two.
• Recessive traits can skip generations in a family tree, only to reappear when two carriers have children. This pattern is a hallmark of X-linked recessive inheritance.

Punnett Square

The Punnett square is a predictive tool that simplifies the puzzle of inheritance. By arranging the possible sperm and egg cells that parents can produce, the grid shows potential combinations in offspring.

It's like creating a lineup of possible genetic outcomes for a child, based on the genes of the parents. For every genetic trait, such as eye color or wing size, there is a designated spot in the square to show each potential combination of parental alleles. The Punnett square brings clarity to the genetic mix-and-match, giving us a visual representation of the probability of various hereditary outcomes.

Visualizing Genetic Crosses with a Punnett Square

• The top and left edges list the possible gametes from each parent.
• Each box within the square shows the combination of alleles that could result from one sperm meeting one egg.
• The Punnett square not only indicates the genotypes but also allows us to infer the phenotypes, the actual traits we can observe, of the offspring.

Genotypes and Phenotypes

The combination of alleles that an individual inherits is called its genotype. This genetic blueprint determines the traits, or phenotypes, that the organism displays. While the genotype is like a behind-the-scenes script for an organism's traits, the phenotype is the actual show — the colors, shapes, and behaviors that we can see and measure.

The Relationship Between Genotypes and Phenotypes

Genotypes are written as letters, where uppercase represents dominant alleles and lowercase represents recessive ones. For example, 'Bb' would be the genotype for an organism with a dominant trait (B), and a hidden recessive trait (b). The phenotype would display the dominant trait.

Although genotypes provide the instructions, the environment can influence how the phenotypes are expressed — a concept known as gene-environment interaction. This interplay is why identical twins, despite having the same genotypes, can sometimes have different phenotypes. In the world of genetics, nature provides the script, but nurture directs the play.