Biology Chapter 7 & 4: Sex Determination

Mode of Sex Determination

• XX/XO, XX/XY, and ZZ/ZW are different modes of sex determination.
• XX/XY: Female gametes contain one X chromosome; male gametes have either X or Y chromosome.
• ZZ/ZW: Females are ZW (heterogametic) and males are ZZ (homogametic); Z chromosome is larger than W, which is smaller.

Homogametic and Heterogametic Sex

• Homogametic sex has two like chromosomes and produces identical gametes.
• Heterogametic sex has two unlike chromosomes, producing two types of gametes (e.g., 50% X and 50% Y in males; 50% Z and 50% W in females).

Y Chromosome and Maleness

• The presence of a Y chromosome determines maleness in humans.
• Humans have 22 pairs of autosomal chromosomes and one pair of sex chromosomes; females are XX and males are XY.

Klinefelter and Turner Syndromes

• Klinefelter syndrome (47,XXY) and Turner syndrome (45,X) result from nondisjunction during meiosis, leading to atypical sexual development.

Triplo-X Syndrome

• Triplo-X syndrome (47,XXX) features three X chromosomes; many individuals are normal, while some may show underdeveloped secondary sex characteristics. Possible sterility and cognitive impairment.

XYY Syndrome

• XYY syndrome (47,XYY) is characterized by tall stature (often over 6 feet) and potential subnormal intelligence; also associated with personality disorders.

Y Chromosome Characteristics

• Y chromosome is smaller and contains fewer genes than X chromosome.
• Pseudoautosomal regions (PARs) on both ends of the Y chromosome allow for pairing with X during meiosis.

Dosage Compensation

• Dosage compensation balances the expression of X-linked genes between males and females.
• Inactive X chromosomes condense into Barr bodies, which are present in females.

Lyon Hypothesis

• Lyon hypothesis states that X-inactivation is random in somatic cells early in embryonic development, resulting in uniform X-expression in descendant cells.
• Example: Calico cats display varied fur patterns due to random X-inactivation affecting color genes.

X-Inactivation

• X-inactivation is the genetic mechanism explaining dosage compensation and follows the N - 1 rule, where N equals the total number of X chromosomes.

X-Linkage

• X-linkage refers to traits determined by genes on the X chromosome, differentiating them from autosomal traits.
• Females can be homozygous or heterozygous for X-linked traits, while males are hemizygous (one dosage).

Color Blindness

• Red/green color blindness is a common X-linked trait in the USA, impacting males more frequently due to their single X chromosome.

Modes of Sex Determination

• XX/XO system: Gender determined by X chromosome dosage; female zygotes have two Xs, males have one X.
• XX/XY system: Female gametes carry one X, while male gametes can carry either X or Y chromosomes.
• ZZ/ZW system: Z chromosome (large) and W chromosome (small); females are ZW and males are ZZ. Example: Chickens.

Homogametic and Heterogametic Sex

• Homogametic sex has two identical chromosomes (e.g., females XX) and produces like gametes.
• Heterogametic sex has two different chromosomes (e.g., males XY) and produces unlike gametes (50% X and 50% Y).

Human Y Chromosome and Maleness

• Human karyotype features 22 pairs of autosomes and one pair of sex chromosomes.
• Females have XX chromosomes, while males have XY chromosomes, with the presence of the Y chromosome determining maleness.

Klinefelter and Turner Syndromes

• Klinefelter syndrome (47,XXY) and Turner syndrome (45,X) result from nondisjunction, leading to abnormal sexual development.
• Nondisjunction is the failure of chromosomes to separate properly during meiosis.

Triplo-X Syndrome

• Characterized by the presence of three X chromosomes (47,XXX) and normal autosome count.
• Can lead to underdeveloped secondary sex characteristics, sterility, and mental retardation in some cases, often due to nondisjunction.

XYY Syndrome

• Notable for males being often over 6 feet tall, along with possible subnormal intelligence and personality disorders.
• Occurs from nondisjunction during meiosis, where Y chromosome sister chromatids fail to separate.

Y Chromosome Characteristics

• The Y chromosome is smaller and contains fewer genes compared to the X chromosome.
• Pseudoautosomal regions (PARs) are found on both ends of the Y chromosome, allowing for pairing and recombination with the X chromosome during meiosis.

Dosage Compensation Mechanism

• Dosage compensation is a genetic mechanism to balance the gene expression of X chromosomes in both males and females.
• Prevents excessive expression of X-linked genes, utilizing Barr bodies (inactive X chromosomes).

Lyon Hypothesis and X-Inactivation

• The Lyon Hypothesis states that X chromosome inactivation is random and occurs early in embryonic development.
• Once an X chromosome is inactivated, all descendant cells retain that inactivation pattern, affecting traits like fur color in calico cats.
• X-inactivation follows the N − 1 rule, where N is the total number of X chromosomes.