Sex Determination and Sex-Linked Characteristics: Structured Study Notes

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Sex Determination Mechanisms

Biological Sex and Gender

Biological sex refers to the physical and genetic characteristics that define male and female phenotypes, while gender is a social and cultural construct that may or may not align with biological sex. The process by which sex is established in an organism is called sex determination.

Sexual phenotype: The observable biological characteristics (male or female).
Gender: Assigned based on identity, behavior, or cultural practices; may differ from biological sex (cisgender, transgender, gender fluid).
Hermaphroditism: Both sexes in one organism (common in some animals).
Monoecious: Both male and female reproductive structures in one plant.
Dioecious: Separate male and female individuals (common in many plants).

Alternation of haploid and diploid stages in sexual reproduction

Chromosomal Sex Determination Systems

Many organisms determine sex by specific chromosomes. Sex chromosomes differ in number and morphology between males and females, while autosomes are the same in both sexes.

Homogametic sex: Produces gametes all the same with respect to sex chromosomes (e.g., XX female).
Heterogametic sex: Produces two types of gametes (e.g., XY male).

XX-XY System

Found in mammals, some plants, insects, reptiles.

Females (XX): Homogametic; all eggs carry X chromosome.
Males (XY): Heterogametic; sperm carry either X or Y chromosome.

Human female karyotype (XX) Human male karyotype (XY) Diagram of gamete formation in XX-XY system Punnett square for sex ratio in XX-XY system

During meiosis, X and Y chromosomes pair through small homologous regions called pseudoautosomal regions.

Pseudoautosomal regions on X and Y chromosomes

XX-XO System

Found in grasshoppers and some insects.

Females (XX): Homogametic; all eggs carry X chromosome.
Males (XO): Heterogametic; sperm carry either X chromosome or no sex chromosome.

Grasshoppers as example of XX-XO system

ZZ-ZW System

Found in birds, snakes, butterflies, some amphibians and fishes.

Females (ZW): Heterogametic; eggs carry either Z or W chromosome.
Males (ZZ): Homogametic; sperm carry only Z chromosome.

Birds as example of ZZ-ZW system Butterfly as example of ZZ-ZW system Snake as example of ZZ-ZW system

Genic Sex Determination

In some plants, fungi, protozoans, and fishes, sex is determined by genes at one or more autosomal loci, not by distinct sex chromosomes.

Fungi as example of genic sex determination Papaya plant as example of genic sex determination

Environmental Sex Determination

Environmental factors such as temperature or position can determine sex in some organisms.

Limpets: Sex determined by position in stack.
Crocodiles and turtles: Sex determined by incubation temperature.

Limpet stack showing environmental sex determination Crocodile as example of environmental sex determination Turtles as example of environmental sex determination

Summary Table: Common Sex-Determining Systems

System	Mechanism	Heterogametic Sex	Organisms
XX-XO	Females XX, Males X	Male	Some grasshoppers, insects
XX-XY	Females XX, Males XY	Male	Many insects, fishes, amphibians, reptiles, mammals
ZZ-ZW	Females ZW, Males ZZ	Female	Butterflies, birds, reptiles, amphibians
Genic sex determination	No distinct sex chromosomes; genes on autosomes	Varies	Plants, fungi, protozoans, fishes
Environmental sex determination	Sex determined by environmental factors	None	Invertebrates, turtles, alligators

Table of common sex-determining systems

Sex Chromosome Abnormalities in Humans

Meiotic Non-Disjunction and Chromosome Number Abnormalities

Non-disjunction during meiosis can result in gametes with abnormal numbers of sex chromosomes, leading to syndromes such as Turner, Klinefelter, Poly-X, and XYY.

Turner syndrome (XO): Females with only one X chromosome; short stature, normal intelligence, usually sterile.
Klinefelter syndrome (XXY, XXXY, etc.): Males with extra X chromosomes; tall, normal intelligence, sterile.
Poly-X females (XXX, XXXX, etc.): Females with multiple X chromosomes; severity of intellectual disability increases with more X chromosomes.
XYY males: Males with extra Y chromosome; generally no distinctive features, sometimes linked to learning disabilities.

Turner syndrome symptoms Turner syndrome karyotype Klinefelter syndrome symptoms Klinefelter syndrome karyotype Poly-X syndrome symptoms XYY syndrome karyotype XYY syndrome symptoms

The Role of Sex Chromosomes and SRY Gene

X chromosome: Essential for both sexes; at least one copy required.
Y chromosome: Contains SRY gene, which determines maleness.
SRY gene: Encodes transcription factor for testes differentiation; absence leads to female phenotype.
Testes: Secrete testosterone and anti-Müllerian hormone, influencing sexual development.

SRY gene location on Y chromosome

Androgen-Insensitivity Syndrome

Individuals with XY chromosomes but defective androgen receptors develop female characteristics, despite having testes and producing testosterone. This illustrates the complexity of human sexual development.

Sex Determination in Drosophila melanogaster

Genic Balance System and X:A Ratio

In Drosophila, sex is determined by the ratio of X chromosomes to sets of autosomes (X:A ratio).

Normal female: AA XX (2 X : 2 A = 1)
Normal male: AA XY (1 X : 2 A = 0.5)
Metafemale: AA XXX (3 X : 2 A = 1.5)
Metamale: AAA XY (1 X : 3 A = 0.3)
Intersex: AAA XX (2 X : 3 A = 0.67)

Drosophila melanogaster

Sex-Linked Inheritance Patterns

X-Linked Characteristics

X-linked traits are encoded by genes on the X chromosome. Males are hemizygous for X-linked loci, while females can be homozygous or heterozygous.

Example: White eyes in Drosophila (X-linked recessive trait).
Classic hemophilia: X-linked recessive trait in humans.
Colour blindness: X-linked recessive trait in humans.

Z-Linked Characteristics

In ZZ-ZW systems, Z-linked traits are inherited similarly to X-linked traits, but the pattern is reversed between males and females.

Y-Linked Characteristics

Y-linked traits: Present only in males; passed from father to son.
Holandric traits: Another term for Y-linked traits.

Dosage Compensation and X-Inactivation

Dosage Compensation Mechanisms

Dosage compensation equalizes the amount of protein produced by X-linked genes in males and females. In humans, one X chromosome is randomly inactivated in females, forming a Barr body.

Lyon hypothesis: All but one X chromosome in a cell is inactivated.
Mosaic expression: Females heterozygous for X-linked genes show mosaic patterns (e.g., calico cats).
Xist gene: Responsible for X chromosome inactivation.

Calico cat showing mosaic expression due to X-inactivation Tortoiseshell cat showing mosaic expression due to X-inactivation

Summary

Sex determination involves chromosomal, genic, and environmental mechanisms.
Sex chromosome abnormalities arise from meiotic non-disjunction.
Sex-linked inheritance patterns are distinctive and can be predicted using genetic crosses.
Dosage compensation ensures equal expression of X-linked genes in both sexes.