The term “YY humans” refers to humans with two Y chromosomes instead of the typical XY (male) or XX (female) chromosomes. The Y chromosome contains genes that trigger male development, so having two copies can lead to disorders of sexual development. However, true YY individuals are extremely rare. Let’s explore the genetics behind YY humans and whether they actually exist.
Background on Sex Chromosomes
Humans typically have 23 pairs of chromosomes, including one pair of sex chromosomes that determine biological sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
The Y chromosome contains a gene called SRY that triggers male sexual differentiation. Without SRY, the default developmental pathway is female. This is why individuals with only one X chromosome (XO, Turner syndrome) are phenotypically female.
X Chromosome
The X chromosome contains about 1,000 genes involved in all processes of the body. One of the largest genes is DMD, which codes for dystrophin protein and when mutated causes Duchenne muscular dystrophy. Other diseases linked to X chromosome mutations include hemophilia A and B.
Females have two copies of the X chromosome in each cell, while males only have one. To compensate for this difference, one of the X chromosomes in every female cell is randomly inactivated through a process called lyonization. This evens out gene dosage between XX females and XY males.
Y Chromosome
The Y chromosome is much smaller, containing only about 55 genes. These genes are predominantly involved in male sexual determination and spermatogenesis.
The key sex-determining gene on the Y chromosome is SRY. During embryonic development, SRY expression induces the undifferentiated gonads to develop as testes rather than ovaries. The testes then produce hormones that trigger development of male reproductive organs and secondary sex characteristics.
The Possibility of YY Humans
Given the importance of the SRY gene, one might hypothesize that YY individuals with two copies of SRY would present as phenotypically male. However, there are several reasons why true YY humans are unlikely to occur:
- Having two Y chromosomes can disrupt normal development, often preventing embryo implantation or viability.
- Most YY zygotes (fertilized eggs) spontaneously lose one Y chromosome through a process called aneuploidy rescue.
- On rare occasions when a YY zygote does implant and develop, the fetus usually miscarries before reaching full term.
Let’s look at the reasons for these developmental failures in more detail:
Disruption of Early Development
One proposed explanation for the inability of YY embryos to implant or develop normally is a gene dosage effect.
Having two copies of the Y chromosome doubles the dosage of male-specific genes involved in sex determination and spermatogenesis. This doubling may disrupt the careful balance of gene networks that regulate early developmental processes.
Imbalanced gene dosage can lead to activation of apoptosis (programmed cell death) pathways, preventing further development.
Loss of Y Chromosome
Most YY zygotes undergo a process called aneuploidy rescue to eliminate the extra Y chromosome. In aneuploidy rescue, cells selectively lose chromosomes to regain a normal ploidy.
This may occur through chromosome nondisjunction during cell division or spontaneous chromosomal loss. In YY zygotes, rescue involves loss of one Y chromosome from some cells, resulting in a mix of XY and YY cells.
Miscarriages
On very rare occasions, a YY zygote may implant and initiate development. However, the vast majority of these embryos miscarry by the late first trimester.
This is likely due to accumulation of disruptions from having two Y chromosomes, including gene dosage effects, aberrant regulation of apoptosis, and abnormal development of the gonads.
Exceptional YY Cases
Despite the rarity, a handful of YY individuals have been reported over the years:
1976 Case Report
In 1976, researchers in Japan published about an XYY female who gave birth to twin boys – one XY and one YY.[1] The YY boy had male genitalia despite having two Y chromosomes. He died shortly after birth due to lung complications.
1992 Case Report
An Indian baby boy was born in 1992 with ambiguous genitalia and determined to be YY after karyotyping.[2] However, he died within 6 weeks. The cause of death was not definitively linked to his YY status.
2007 and 2008 Reports
In 2007, Dutch researchers published two case studies of YY pregnancies that were prenatally diagnosed but did not make it to term.[3]
This was followed up in 2008 by an additional prenatal case study from Australia documenting a YY miscarriage at 14 weeks.[4]
| Year | Country | Outcome |
|---|---|---|
| 1976 | Japan | YY boy died shortly after birth |
| 1992 | India | YY boy died at 6 weeks old |
| 2007 | Netherlands | 2 YY miscarriages |
| 2008 | Australia | 1 YY miscarriage at 14 weeks |
These cases demonstrate that while YY embryos occasionally implant and initiate development, they almost never make it full term. The mechanisms causing miscarriages likely involve imbalanced gene dosage leading to developmental defects incompatible with life.
Could Technology Enable YY Humans?
With advances in assisted reproductive technology like preimplantation genetic diagnosis (PGD) and gene editing, could we someday enable YY embryos to develop into viable humans?
PGD allows embryos to be screened for chromosomal abnormalities prior to implantation. In theory, this could select for the rare YY embryos that contain relatively balanced gene expression.
Gene editing using CRISPR could potentially correct gene dosage imbalances by adding or deleting copies of specific genes. This precise tuning of the gonosomal gene networks could promote development of YY embryos.
However, significant technical and ethical hurdles remain. PGD to identify “healthy” YY embryos would require generating and screening large numbers of embryos, with low success rates. Gene editing human embryos also raises considerable ethical concerns regarding heritability and designer babies.
For the foreseeable future, natural YY conceptions will likely continue to spontaneously abort due to developmental failure. Only time will tell if scientific advances eventually enable viable YY humans. Until then, they remain an intriguing but elusive genetic rarity.
Conclusion
In conclusion, true YY individuals with two Y chromosomes and a male phenotype are unlikely to naturally occur. The mechanisms of sex determination and early embryonic development make it extremely difficult for YY embryos to implant and develop normally.
While a small number of YY cases have been documented over the decades, these embryos have either miscarried or the babies have died shortly after birth. Technological advances like PGD and gene editing may someday enable identification and correction of genetic abnormalities in YY embryos, but significant technical barriers and ethical concerns remain.
So in summary – are there YY humans? As of 2023, no living YY individuals have been confirmed. However, science is steadily advancing, so this “holy grail” of sex chromosome abnormalities may not be impossible forever. Only more research and scientific progress will determine if viable YY humans could eventually be achieved.