Sex-Linked Inheritance: Videos & Practice Problems

Sex-linked inheritance involves genes located on sex chromosomes, primarily the X chromosome, which is larger and contains more genes than the Y chromosome. Females have two X chromosomes, allowing for homozygous or heterozygous conditions, while males have one X and one Y chromosome, expressing whatever allele is present on their single X. X-linked recessive disorders, like hemophilia, affect males more frequently since they require only one recessive allele to express the disorder. Pedigrees for these disorders typically show a higher prevalence in males, with specific inheritance patterns for affected females.

Sex-Linked Inheritance

Sex-linked inheritance is a crucial concept in genetics that revolves around the sex chromosomes, specifically the X and Y chromosomes, which determine the sex of an organism. Typically, females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). This distinction is essential as it influences the inheritance patterns of certain traits linked to these chromosomes.

Genes located on the X chromosome are referred to as X-linked genes, whereas those on the Y chromosome are known as Y-linked genes. Notably, the X chromosome is significantly larger than the Y chromosome, containing approximately 1,100 X-linked genes compared to around 100 Y-linked genes. This disparity highlights the greater genetic complexity associated with the X chromosome.

In terms of inheritance, the presence of the Y chromosome is critical for the development of male reproductive systems. During fertilization, there is a 50% probability of producing a female (XX) and a 50% probability of producing a male (XY). This can be illustrated using a Punnett square, which demonstrates the potential combinations of parental gametes. Each fertilization event results in these equal chances, emphasizing the random nature of sex determination.

As we delve deeper into the study of sex-linked inheritance, the focus will primarily be on X-linked genes, given their prevalence and significance in various genetic traits and disorders. Understanding these concepts lays the groundwork for exploring more complex genetic interactions and their implications in biology.

X-Linked Inheritance

X-linked inheritance is a genetic pattern that is crucial for understanding how certain traits are passed down through generations, particularly in relation to sex chromosomes. In humans and many other organisms, females possess two X chromosomes, allowing them to inherit two alleles for each X-linked gene—one from each parent. This means females can be homozygous dominant (two dominant alleles), homozygous recessive (two recessive alleles), or heterozygous (one dominant and one recessive allele) for X-linked traits.

In contrast, males have one X and one Y chromosome, which means they only inherit one allele for each X-linked gene from their mother. Consequently, males cannot be homozygous or heterozygous; they express whatever allele is present on their single X chromosome. This fundamental difference in inheritance patterns leads to distinct expressions of traits between males and females.

For example, in fruit flies, the gene responsible for eye color is located on the X chromosome, with two alleles: the dominant allele (denoted as X^R for red eyes) and the recessive allele (denoted as X_r for white eyes). When conducting genetic crosses, such as between a homozygous red-eyed female (X^RX^R) and a white-eyed male (X_rY), the resulting offspring can be analyzed using a Punnett square.

In this specific cross, all female offspring will inherit a red-eye allele from their mother, resulting in 0% of females having white eyes. However, all male offspring will inherit the white-eye allele from their father, leading to 100% of males having white eyes. This pattern continues across various crosses, demonstrating that males are more likely to express recessive traits linked to the X chromosome.

In another example, crossing a heterozygous red-eyed female (X^RX_r) with a white-eyed male results in 50% of the males having white eyes, while females remain unaffected. This consistent trend highlights that males are generally more susceptible to X-linked disorders, as they lack a second X chromosome that could mask the effects of a recessive allele.

Overall, the study of X-linked inheritance not only reveals the genetic mechanisms behind specific traits but also underscores the differences in how males and females inherit and express these traits. Understanding these patterns is essential for further exploration of genetic disorders and their implications in various species.

Wild type fruit flies have red eyes. A white-eyed female fly is crossed with a red-eyed male fly. All of the females from the cross are red-eyed and all of the males, white-eyed. What type of inheritance pattern is this?

• A. Autosomal recessive.
• B. Autosomal dominant.
• C. Sex-linked on X chromosome.
• D. Incomplete dominance.
• E. Sex-linked on Y chromosome

Answer: (C)

When Thomas Hunt Morgan crossed red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

• A. The gene involved is located on the Y chromosome.
• B. The gene involved is located on the X chromosome.
• C. The gene involved is located on an autosome, but only in males.
• D. Other male-specific factors influence eye color in flies.

Answer: (B)

X-Linked Recessive Disorder: Hemophilia Inheritance

Hemophilia is an example of an X-linked recessive disorder, which means it is associated with a gene on the X chromosome and is expressed only when an individual has the recessive allele. In this context, females must be homozygous recessive, possessing two copies of the recessive allele (X_h), to exhibit the disorder. In contrast, males, who have only one X chromosome, need only one recessive allele to be affected, making them more susceptible to X-linked recessive disorders like hemophilia.

