Unit 3: Sex Determination, Sex-linked Inheritance, and Extrachromosomal Genetics

1. Sex Determination

Sex determination refers to the biological system that determines the development of sexual characteristics in an organism. There are various mechanisms by which sex can be determined, involving genetic, environmental, and physiological factors.

Chromosomal Basis of Sex Determination

Many organisms have specific chromosomes, called sex chromosomes, that determine an individual's sex. These systems differ across species:

• XX-XY System (e.g., Humans, most mammals, Drosophila):

  Females typically have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The presence of the Y chromosome, specifically the SRY gene (Sex-determining Region on Y), usually determines maleness.

  Important Observation: The SRY gene encodes a transcription factor that initiates the development of male characteristics. In its absence, female development occurs.

• ZW-ZZ System (e.g., Birds, some reptiles, some fish, butterflies):

  Females are the heterogametic sex (ZW), while males are homogametic (ZZ). Here, the presence of the W chromosome determines femaleness.

• XO-XX System (e.g., Grasshoppers, crickets):

  Females have two X chromosomes (XX), and males have only one X chromosome (XO). The number of X chromosomes determines sex.

• Haplo-diploidy System (e.g., Bees, ants):

  Females develop from fertilized eggs and are diploid (2n), while males develop from unfertilized eggs and are haploid (n).

Cytoplasmic Basis of Sex Determination

In some organisms, factors within the cytoplasm of the egg, rather than specific nuclear chromosomes, can influence sex determination. These cytoplasmic factors are often maternally inherited.

Example: In certain plants and some microorganisms, cytoplasmic components or genes within organelles like mitochondria can influence the sexual phenotype or fertility.

Hormonal Basis of Sex Determination

Hormones play a crucial role in the development and differentiation of sexual characteristics, especially in vertebrates. While chromosomal mechanisms often initiate sex determination, hormones fine-tune and complete the process.

• Mammals: After genetic sex is established, embryonic gonads develop into testes or ovaries, which then produce sex hormones (e.g., testosterone, estrogen). These hormones drive the development of secondary sexual characteristics.
• Sequential Hermaphroditism: Some fish species can change their sex during their lifetime, often triggered by social cues, where hormonal shifts facilitate the transformation. For example, a dominant female in a group might become male if the existing male dies (e.g., clownfish).

Environmental Basis of Sex Determination

In some species, environmental factors directly influence the sex of an offspring, overriding or interacting with genetic predispositions.

• Temperature-Dependent Sex Determination (TSD):

  Common in many reptiles (e.g., crocodiles, alligators, many turtles, some lizards). The incubation temperature of eggs determines the sex of the hatchlings. Different species have different patterns:

  • Pattern Ia: Low temperatures produce males, high temperatures produce females (e.g., some turtles).
  • Pattern Ib: Low temperatures produce females, high temperatures produce males (e.g., alligators).
  • Pattern II: Intermediate temperatures produce males, while extreme low and high temperatures produce females (e.g., some crocodiles).
• Social Factors:

  In certain fish species, the social structure of a group can determine the sex of individuals. For instance, in some reef fish, the removal of a dominant individual can cause another individual to change sex.

2. Sex-linked Inheritance

Sex-linked inheritance refers to the inheritance pattern of genes located on the sex chromosomes, typically the X or Y chromosome in species with an XX-XY or ZW-ZZ system.

Definition: Sex-linked genes are genes located on the sex chromosomes. Traits determined by these genes are called sex-linked traits.

Characteristics of Sex-linked Inheritance

• X-linked inheritance: Genes are located on the X chromosome. Because males have only one X chromosome, they are hemizygous for X-linked genes.
• Y-linked inheritance: Genes are located on the Y chromosome. Only males can express and pass on Y-linked traits, and they are passed directly from father to all sons.
• Reciprocal Crosses: Often yield different results when dealing with sex-linked traits, unlike autosomal traits.
• Criss-cross Inheritance: X-linked recessive traits are often passed from a father to his daughters, who then pass it to their sons.

Examples of Sex-linked Traits (X-linked Recessive)

Most commonly studied sex-linked traits are X-linked recessive, meaning that a female needs two copies of the recessive allele to express the trait, while a male needs only one copy.

• Red-Green Color Blindness:

  An inability to distinguish between shades of red and green. It is much more common in males because they need only one recessive allele on their single X chromosome. Females require two recessive alleles (one on each X chromosome) to be colorblind, making it rare.

• Hemophilia:

  A bleeding disorder caused by a deficiency in certain blood clotting factors. Like color blindness, it is an X-linked recessive disorder and is more prevalent in males.

• Duchenne Muscular Dystrophy (DMD):

  A severe, progressive muscle-wasting disease caused by a defect in the dystrophin gene on the X chromosome. Primarily affects males.

Common Mistake: Confusing sex-linked inheritance with sex-influenced or sex-limited traits. Sex-influenced traits are on autosomes but expressed differently in males and females (e.g., pattern baldness). Sex-limited traits are also on autosomes but expressed only in one sex (e.g., milk production).

3. Extra-chromosomal Inheritance (Cytoplasmic Inheritance)

Extra-chromosomal inheritance, also known as cytoplasmic inheritance or maternal inheritance, refers to the transmission of genes located outside the nucleus, typically in organelles like mitochondria and chloroplasts.

Definition: Extra-chromosomal inheritance is the inheritance of genetic material that is not part of the nuclear chromosomes. This usually involves DNA found in cytoplasmic organelles.

Principles of Extra-chromosomal Inheritance

• Maternal Inheritance: In most cases, the organelles (mitochondria in animals, mitochondria and chloroplasts in plants) are inherited solely from the mother's egg cell, as the sperm usually contributes only its nucleus (or very little cytoplasm) to the zygote.
• No Mendelian Ratios: Traits inherited extra-chromosomally do not follow Mendelian laws of segregation and independent assortment, nor do they show typical Mendelian phenotypic ratios.
• Non-reciprocal Cross Results: Reciprocal crosses (swapping which parent has the trait) often yield different results, highlighting the maternal influence.

Mitochondrial Inheritance

Mitochondria contain their own circular DNA (mtDNA) and are primarily inherited from the mother. All children of an affected mother will inherit the trait, regardless of their sex. Affected fathers do not pass the trait to their children.

• Characteristics:
  • Passed from mother to all offspring (sons and daughters).
  • Affected fathers do not pass the trait.
  • All offspring of an affected mother are affected.
• Examples of Mitochondrial Diseases:
  • Leber's Hereditary Optic Neuropathy (LHON): Causes sudden vision loss, primarily in young adult males.
  • MELAS syndrome (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes).
  • MERRF syndrome (Myoclonic Epilepsy with Ragged Red Fibers).

Chloroplast Inheritance

Chloroplasts, found in plant cells, also contain their own DNA (cpDNA) and are typically inherited maternally in many plant species, similar to mitochondria.

• Characteristics: Similar to mitochondrial inheritance, often maternal transmission.
• Example: Variegation in Mirabilis jalapa (Four O'Clock Plant):

  Leaf color (green, white, or variegated) is determined by the chloroplasts inherited from the maternal parent. A branch from a white area of a variegated plant will produce all white offspring (if self-pollinated), green from a green area, and variegated from a variegated area, irrespective of the pollen parent's color.

Key Differences from Mendelian (Nuclear) Inheritance