Mathematical Monday: Family Genetics

I’m back, sort of, or at least this week, at least through Thursday. Anyway, time for posts, yay!

First of all, it’s 02-21-2011 or 2-21-11, that combination of digits amuses me for no significant reason. Tomorrow will be 02-22-2011. That’s a lot of two’s.

Okay, onto the real Math Monday. Dawn is a miracle child in my family. Why do I say that? Well, because the probability of her features as they exist is only 1.6%. Yup, today I want to talk about genetics and probabilities. Partially because I have been talking a bit about genetics on my job interviews, and partially because my family has always been a bit obsessed with them. Salient points:
– We call Dawn the “recessive” child due to her very visible blond hair and blue eyes, both recessive genes. She’s also a lefty. Skinny might be recessive too, we’ll see.

Ravenclaws say Caw!

– Gardella used to spend hours putting together families by drawing parents then determining their genotype and using random numbers and probabilities to determine what their children would look like.

– I’m a molecular (micro)biologist. That means I work with DNA on a daily basis. I think Mendel was one awesome monk. Did you know he used peas because the monastery frowned about cross-breeding animals on their property? Good thing too, because peas have simple genes, and mouse fur colors are much more complex

Anyway, you know how a basic Punnet Square works? If not, I’ll imagine you clicked the link and now you do. Well, in our family, everyone has brown hair except for Dawn, who’s blond. That means my parents much have to be recessive gene carriers like this:

Don't it make your brown eyes blue?

So, B is brown dominant, so BB and Bb make brown hair and bb makes blond (yes, I’m ignoring red right now, but we have no ginger genes in my family: Jewish and German!). Therefore, my parents only have a 25% chance of having a blond child.

You can draw similar squares for eye color (Mom and me are dark brown, Dad and Gardella are hazel, only Dawn has blue) and for handedness (all right-handed except Dawn). That leaves a 25% of each of those traits. So, when you combine the three 25% chances (25% * 25% * 25%) you get 1.6% or 1/64 chance. That’s why Dawn is so awesome. So is probability.

As a side note. I married a left-handed man. My chances of carrying a left-handed gene (knowing both parents are carriers) is 67%. He has two. Therefore, the chance of our child being left-handed is 33%. That’s pretty high. He also carries blond and blue-eyes genes from his father, so I may have a daughter that reminds me of my sister. Now you know how that identical cousins thing happens!


13 Comments (+add yours?)

  1. Sarah
    Feb 21, 2011 @ 11:25:15

    Genetics has always fascinated me as well! I’m glad to see I’m not the only one.

    I’m psyched because there’s a chance I could have a green-eyed child, since my husband’s brother has a green-eyed girl (I’m green-eyed, my hubby is blue).

    But we’re not for sure on the percentage because they are half-brothers only but it’s still a chance!


  2. K_Commenter
    Feb 21, 2011 @ 11:45:13

    I am a tall, red-haired, blue-eyed child of two short, dark-haired, brown-eyed parents (with mainly dark-haired, brown-eyed aunts and uncles as well). The math of recessive genes has fascinated me since I first learned about the concept as a kid.

    (I married a guy with light brown hair, with red in his beard, and hazel eyes. Whatever our kids turn out like, they’ve certainly got a mix of genes to pull from.)


  3. SDC
    Feb 21, 2011 @ 13:38:19

    My daughter has a blue eye and a green eye. I’m not sure what the probability of that was, but my eyes are green and my wife’s are blue.


  4. Edana
    Feb 21, 2011 @ 14:54:03

    Huh, I’ve always learned that none of those are single-gene traits–do punnet squares really work for them? And handedness isn’t entirely genetic, is it? I read somewhere that a pair of identical twins with one lefty only had a 75%-ish chance of both being lefties.


    • GirlsAreGeeks
      Feb 21, 2011 @ 21:48:37

      It’s true that things are generally more complicated than I put in this post, but the basics still generally work out, or at least are fun to play with even knowing they are quite accurate. However, I do have to mention that our recessive Dawn has dominant dimples that no one else in the immediate family has!


  5. Neeni
    Feb 21, 2011 @ 17:44:47

    Genetic probability always amused me. My little sister looks nothing like the rest of the family. And none of the kids look anything alike. I seemed to have picked up the Irish/Scottish genes from my mother’s side, my sister picked up the Native and British genes from my father and my brother picked up the Scottish and German genes from my mother. I have dark brown hair and green eyes, my sister has blonde hair and blue eyes, and my brother has copper-y red-brown hair with brown eyes. It’s just weird.


  6. Jennifer
    Feb 21, 2011 @ 19:56:17

    talk about recessive genes. both of my parents are fair skinned (freckled even) and have green eyes, my dad while I give him some credit for the black hair is really the only thing I get from either. My olive skin, and brown eyes come from what I am apparently told is my paternal grandmother’s father. Even those in attendance at my Nanny’s funeral said I look like him. That’s 3 generations. DNA is odd.


  7. Eleni
    Feb 22, 2011 @ 03:01:48

    Cool! I wonder about the complexity of eye color genes. My mom is Chinese and has only dark brown eye genes, and my dad has blue eyes, and while my brothers have dark brown eyes, mine are a lighter sort of hazel. Anyway, even thought I have no aesthetic preference for blue vs. brown vs. other eye colors, for the sake of genetics I have a secret wish to marry a man with blue eyes or at least a blue-eyed parent, so I have a chance of having a blue-eyed child.

    Your genetics Mathematical Monday post has inspired me to write a Mathematical Monday post of my own. Math is great!


  8. Avalyn
    Feb 22, 2011 @ 12:47:26

    This is weird because I was just thinking about genetics yesterday, but on a different topic. I was trying to figure out how closely related I am to my double first cousins (that means we are cousins twice because our dads are brothers and our moms are sisters, so we share the same geneological background except for our immediate parents). I’ve read that we are considered as close a an uncle/aunt to a nephew/neice and that we have twice the consanguinity as first cousins and are as related as half-siblings. However, couldn’t the actual genetic relationship be anywhere from 0% to 100%, depending on which genes we happened to get passed down from our shared 4 grandparents? Of course we would have a much larger chance of 100% similarity if our dads and/or moms had been identical twins. Since they are not, our similarities will be closer to 25% than 100%. I think a DNA analysis is really the only thing that could tell.


  9. J
    Mar 08, 2011 @ 20:02:02

    I LOVE this topic – mostly because my three older sisters and I pretty much make our own Punnet Square.

    Funny thing about eye color, though. My eyes are green – now. However, they were very clearly blue, not just when I was a baby, but all the way until I was at least eight or nine years old, when they began changing. By thirteen, they were the color they are today. And I’m the only one.

    I’ve found two possible explanations. One, it’s a weird sex-linked trait linked to puberty. The other, it’s due to sudden increased sun exposure. As someone who has one blue-eyed parent, and one blue-or-maybe-green parent, AND who moved to a sunny climate when I was six, both are distinct possibilities.

    Thus, I have no idea if I’m handing down a blue (green) gene, or a blue (blue) gene. And that drives me crazy!


    • GirlsAreGeeks
      Mar 08, 2011 @ 20:59:11

      That is interesting! Of course, unless you marry either a blue or green eyed man, you may not know any time soon! Then again, green-eyed men are quite sexy, both the oh so hot boys of Supernatural have green eyes.


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