Sex Determination and Sex Chromosomes: Dosage Compensation, Chromosomal Ratios, and Environmental Effects

Sex Determination and Sex Chromosomes

Dosage Compensation in Mammals

Dosage compensation is a genetic mechanism that ensures equal expression of X-linked genes in males (XY) and females (XX), preventing excessive expression in females. This process is crucial for maintaining genetic balance and normal development.

• Dosage Compensation: Balances the dose of X chromosome gene expression between sexes.

• Barr Bodies: Inactive X chromosomes in female cells, visible as darkly stained bodies in interphase nuclei.

• X-Inactivation: Random inactivation of one X chromosome in each somatic cell, following the N-1 rule (N = total number of X chromosomes; number of Barr bodies = N-1).

• Lyon Hypothesis: States that X-inactivation is random and occurs early in embryonic development; all descendant cells retain the same inactivated X.

• Exceptions: Not all X chromosomes are fully inactivated; about 15% escape inactivation. X-inactivation does not occur in early gonadal tissue development, explaining syndromes like Turner (XO) and Klinefelter (XXY).

• X-inactivation Center (Xic): Region on the X chromosome containing the XIST gene, which is critical for X-inactivation. Other noncoding genes (Tsix, Xite) also play roles.

Example: Calico cats exhibit mosaic fur patterns due to random X-inactivation, leading to different expression of coat color genes.

Example: Barr bodies can be observed in interphase nuclei, and their number follows the N-1 rule. For instance, XXY individuals (Klinefelter syndrome) have one Barr body, while XO individuals (Turner syndrome) have none.

Additional info: The N-1 rule is used to predict Barr body number: $\text{Number of Barr bodies} = N - 1$, where N is the number of X chromosomes.

Chromosomal Ratios and Sex Determination in Drosophila

In Drosophila melanogaster, sex is determined by the ratio of X chromosomes to sets of autosomes (A). The Y chromosome does not determine sex in this species.

• Normal Female: 2X:2A ratio (1.0)

• Normal Male: XY:2A ratio (0.5)

• Intersex: Ratios between 0.67 and 0.75

• Metafemale: Ratio exceeds 1.0 (e.g., 3X:2A)

• Dosage Compensation in Drosophila: Unlike mammals, X-inactivation does not occur. Instead, male X-linked genes are transcribed at twice the rate of females.

• Sxl (Sex-lethal) Gene: Master switch gene controlling female differentiation and dosage compensation.

Chromosome Formulation	Ratio of X chromosomes to autosome sets	Sexual Morphology
3X:2A	1.5	Metafemale
3X:3A	1.0	Female
2X:2A	1.0	Female
3X:4A	0.75	Intersex
2X:3A	0.67	Intersex
X:2A	0.50	Male
XY:2A	0.50	Male
3X:2A	0.33	Metamale

Additional info: The formula for sex determination in Drosophila is $\text{Sex ratio} = \frac{\text{Number of X chromosomes}}{\text{Number of sets of autosomes}}$.

Sex Determination in Caenorhabditis elegans

Caenorhabditis elegans is a nematode worm used as a model organism in genetics. It exhibits two sexual phenotypes: males and hermaphrodites.

• Males: Have only testes.

• Hermaphrodites: Have both testes and ovaries, capable of self-fertilization.

• Self-Fertilization: Hermaphrodites produce eggs fertilized by stored sperm, resulting in >99% hermaphrodite offspring.

• Cross-Fertilization: Males can mate with hermaphrodites, producing 50% male and 50% hermaphrodite offspring.

Temperature-Dependent Sex Determination in Reptiles

In many reptiles, sex determination is controlled by the temperature at which eggs are incubated during embryonic development. This phenomenon is known as temperature-dependent sex determination (TSD).

• TSD Patterns: Three distinct patterns exist, varying by species.

• Species: Crocodiles, most turtles, and some lizards exhibit TSD.

• Mechanism: Incubation temperature during a critical period determines whether the embryo develops as male or female.

Additional info: TSD is an example of environmental sex determination, contrasting with genotypic sex determination found in mammals and birds.

Review Questions and Key Concepts

• Barr Body Calculation: For XXY individuals, expect one Barr body; for XO individuals, expect zero.

• X-inactivation Center: The Xic region is the major control center for X-inactivation in mammals.

• Drosophila Sex Determination: Sex is determined by the balance between X chromosomes and autosome sets.

• Genotypic vs. Environmental Sex Determination: Not all organisms use genotypic sex determination; reptiles use environmental cues (temperature).