Sex-Linked Inheritance

Introduction to Sex Chromosomes

Sex chromosomes, X and Y, determine the biological sex of an individual and carry genes that influence various traits. These chromosomes are not homologous and differ in size, gene content, and function.

• X Chromosome: Contains approximately 1000 genes, including those necessary for female development and other traits not related to sex determination. Females have two X chromosomes (XX), providing a backup if one is defective.

• Y Chromosome: Contains fewer genes, primarily those required for male development (e.g., formation of male genitalia, sperm production, testosterone synthesis). Males have one X and one Y chromosome (XY), lacking a backup for X-linked genes.

Sex-Linked Inheritance Patterns

Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes. These patterns differ from autosomal inheritance due to the unique pairing of X and Y chromosomes in males and females.

• Y-linked inheritance: Traits are passed from father to son via the Y chromosome. Few examples exist, such as Y-linked infertility, since the Y chromosome mainly determines male traits.

• X-linked inheritance: Traits are associated with genes on the X chromosome. Many X-linked traits exist because the X chromosome carries genes unrelated to sex determination.

X-Linked Inheritance in Males and Females

The expression of X-linked traits differs between males and females due to their chromosomal composition:

• Females (XX): X-linked traits behave like recessive autosomal traits. Two copies of the affected gene are required for the trait or disorder to be expressed.

• Males (XY): Only one copy of the affected gene (from the mother) is needed for the trait or disorder to be expressed, as there is no second X chromosome to mask the effect.

Example: Hemophilia

• Hemophilia is a well-known X-linked recessive disorder, also called "bleeder's disease." Affected individuals lack a blood clotting factor, leading to excessive bleeding.

Pedigree Analysis of X-Linked Traits

Pedigree charts help trace the inheritance of X-linked traits through generations. Key points include:

• Females are rarely affected but often serve as carriers (heterozygous for the trait).

• Males inherit X-linked traits from their carrier mothers.

• Affected fathers cannot pass X-linked traits to their sons but will pass the affected gene to all daughters, who may become carriers or be affected depending on the mother's genotype.

Sex-Influenced Traits

Some traits are influenced by the sex of the individual but are not located on the sex chromosomes. These are called sex-influenced traits.

• Baldness: The gene is found on an autosome (not a sex chromosome). The allele for baldness is recessive in females (bb) but can be expressed in males with either Bb or bb due to the influence of testosterone.

Chromosome Abnormalities

Types of Chromosome Abnormalities

Chromosome abnormalities can occur during DNA replication, mitosis, or meiosis, leading to an abnormal number or structure of chromosomes in daughter cells. Major mechanisms include:

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during cell division, resulting in gametes or cells with too many or too few chromosomes.

• Deletions: A segment of a chromosome is lost, which can be lethal or cause severe disorders.

• Translocations: A chromosome segment breaks off and attaches to a different chromosome, potentially altering gene expression and increasing disease risk.

Nondisjunction and Its Consequences

• Mitosis: Nondisjunction usually results in nonviable daughter cells.

• Meiosis: Nondisjunction can lead to gametes with abnormal chromosome numbers, causing disorders if involved in fertilization.

Example: Down Syndrome (Trisomy 21)

• Down Syndrome is caused by an extra copy of chromosome 21, most often due to nondisjunction during meiosis.

• Incidence increases with maternal age.

Deletions and Translocations

• Deletions: Loss of a chromosome segment. Example: Cri-du-chat syndrome (deletion on chromosome 5) causes intellectual disability and a characteristic cry in infants.

• Translocations: Rearrangement of chromosome segments, which can subtly alter gene function and increase the risk of certain cancers, such as chronic myelogenous leukemia.

Genetic Disorders

Recessive Genetic Disorders

Recessive disorders are expressed only when an individual inherits two defective alleles (one from each parent). Carriers have one normal and one defective allele and do not express the disease.

• Phenylketonuria (PKU): Mutation on chromosome 1 leads to accumulation of phenylalanine, causing intellectual disability if untreated. Detected by the Guthrie test at birth.

• Tay-Sachs Disease: Mutation on chromosome 15 causes lipid accumulation in the brain, leading to neurodegeneration. Common in Ashkenazi Jewish populations.

• Cystic Fibrosis: Mutation on chromosome 7 affects chloride transport, resulting in thick mucus that clogs lungs and pancreas.

Dominant Genetic Disorders

Dominant disorders require only one defective allele for the disease to be expressed. These disorders are less common because they often reduce reproductive fitness.

• Huntington Disease: Progressive neurodegenerative disorder caused by a dominant allele. Symptoms include physical and mental decline, with no known cure. Genetic testing is available.