Sex-linked inheritance refers to the transmission of traits determined by genes located on the sex chromosomes, which in humans are the X and Y chromosomes. This type of inheritance is distinct from autosomal inheritance, which involves genes on non-sex chromosomes.

Types of Sex-Linked Inheritance

1. X-Linked Inheritance: Involves genes on the X chromosome.

2. Y-Linked Inheritance: Involves genes on the Y chromosome.

X-Linked Inheritance

X-linked inheritance involves genes found on the X chromosome. This can be divided into two categories: X-linked recessive and X-linked dominant.

X-Linked Recessive Inheritance

Key Features:

• More common in males: Males have only one X chromosome, so a single recessive allele on that X chromosome will cause the trait to be expressed.

• Females are often carriers: Females have two X chromosomes, so a recessive allele on one X chromosome can be masked by a dominant allele on the other X chromosome.

Examples:

1. Hemophilia:

   • A bleeding disorder where blood does not clot properly.

   • More common in males.

2. Duchenne Muscular Dystrophy (DMD):

   • A severe muscle-wasting disease.

   • Typically affects males and is rare in females.

3. Red-Green Color Blindness:

   • Affects the ability to distinguish between red and green colors.

   • Much more common in males.

Inheritance Pattern:

• Mother as a Carrier (XN^NNXn^nn) and Unaffected Father (XN^NNY):

  • Sons have a 50% chance of being affected (Xn^nnY).

  • Daughters have a 50% chance of being carriers (XN^NNXn^nn) and a 50% chance of being unaffected (XN^NNXN^NN).

X-Linked Dominant Inheritance

Key Features:

• Affects both males and females, but can be more severe in males.

• A single copy of the dominant allele on the X chromosome causes the trait to be expressed.

Examples:

1. Rett Syndrome:

   • A neurodevelopmental disorder that primarily affects females.

   • Males with the mutation typically do not survive to birth.

2. Fragile X Syndrome:

   • Leads to intellectual disability and developmental issues.

   • More severe in males.

Y-Linked Inheritance

Y-linked inheritance involves genes located on the Y chromosome, which is only found in males. Since the Y chromosome is much smaller and carries fewer genes compared to the X chromosome, Y-linked traits are rare.

Key Features:

• Only affects males.

• Traits are passed directly from father to son.

Examples:

1. Y-Linked Infertility:

   • Genetic conditions affecting spermatogenesis and resulting in male infertility.

2. Hairy Ears:

   • A rare trait where males have excessive hair growth on the ears, linked to a gene on the Y chromosome.

Key Differences Between X-Linked and Y-Linked Inheritance

• X-Linked Inheritance:

  • Can affect both males and females.

  • More complex inheritance patterns due to the presence of two X chromosomes in females.

  • Traits can be dominant or recessive.

• Y-Linked Inheritance:

  • Only affects males.

  • Simple inheritance pattern: passed directly from father to son.

  • No equivalent traits in females.

Sex-Limited Inheritance

Sex-limited inheritance refers to traits that are expressed in only one sex, even though the genetic information for the trait may be present in both sexes. These traits are not necessarily linked to sex chromosomes but are often influenced by sex hormones and physiological differences between males and females.

Key Characteristics of Sex-Limited Inheritance

1. Expression Limited to One Sex:

   • The traits are expressed in only one sex, regardless of whether the genes are present in both sexes.

2. Influence of Hormones:

   • The expression of sex-limited traits is often influenced by sex hormones such as testosterone, estrogen, and progesterone, which can activate or suppress gene expression.

3. Not Restricted to Sex Chromosomes:

   • These traits can be carried on any chromosome (autosomes or sex chromosomes), but their expression is confined to one sex due to physiological or hormonal reasons.

Examples of Sex-Limited Traits

1. Milk Production in Mammals

Description:

• Only females produce milk, even though both males and females have the genes necessary for milk production.

• The trait is activated in females due to the presence of specific hormones such as prolactin and oxytocin during and after pregnancy.

Inheritance Pattern:

• Genes related to milk production are present in both sexes but are only expressed in females.