To illustrate the inheritance pattern of hemophilia, consider a Punnett square involving a heterozygous mother (X_HX_h) and an unaffected father (X_HY). The mother carries one normal allele (X_H) and one recessive allele (X_h

1. **Females**: The top half of the Punnett square shows that all female offspring will inherit at least one normal allele (X_H), making them unaffected by hemophilia.

2. **Males**: The bottom half indicates that 50% of male offspring will inherit the normal allele (X_H) and be unaffected, while the other 50% will inherit the recessive allele (X_h) and be affected by hemophilia.

In summary, hemophilia is characterized by abnormal blood clotting due to the inability to form blood clots, and its inheritance pattern highlights the greater likelihood of males being affected by X-linked recessive disorders. Understanding these genetic principles is crucial for predicting the occurrence of such disorders in families.

Women with X-linked disorders always pass the genes for the disorder on to ______. While men with X-linked 
disorders always pass the genes for the disorder on to _______.

• A. only their daughters; only their daughters.
• B. both their daughters and sons; only their sons.
• C. both their daughters and sons; only their daughters.
• D. both their daughters and sons; both their daughters and sons.

Answer: (C)

X-Linked Recessive Pedigrees

X-linked recessive pedigrees are essential for understanding the inheritance patterns of X-linked recessive disorders, which predominantly affect males. In these pedigrees, it is common to observe a higher prevalence of the disorder among males compared to females. This is due to the fact that males possess only one X chromosome, meaning that the presence of a single recessive allele (denoted as x_b) will result in the expression of the disorder. In contrast, females require two copies of the recessive allele (x_bx_b) to exhibit the disorder, as they have two X chromosomes.

For a female to be affected by an X-linked recessive disorder, her father must be affected, and her mother must either be affected or at least a carrier (heterozygous). This means that the mother carries one normal allele (X_B) and one recessive allele (x_b). In a pedigree illustrating red-green color blindness, an example of an X-linked recessive disorder, the normal vision allele is represented as X_B, while the allele for color blindness is x_b.

In the pedigree, affected individuals are typically represented with filled-in symbols, indicating that they possess the recessive allele. The pattern observed in these pedigrees shows that most affected individuals are male, with only a few females being affected. This pattern serves as a significant clue for identifying X-linked recessive disorders. Additionally, all sons of an affected female will inherit the disorder, as they receive their X chromosome from their mother.

In summary, recognizing the predominance of affected males and understanding the inheritance requirements for females are crucial for analyzing X-linked recessive pedigrees. This knowledge aids in identifying and predicting the occurrence of such genetic disorders in families.

Sex-Linked Inheritance Example 1

In analyzing a pedigree to determine the inheritance pattern of a genetic disorder, it is essential to observe the distribution of affected individuals across genders. In this example, there are 5 affected females (represented by shaded circles) and 7 affected males (represented by shaded squares). The relatively balanced distribution suggests that the disorder is not X-linked, as X-linked disorders typically show a significant disparity between affected males and females.

This leads us to consider autosomal inheritance patterns, specifically autosomal dominant and autosomal recessive. A key characteristic of autosomal dominant disorders is their presence in every generation. Upon examining the pedigree, it becomes evident that while the disorder appears in the first generation, it skips the second generation due to individual number 7, who is not directly linked to the affected parents. This skipping of generations is indicative of an autosomal recessive inheritance pattern.

In autosomal recessive inheritance, individuals must be homozygous recessive to express the disorder. Therefore, unaffected individuals must possess at least one dominant allele. By assigning alleles to individuals in the pedigree, one can verify the consistency of this inheritance pattern throughout the generations. The presence of affected individuals in the first and third generations, with a notable absence in the second, reinforces the conclusion that the disorder follows an autosomal recessive pattern.

In summary, when analyzing pedigrees, look for clues such as the distribution of affected individuals and the presence or absence of the disorder across generations. These observations are crucial for determining the mode of inheritance, and in this case, the disorder is confirmed to be autosomal recessive.

The following pedigree is for the X-linked-recessive trait for color blindness. Using XN for the normal allele and Xn for the color blindness allele, fill in the top half of the boxes/circles with the genotype. Also, fill in the bottom half of the boxes/circles with the phenotype (Normal vision or color blind). If it is impossible to know for certain a specific allele in the genotype, then place a '?' to represent the allele that is in question.

