Overview
Today's lecture explored the types of sperm that can be formed by an XYY male, their genetic and phenotypic consequences, and real-life frequencies of related genetic disorders.
Gamete Formation in XYY Male
- An XYY male has a karyotype with two Y chromosomes and one X chromosome.
- Meiosis in XYY males can produce four types of sperm: X, Y, YY, and XY.
- These arise from unusual chromosomal separation during meiosis.
Punnett Square & Offspring Outcomes
- Crossing an XYY male with a normal XX female yields potential offspring with XX, XYY, XY, and XXY genotypes.
- XX and XY produce typical females and males, respectively.
- XYY and XXY result in genotypically abnormal, but often phenotypically normal, males.
Genotypic & Phenotypic Ratios
- Theoretical genotypic ratio: 25% XX (female), 25% XY (male), 25% XYY (male), 25% XXY (male).
- Theoretical phenotypic ratio: 1 female : 3 males since all with at least one Y are male.
- In reality, abnormal genotypes like XYY and XXY are much less frequent due to high rates of self-abortion.
Real-World Frequencies & Disorders
- Turner syndrome (XO): females with one X chromosome, occurs in 1/2,500 live births; higher rates seen in early miscarriages.
- Actual occurrence of XYY and XXY is much less than predicted, often closer to 1% of offspring.
- XYY males (sometimes called "super males") are typically fertile, phenotypically normal, and taller on average.
Myths & Misconceptions
- Claims linking XYY males to violent behavior or higher presence in prisons are unfounded and based on flawed studies.
Sex Ratios in Population
- At birth, sex ratios are roughly 49% female to 51% male, due in part to lighter/faster Y-bearing sperm.
- By adulthood, the sex ratio evens out to about 1:1.
Key Terms & Definitions
- Karyotype — the complete set of chromosomes in an individual.
- XYY male — a male individual with two Y chromosomes and one X chromosome.
- Turner syndrome (XO) — a disorder where a female has only one X chromosome.
- Genotype — genetic constitution of an individual.
- Phenotype — observable physical or physiological traits.
Action Items / Next Steps
- Review Punnett square practice problems for unusual karyotypes.
- Read about sex chromosome aneuploidies and related syndromes.