Genetic Disorders: Videos & Practice Problems

Autosomal disorders can be inherited as either autosomal dominant or autosomal recessive. Autosomal dominant disorders require only one dominant allele for expression, appearing in every generation, while autosomal recessive disorders require two recessive alleles and may skip generations. X-linked recessive disorders, like hemophilia, predominantly affect males, as they need only one recessive allele. Females require two recessive alleles to express the disorder. Understanding these inheritance patterns is crucial for genetic counseling and predicting disease occurrence in families.

Autosomal Disorders

Autosomal disorders are genetic conditions linked to autosomes, which are non-sex chromosomes. These disorders can be inherited in two primary ways: as autosomal dominant or autosomal recessive disorders.

In autosomal dominant disorders, individuals with at least one dominant allele exhibit the disorder. This means that both homozygous dominant (AA) and heterozygous (Aa) genotypes will show the trait. A key characteristic of these disorders is that they appear in every generation, with affected individuals often having an affected parent. An example of an autosomal dominant disorder is polydactyly, where individuals have extra fingers or toes. In this case, the presence of at least one dominant allele (F) results in the disorder, while individuals with two recessive alleles (ff) have the normal phenotype.

Conversely, autosomal recessive disorders require individuals to be homozygous recessive (aa) to express the disorder. Those with at least one dominant allele (AA or Aa) will not show the disorder and are considered healthy carriers. Cystic fibrosis is a well-known example of an autosomal recessive disorder. In pedigrees illustrating this condition, affected individuals are homozygous recessive, while those with at least one dominant allele are unaffected. A notable feature of autosomal recessive disorders is that they can skip generations, meaning that the disorder may not appear in every generation.

Understanding the inheritance patterns of these disorders is crucial for genetic counseling and predicting the likelihood of occurrence in future generations. As you continue your studies, you will have opportunities to apply these concepts through various exercises and examples.

If a genetic counselor was examining a pedigree chart and noticed an occurrence of a disease in every generation, the counselor would most likely assume that the disease was caused by:

• A. A new reoccurring mutation.
• B. An autosomal recessive disorder.
• C. A chromosomal abnormality.
• D. An autosomal dominant disorder.
• E. Having an extra set of chromosomes.

Answer: (D)

X-Linked Recessive Disorder: Hemophilia Inheritance

Hemophilia is an example of an X-linked recessive disorder, which is characterized by abnormal blood clotting. This type of disorder is linked to a gene on the X chromosome, and its recessive nature means that it is expressed only when an individual possesses two recessive alleles on the X chromosomes. In females, who have two X chromosomes, being affected by hemophilia requires them to be homozygous recessive, meaning they must have two copies of the recessive allele (denoted as X_h). In contrast, males, who have only one X chromosome, need only one recessive allele to express the disorder, 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. Female offspring can inherit either X_H from the mother and X_H from the father (X_HX_H), or X_h from the mother and X_H from the father (X_HX_h), resulting in two unaffected females.

2. Male offspring can inherit X_H from the mother and Y from the father (X_HY), or X_h from the mother and Y from the father (X_hY). This results in a 50% chance of being unaffected (X_HY) and a 50% chance of being affected (X_hY).

Thus, the likelihood of males being affected by hemophilia is significantly higher than that of females, due to the requirement of only one recessive allele for expression in males. This understanding of X-linked recessive inheritance is crucial for predicting the occurrence of hemophilia in future generations.

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 that more males are affected than females. This is due to the fact that males have only one X chromosome, so a single recessive allele on that chromosome will result in the disorder. In contrast, females have two X chromosomes and require two copies of the recessive allele to express the disorder.

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 if a female has one affected X chromosome, she can only express the disorder if her other X chromosome also carries the recessive allele. In the case of red-green color blindness, which serves as a classic example of an X-linked recessive disorder, the normal vision allele is represented as \(X^{B}\) and the color blindness allele as \(X^{b}\). Affected males will have the genotype \(X^{b}Y\), while affected females will have the genotype \(X^{b}X^{b}\).

