Human fertilisation and embryology bill

MPs are voting on a series of interlinked ethical issues about science's role in the creation of life today and tomorrow.

The government has permitted free votes on four issues, all of which have important implications for the way humans start their lives.

Hybrid embryos combine human DNA with animal eggs and could be used to help treat diseases like Parkinson's disease.

'Saviour siblings' involve the creation of a new child with their genes altered, allowing children to be created with a tissue match that can help treat their sick brother or sister.

MPs will also vote on whether lesbians should be awarded equal rights in terms of IVF treatment.

And there is also another challenge looming on the perennial abortion issue. Should the upper limit, currently at 24 weeks, be lowered to 22, 20 or 16 weeks?