Zygosity & Vanishing Twins

Question 1

What is definition of zygosity?

Answer 1

Degree of similarity of the alleles for a trait in an organism.

Question 2

What are the 2 types of twins?

Answer 2

• Dizygotic (fraternal)
• Or Monozygotic
• Dizygotic: Develop from 2 eggs each fertilised by different sperm.
• male/female more common.
• Genes In TGF9 signalling pathway
• Incidence increases with maternal age
• Monozygotic: develops from same egg, which splits & forms 2 embryos
• No known maternal age effect.
• Share 100% genes but rarely absolutely identical. Due to epigenetic mechanisms.

Question 3

What is a vanishing twin?

Answer 3

• One twin suffers early fetal death & isn’t aborted, can result in vanishing twin.
• 1 twin suffering early death due to chromosomal abnormality!
• NIPT counting methods can’t distinguish fetal haplotypes assoc with vanishing twins leading to false positives (~15%). cffDNA from demised twin can be detected 8wks or longer after demise.
• ? Some females could be wrongly diagnosed as male due to presence of vanishing (male) twin in female pregnancy.
• Could produce false result in 0.3-0.7% pregnancies.