In this video, we're going to talk about X-linked inheritance. It's important to note that because females have 2 X chromosomes, they have 2 alleles for each X-linked gene, where the females inherit one allele from each of their parents. Females can either be homozygous dominant, homozygous recessive, or heterozygous for the X-linked genes. However, this is not the case for males, because males only have one X chromosome and a Y chromosome, therefore, they only have one allele for each X-linked gene instead of having 2 alleles like females do. Males inherit one of their alleles from their mother and none of the alleles from their father because they inherit a Y chromosome from the father. So, males cannot be homozygous dominant, recessive, or heterozygous. Instead, males express whatever X-linked allele is on their single X chromosome inherited from their mother.
In our example below, this experiment tracking eye color in fruit flies first revealed the X-linked inheritance pattern. There are two possible eye colors in this experiment: red eyes and white eyes. The gene for eye color is on the X chromosome, making it an X-linked gene with two alleles. The dominant allele is \(X_R\) which results in red eyes, and the recessive allele is \(X_r\) which leads to white eyes, with \(r\) being recessive to \(R\) which is dominant.
Let’s look at the different Punnett Squares we have. In the first cross, we have a homozygous red-eyed female (\(X_RX_R\)) crossed with a white-eyed male (\(X_r\)). When completing the Punnett Squares, keep in mind that males will pass on their Y chromosome in 50% of the cases. What results is that \(X_R\) is brought down in the Punnett Square, while the \(X_r\) and the Y chromosome are passed on. Therefore, 50% of the offspring will be males and 50% females, with none of the females having white eyes and none of the males too in this specific cross.
.In another cross, we have a homozygous white-eyed female (\(X_rX_r\)) with a red-eyed male (\(X_R\)). Here, all females inherit \(X_R\) from the father and \(X_r\) from the mother, leading to a heterozygous red-eyed outcome. However, for males, since they inherit the mother’s \(X_r\) and the father’s Y, 100% of the males will have white eyes.
In a different scenario where a heterozygous red-eyed female (\(X_RX_r\)) is crossed with a red-eyed male (\(X_R\)), you’ll find that 50% of the males have white eyes but no females do. In the last situation, crossing a heterozygous red-eyed female with a white-eyed male (\(X_r\)), results in 50% of both females and males having white eyes.
This experiment illustrates the different inheritance patterns for males and females, which was a key observation by scientists studying these traits. We'll discuss further how X-linked disorders typically affect males more than females as we progress in our course. These patterns are foundational for understanding genetic inheritance and will help us apply these concepts moving forward. See you all in our next video.