A bid to stop parents having "saviour siblings" - babies selected to provide genetic material for seriously ill relatives - has been defeated by MPs.
The Human Fertilisation and Embryology Bill would allow the selection of embryos that are a tissue match for a sick older brother or sister.
But Tory David Burrowes said it was wrong to create a child for the benefit of another, regardless of "the need".
His bid to ban tissue typing outright was voted down by 342 votes to 163.
A further amendment by shadow health secretary Andrew Lansley to only allow tissue typing and sex selection in cases where the other sibling is suffering a life threatening illness was defeated by 318 votes to 149.
The debate followed a vote by MPs to back moves to create hybrid human-animal embryos.
A bid by Conservative ex-minister Edward Leigh to outlaw the measure was defeated by 336 votes to 176 - a government majority of 160.
They are the first in a series of critical free votes on emotive issues in committee stage debate of the Human Fertilisation and Embryology Bill, aimed at updating laws from 1990 in line with scientific advances.
In the "saviour sibling" debate, Mr Burrowes said it was an "important principle" that a child should not be "deliberately created to be used for the benefit of another, no matter how pressing the need".
But Labour's Des Turner said if the Tory MP's bid was successful, "a few children will die because they cannot be treated".
"There is a moral imperative, if therapeutic measures exist, as they do in this case...to use therapies to save lives," he said.
Labour ex-minister George Howarth said: "There is a powerful moral argument in support of the measures contained in the Bill.
"Subject to the safeguards it contains, we should allow medicine to intervene to save more lives and reduce suffering wherever possible."
Dr Evan Harris, the Lib Dems science spokesman, conceded there were strong feelings about the ethical implications of creating saviour siblings.
But he told MPs: "I don't accept the allegation that there will be a burden on the saviour sibling.
"There's no evidence of that. There is evidence that if they are not born in this way then they will suffer bereavement because the older child may die."
He added: "Even if you could forecast circumstances where there would be such a burden, that potential circumstance has to be balanced against the certainty of harm if they are not allowed to do it."
The Human Fertilisation and Embryology Authority has made it clear it would not allow the procedure if there was an alternative which did not involve the destruction of embryos.
The law already allows parents to use IVF procedures to select embryos that will be a genetic match to older siblings, but only when they have a life-threatening disease such as rare blood disorders.