In a pedigree chart, affected individuals are typically represented with filled-in symbols. Observing the distribution of affected individuals can provide insights into the inheritance pattern. For instance, if a pedigree shows a significant number of affected males compared to females, it is indicative of an X-linked recessive disorder. Additionally, all sons of an affected female will also be affected, as they inherit their X chromosome from their mother.

In summary, recognizing the patterns in X-linked recessive pedigrees—such as the predominance of affected males and the specific inheritance requirements for females—can greatly aid in identifying these genetic disorders. Understanding these principles is crucial for genetic counseling and studying hereditary conditions.

Here’s what students ask on this topic:

What is the difference between autosomal dominant and autosomal recessive disorders?

Autosomal dominant disorders require only one dominant allele for the disorder to be expressed. This means individuals with either a homozygous dominant (AA) or heterozygous (Aa) genotype will exhibit the disorder. These disorders typically appear in every generation without skipping. An example is polydactyly, which causes extra fingers or toes.

In contrast, autosomal recessive disorders require two recessive alleles (aa) for the disorder to manifest. Individuals with one dominant allele (Aa) are carriers but do not show symptoms. These disorders often skip generations, as carriers do not exhibit the condition. An example is cystic fibrosis, which affects the respiratory and digestive systems.

Understanding these inheritance patterns is crucial for predicting the likelihood of genetic disorders in families.

Why are males more likely to be affected by X-linked recessive disorders?

Males are more likely to be affected by X-linked recessive disorders because they have only one X chromosome (XY). If a male inherits a recessive allele on the X chromosome, there is no corresponding allele on the Y chromosome to mask its effects, so the disorder is expressed. For example, in hemophilia, a male with the recessive allele (X_hY) will have the disorder.

In contrast, females have two X chromosomes (XX). To express an X-linked recessive disorder, a female must inherit two recessive alleles (X_hX_h). If she has one dominant allele (X_HX_h), she will be a carrier but unaffected. This difference in chromosome composition explains why males are more frequently affected by X-linked recessive disorders.

What are some examples of X-linked recessive disorders?

Examples of X-linked recessive disorders include:

• Hemophilia: A disorder characterized by impaired blood clotting, leading to excessive bleeding from minor injuries.
• Red-green color blindness: A condition where individuals have difficulty distinguishing between red and green hues.
• Duchenne muscular dystrophy: A progressive muscle-wasting disorder caused by mutations in the dystrophin gene.

These disorders are more common in males because they only need one copy of the recessive allele on their single X chromosome to be affected. Females, with two X chromosomes, must inherit two recessive alleles to express the disorder, making it less common in females.

How can pedigrees help identify X-linked recessive disorders?

Pedigrees are useful tools for identifying X-linked recessive disorders by analyzing patterns of inheritance across generations. Key features include:

• More males affected: Males are more frequently affected because they only need one recessive allele on their X chromosome.
• Affected females: Females can only be affected if their father is affected and their mother is a carrier or affected.
• All sons of affected females: Sons of affected females will inherit the disorder because they receive the X chromosome carrying the recessive allele.

By observing these patterns, geneticists can determine whether a disorder is X-linked recessive and predict its likelihood in future generations.

What is the inheritance pattern of cystic fibrosis?

Cystic fibrosis is inherited in an autosomal recessive pattern. This means an individual must inherit two recessive alleles (aa) to have the disorder. If a person has one dominant allele and one recessive allele (Aa), they are a carrier but do not exhibit symptoms.

Carriers can pass the recessive allele to their offspring. If two carriers have children, there is a 25% chance the child will inherit both recessive alleles and have cystic fibrosis, a 50% chance the child will be a carrier, and a 25% chance the child will inherit two dominant alleles and be unaffected.

Cystic fibrosis primarily affects the respiratory and digestive systems due to thick mucus buildup, making early diagnosis and management essential.