2. Beard Growth in Humans

Description:

• Beard growth is typically seen in males, driven by androgens like testosterone.

• Males and females both carry the genes responsible for hair growth, but in females, the lower levels of androgens prevent significant beard growth.

Inheritance Pattern:

• Genes for beard growth are autosomal and can be passed to both sexes, but significant expression is limited to males due to higher levels of androgens.

3. Male Sexual Characteristics in Animals

Description:

• Characteristics such as the presence of antlers in male deer or bright plumage in male birds are sex-limited traits.

• These traits are influenced by androgens and are typically used in mating displays or competitions.

Inheritance Pattern:

• Genes for these traits are autosomal and can be inherited by both sexes, but expression is limited to males due to hormonal triggers.

How Sex-Limited Traits Work

• Genetic Basis: The genes for sex-limited traits are typically autosomal, meaning they are not located on the sex chromosomes (X or Y).

• Hormonal Influence: The expression of these genes is regulated by hormones, which are differentially present in males and females.

  • Example: In the case of milk production, prolactin and oxytocin play crucial roles in females, triggering the expression of genes involved in lactation.

• Physiological Factors: Physiological differences between sexes also contribute to the limitation of these traits to one sex.

  • Example: Male deer have the physiological ability to grow antlers, while females do not, even though both may carry the necessary genes.

Sex-limited inheritance exemplifies how genetic and hormonal interactions contribute to the diversity of trait expression in different sexes, and it underscores the complexity of genetic inheritance beyond the simple presence of specific genes. 

Sex-influenced inheritance refers to traits that are determined by genes located on autosomes (non-sex chromosomes), but their expression is influenced by the sex of the individual. Unlike sex-linked traits, which are tied to genes on the sex chromosomes, sex-influenced traits can appear in both males and females but are expressed differently depending on the individual's sex due to hormonal differences.

Key Characteristics of Sex-Influenced Inheritance

1. Expression Varies by Sex:

   • Traits are present in both sexes but their expression varies in prevalence or intensity between males and females.

2. Hormonal Influence:

   • The different hormonal environments in males and females, particularly involving androgens (male hormones) and estrogens (female hormones), can lead to differing expressions of the same trait.

3. Not Sex Chromosome Linked:

   • The genes for these traits are located on autosomes, meaning they are not linked to the X or Y chromosomes.

Examples of Sex-Influenced Traits

1. Pattern Baldness

Description:

• Pattern baldness, or androgenic alopecia, is a common example of a sex-influenced trait.

• The trait can be seen in both males and females but is more commonly expressed in males and often in a more severe form.

Influence of Hormones:

• The expression of pattern baldness is influenced by androgens (such as testosterone), which are present at higher levels in males.

• Males typically need only one allele for baldness to show the trait, while females generally need two alleles for the trait to be expressed.

Inheritance Pattern:

• Both males and females can carry the gene for baldness, but its expression depends on androgen levels, which are higher in males.

2. Voice Pitch

Description:

• The pitch of the human voice is a trait influenced by sex. Both males and females have genes that affect voice pitch, but males typically have lower-pitched voices.

Influence of Hormones:

• During puberty, increased levels of testosterone in males cause the larynx to enlarge and the vocal cords to lengthen and thicken, resulting in a deeper voice.

• In females, lower levels of testosterone and higher levels of estrogen prevent such significant changes, leading to higher-pitched voices.

Inheritance Pattern:

• Both sexes inherit genes related to voice pitch, but hormonal differences during puberty lead to the characteristic differences in voice pitch between males and females.

3. Cleft Chin

Description:

• The presence of a cleft chin is a sex-influenced trait, often more prominent in males than females.

Influence of Hormones:

• Androgens can influence the development of facial bone structure, making cleft chins more common or noticeable in males.

Inheritance Pattern:

• Both sexes can inherit the gene for a cleft chin, but the trait's expression is more prominent in males due to higher androgen levels.

Sex-influenced inheritance highlights how genetic traits can manifest differently in males and females due to hormonal differences. Understanding these patterns is vital for accurately predicting and managing traits and conditions in both humans and animals.