A/N I don't own Harry Potter. Any errors in the science are mine. Multiple miscarriages mentioned.
February 1973
Narcissa Black was sat in the Hogwarts library writing her Ancient Runes essay. In five short months, she would have graduated. In the month following her graduation, Narcissa Black would be marrying the most eligible bachelor in society. It was an arranged marriage, the contracts signed and sealed, not even Andromeda running off with a Muggle-born had broken them. She had butterflies thinking about it.
Narcissa missed her sister. She couldn't tell anyone that, and she knew that burning in her from the family tree was the only way their family could keep their standing. Bella was intense and preferred her friend's company to her little sister. Andromeda had spent time with Narcissa and not made it feel like a chore. It was Andromeda who had shown Narcissa as a first year how not to get lost or caught on the moving staircases of the castle. How to get around to all her classes. How to hold her head up with pride in the Great Hall, and now she was gone. The loss of her eldest sister was still new and raw, but it would never show on Narcissa's face. She was a Black after all.
Narcissa sighed wistfully and wrote another sentence. Summer was coming, and with it, her new life stretched out before her. In the meantime, she would be here completing essays. She'd be burnt off the family tree herself before anyone thought her a trophy wife - all looks and no brain. Lucius liked that she was clever, liked that she was cunning; growing up a daughter of the House of Black had ensured both.
Narcissa glanced idly around the library looking for inspiration for the rest of the Runes essay. It wasn't her strongest subject, and she privately thought four feet of parchment discussing why the chosen alphabet was no longer used in favour of another, an exercise in futility.
In front of her table, there were two Ravenclaws. A pureblood of a minor but popular house and a girl she didn't know. Muggle-born or half-blood she supposed.
The witch she didn't know was gesturing to her parchment "... No look here, can you see if I draw the tree this illustrates the lack of diversity."
"So, this is the reason for the low birth rates?" her friend asked.
"Yes, you remember I told you my Dad was a geneticist? I asked him to run my blood to map my DNA so we could examine the chromosomes that match his. Then by mapping what I got from him, we could isolate the chromosomes that matched my Mum and maybe eventually find the gene for magic."
"But how does this relate to your Arithmancy? I thought you were doing something with dragons?"
"Oh well I am; only I'm not. The gene pool I'm looking at is the magical one. The birth rates are low and have been so for generations. I know everyone sniggers about the Houses being interbred, but it's true. I started mapping the ways that they are related to find out how small their genetic sample was, and well, it's not good."
"What do you mean not good?"
"The more a group of people have children with each other the smaller the selection of genes are. Your genes are what makes you, you. Responsible for characteristics like red hair or green eyes. They are made up of twenty-three chromosomes. You get half your chromosomes from each of your parents. Who in turn get theirs from their parents all the way back. It's why the Weasley's have red hair or the Black's have grey eyes. It's a characteristic that dominates in their genes which are passed down. The problem lies in the lack of well, variety in the gene pool. The more they stay the course of 'pure' the more birth defects and failed pregnancies they will have. The more diverse the gene pool, the more diverse the genes, reducing the likelihood of birth defects and miscarriage and infant mortality in a broad sense."
"But won't it dilute the magic, my Mum…"
"Has never heard of genetics. If purebloods have children with half-bloods or Muggleborn the genes won't dilute, there's no such thing. It's twenty-three chromosome pairs nothing else. What you will find is that the familiar traits such as blonde hair or blue eyes would become less common. But do you want healthy magical children or clones? Because the clones are what are killing the magical communities."
"But still, I thought you were doing dragons."
"My Arithmancy professor is a pureblood who can trace her lineage back to the 1400's. What do you think she would say if I handed her an extra credit project that detailed why her chances of having a healthy magical child were less than one in ten."
"What, really? Merlin, you can't hand that in!"
"The numbers don't lie, look." She tapped her wand on something Narcissa couldn't see and a perfectly solved equation span slightly above the two witches.
"That's why I've changed everything to dragons, the numbers are sound, it's true for dragons as much as anyone."
"What are you going to do with it, though? You've been working on this for years."
"Take it to St Mungo's. I've got my apprenticeship sorted with them, if I help just one person then it will be worth it."
"You want to help the pureblood keep the power they use to treat you as less?" her companion asked sceptically
"No one deserves to go through a miscarriage let alone as many as some of them do, and death in childbirth is still happening. Snotty pureblood or not."
Narcissa stopped listening, she hadn't understood most of what they had been discussing, but she'd heard the highlights. She knew of the problems of producing an heir that some of the pureblood families had. Lucius himself was an only son, and his parents had been in their forties when he'd been born.
Narcissa shook herself. It didn't matter what the Muggleborn said, she would be fine. She would have children with Lucius, and they would be beautiful. She packed up her things giving up on the essay, for now, she'd come back to it. She wanted to be anywhere but here listening to these witches tell her she wouldn't be able to have healthy children, wouldn't be able to fulfil the duty of a pureblood wife.
It was an arranged marriage, but that didn't mean love couldn't develop. Lucius liked her, that was a start, and she'd be a good wife. She'd learn to love him and him her, and they would have happy, healthy children, they would.