If identical twins are heterozygous for a recessive mutation on an x-linked gene and one inactivates mostly the healthy X and the other mostly the good X, does that mean they are no longer identical?
This is a great question! They will not have the same phenotype as the twin who inactivates mostly healthy Xs will develop the disorder to an extent depending on the ratio of her cells using the mutated and healthy X’s. They will still have the same genotype so what has happened is that the expression of the genotype has altered. .X inactivation occurs in early embryogenesis about the 16 to 64 cell stage. This means that by chance it is possible for an embryo to inactivate in a skewed or non random way. About 5% of carriers of X-linked Duchenne muscular dystrophy have muscular symptoms. Doctors have noticed that identical twins who carry an X-linked recessive disease are often discordant for the phenotype (one is affected, the other is healthy). Since twinning often occurs about the same stage as X-inactivation, it is thought that phenotypic differences between the cell lines using each X chromosome may precipitate the twinning process.
Related Questions
- If identical twins are heterozygous for a recessive mutation on an x-linked gene and one inactivates mostly the healthy X and the other mostly the good X, does that mean they are no longer identical?
- Why does a mutation that deletes one or two DNA nucleotides changes gene function more drastically than a substitution of one nucleotide for another type?
- Do identical twins have the same gene i.e. the same type of DNA?