In this exercise, we explore a pedigree analysis for an X-linked recessive trait, specifically color blindness. The normal allele is represented as X^M and the color blindness allele as Xⁿ. Understanding the inheritance patterns of X-linked traits is crucial, as they exhibit different probabilities in males and females due to the presence of only one X chromosome in males (XY) compared to two in females (XX).

To begin, we identify individuals in the pedigree based on their phenotypes. A color-blind female must possess the genotype XⁿXⁿ, as she has two recessive alleles. Her normal vision brother has the genotype X^MY, indicating he carries the normal allele. Since the color-blind daughter received one color-blind allele from each parent, her father must also be color blind, with the genotype XⁿY. The mother, who has normal vision but has a color-blind daughter, must be heterozygous (X^MXⁿ), as she contributed the recessive allele.

Next, we analyze the mother's family. Her brother, being normal, has the genotype X^MY, while her sister, also normal, is heterozygous (X^MXⁿ). The mother’s father is normal (X^MY), confirming that the mother must be a carrier (X^MXⁿ) since she has a color-blind daughter. The normal daughter from this family has a question mark for her second allele, as we cannot determine if she is homozygous normal or a carrier without further information.

Moving to another family, the color-blind mother (XⁿXⁿ) has a normal husband (X^MY). Their normal daughter must be heterozygous (X^MXⁿ), receiving the normal allele from her father. The son, however, inherits the X chromosome from his mother, making him color blind (XⁿY).

Finally, we assess the last family in the pedigree. The color-blind daughter (XⁿXⁿ) indicates that her father must also be color blind (XⁿY). The normal son has the genotype X^MY, confirming he received the normal allele from his mother. The mother, who has normal vision, must be a heterozygous carrier (X^MXⁿ) since she has a color-blind daughter.

Through this analysis, we have filled in the genotypes and phenotypes for all individuals in the pedigree, demonstrating the complexities of X-linked inheritance. Understanding these patterns is essential for predicting the likelihood of traits in future generations.

Here’s what students ask on this topic:

What is sex-linked inheritance and how does it differ between males and females?

Sex-linked inheritance involves genes located on the sex chromosomes, primarily the X chromosome. Females have two X chromosomes, allowing them to be homozygous or heterozygous for X-linked genes. This means they can carry two alleles for each gene, one from each parent. Males, on the other hand, have one X and one Y chromosome. They only have one allele for each X-linked gene, inherited from their mother, and express whatever allele is present on their single X chromosome. This difference makes males more susceptible to X-linked recessive disorders, as they only need one recessive allele to express the disorder, whereas females would need two.

Why are X-linked recessive disorders more common in males than females?

X-linked recessive disorders are more common in males because males have only one X chromosome. For a male to express an X-linked recessive disorder, he needs only one copy of the recessive allele on his single X chromosome. In contrast, females have two X chromosomes and would need two copies of the recessive allele to express the disorder. This makes males more likely to be affected by X-linked recessive disorders, such as hemophilia and red-green color blindness, as they have a higher probability of inheriting the single necessary recessive allele.

How does a Punnett square illustrate the inheritance of X-linked traits?

A Punnett square can illustrate the inheritance of X-linked traits by showing the possible combinations of sex chromosomes and alleles from each parent. For example, if a mother is a carrier for an X-linked recessive disorder (X^HX^h) and the father is unaffected (X^HY), the Punnett square will show the potential genotypes of their offspring. The possible combinations are X^HX^H (unaffected female), X^HX^h (carrier female), X^HY (unaffected male), and X^hY (affected male). This helps visualize the 50% chance of having a male or female and the likelihood of inheriting the disorder.

What are some examples of X-linked recessive disorders?

Examples of X-linked recessive disorders include hemophilia, red-green color blindness, and Duchenne muscular dystrophy. Hemophilia is characterized by abnormal blood clotting due to a deficiency in clotting factors. Red-green color blindness affects the ability to distinguish between red and green colors. Duchenne muscular dystrophy is a severe form of muscular dystrophy that leads to muscle degeneration and weakness. These disorders are more common in males because they only need one copy of the recessive allele on their single X chromosome to express the disorder.

How can pedigrees be used to track X-linked recessive disorders?

Pedigrees can be used to track X-linked recessive disorders by illustrating the inheritance patterns across generations. In these pedigrees, affected individuals are typically shaded. More males than females are usually affected due to the nature of X-linked recessive inheritance. Females can only be affected if their father is affected and their mother is at least a carrier. All sons of an affected female will be affected. By analyzing these patterns, one can determine the likelihood of individuals being carriers or affected by the disorder.