Tuesday, 3 February 2009

Please don't make your children eat Brussels Sprouts

I hate Brussels Sprouts and as a child my mom would make me eat them. Just putting them on my tongue would make me gag. There's always been this love-hate aura around brussel sprouts; some people love them, others hate them - they're best left off menus. The new (Feb 2009) issue of Runner's World magazine has a piece on "Eat This Now: Brussels Sprouts" and how nutritious these little cabbage-relatives are. If you like them, fine, but I want to plead with you not to force your children to eat them if they don't like them. And this is why...

Gregor Mendel and inheritance
At university my study path was in genetics, cell biology, zoology... It was in a first year "Introduction to Genetics" block that Mendel's Laws of Heredity (segregation, traits and inheritance) were drummed into us. Yes, the same stuff as those sweet pea case studies that you did in school biology with the pink and white flowers crossing and tall and short plants crossing and what their progeny ("children") would look like.

It was here that I learned that my hatred for Brussels Sprouts has a genetic element (more on this later).

Dominant and recessive
Everyone has two copies of genes; one from your mom and one from your dad, contributed by the egg and sperm respectively. Some traits are controlled by single genes; some are controlled by multiple genes and present variable results (like hair colour and eye colour).

In this posting I'm talking about traits that are expressed by a a single gene.

Some traits are dominant and others are recessive. It's a bit like a game of "ching-chong-cha" (rock-paper-scissors). Rock beats scissors because rock is dominant to the recessive scissors (if rock and scissors were tie, they would be codominant traits; like a red plant and white plant producing pink progeny). This post only related to dominant trumping recessive hands-down, not sharing the spoils.

If your mom passes on a dominant copy of a gene to you and dad passes on a recessive form of the gene, you'll present your mom's trait - "You've got such a cute cheek dimples when you smile; just like your mom" is a perfect example.

What you'll notice is that you can display a trait - like dimples - even though you've only got one copy of the gene that says "give him dimples" and one that says "don't give him dimples". So, those with 2 x dimple genes (from mom and dad) and those with 1 x dimple gene (from mom only), will have dimples. Those with 2 dimple genes we call homozygous dominant (both the same), those with 1 dimple gene we call heterozygous (mixed; half-half) and those with 2 x copies of genes that don't make dimples are homozygous recessive.

Using the rock and paper example... If mom is a rock, she is either homozygous (two dominant copies from her parents) or heterozygous (one dominant and one recessive from her parents) dominant. Dad is paper so he is homozygous recessive (two recessive copies from his parents). Taking that each parent contributes one copy of their genes, their children will present with the following:

What Mendel discovered, through his painstaking work with pea plants and unbelievable patience and attention to detail (that he was a monk probably helped), is that the traits presented by progeny are directly determined by their parents - back then they didn't know about genes (Mendel called these hereditary elements "factors") and that you got one from mom and one from dad; he figured this out. And, he also worked out the statistical likelihood that progeny would have at trait (like dimples) if mom had them and dad didn't.

What does this have to do with Brussels Sprouts?
Like cheek dimples, blood type (think paternity; if mom is O and dad is O and you are A, dad isn't your daddy), cleft chin, albinism, preference for Brussels Sprouts is genetically inherited. OK, so it isn't exactly about Brussels Sprouts (BS), but about an organic compound in these awful things, phenylthiocarbamide, also known as PTC, or phenylthiourea. Actually, food doesn't contain pure PTC, which is toxic, but related compounds.
The ability to taste PTC (and related compounds) is genetically determined by a a specific gene. To those with dominant copies (homozygous dominant or heterozygous) of the gene PTC tastes very bitter. To those with the recessive form of the gene PTC is virtually tasteless. And logically, the ability to taste the bitterness of thiourea compounds will determine what foods people do and don't consume and enjoy i.e. I don't like Brussels Sprouts so I don't eat them.
Guess what? An 50-70% of people can taste PTC; and this varies according to population group. There's a test they do with filter paper saturated in PROP, a compound closely related to PTC but with lower toxicity. We did this in first year; the paper had hardly touched my tongue before I ran retching to the basin in the lab. I'm evidently a supertaster for this compound and my reaction to it is strong.
It is your fault that your children don't like Brussels Sprouts
My mom thinks Brussels Sprouts taste like cabbage; she is a recessive. I think they taste vile. Because my mom is a recessive I would have to be heterozygous, getting a dominant gene from my dad, who would be either homo- or heterozygous. Oddly enough, my dad eats them, although they're not his favourite vegetables. But, he is a smoker, and has been for most of his life. Smokers are relatively insensitive to PTC and PROP. It's either this, or he isn't my dad.
I'm all for making children eat peas, carrots, beans and loads of other veggies. But when they say that Brussels Sprouts taste bad, they're not just being picky - Brussels Sprouts can taste really bad to them, and not you.
--> Great paper on the whole discovery of this PTC thing, Phenylthiocarbamide: A 75-Year Adventure in Genetics and Natural Selection by Stephen Wooding, published in the journal Genetics in April 2006


N said...

Working with the rock-paper-scissors analogy, surely paper should be dominant and rock recessive? :-D

adventurelisa said...

Yes, you're right... if we're talking of rock vs paper. I was talking rock vs scissors.
Ok, so my analogy has its flaws but whilst I was in the spirit of writing this I thought it was quite brilliant ;)

Danel said...

Well, brussels sprouts do taste bitter to me but I absolutely love them. I think the bitterness works well with the nuttiness. How does that make any sense?

adventurelisa said...

Nope, doesn't make any sense Danel. I think you're like my dad who doesn't think they're great but he eats them anyway. I have a feeling that like hair and eye colour there is a degree of variation in how you perceive taste - either genetic or environmental, the latter according to what you're used to and tastes (sweet, sour, salty etc)you enjoy.