Calculate how tall your children will be

I realized today that my post about doing quantitative genetic back-of-the-envelopes was rather wordy. And, I have a hunch that those who “got it” already have an intuitive feel for what I’m talking about, so I thought perhaps the easiest way to get people to develop a better feel was to roll up the calculations into a small Javascript calculator. You can find it below the fold. The terms are defined in the earlier post, but I plugged in the default values (you can change them if you want). Obviously you enter in inches or centimeters depending on whether you’re normal or Canadian. Finally, if you are from another dimension where heritabilities are outside of the 0 to 1 interval and heights are negative then you will encounter problems. Remember the garbage-in-garbage-out principle; I did this in a hurry so no catching your errors and throwing back intelligible warnings.

<!—->

input.calcNorm{ width:50px; clear:none;}
.unitsCm { display:none;}
.unitsIn { display:block; }
span { clear:none;}
div.widG{ padding-bottom:25px;}
#calcOutput {display:none;}

function init()
{
doc = window.document.form;
}
function unitConvert(units)
{
document.getElementById(“calcOutput”).style.display = “none”;
doc = window.document.form;
if (units == “Cm”)
{
doc.heightMother.value = Math.round(doc.heightMother.value/(0.393700787402)*100)/100;
doc.heightFather.value = Math.round(doc.heightFather.value/(0.393700787402)*100)/100;
doc.meanFather.value = Math.round(doc.meanFather.value/(0.393700787402)*100)/100;
doc.meanMother.value = Math.round(doc.meanMother.value/(0.393700787402)*100)/100;
doc.standardDeviation.value = Math.round(doc.standardDeviation.value/(0.393700787402)*100)/100;
}
if (units == “In”)
{
doc.heightMother.value = Math.round(doc.heightMother.value*(0.393700787402)*100)/100;
doc.heightFather.value = Math.round(doc.heightFather.value*(0.393700787402)*100)/100;
doc.meanFather.value = Math.round(doc.meanFather.value*(0.393700787402)*100)/100;
doc.meanMother.value = Math.round(doc.meanMother.value*(0.393700787402)*100)/100;
doc.standardDeviation.value = Math.round(doc.standardDeviation.value*(0.393700787402)*100)/100;
}
}
function calcHeights()
{
doc = window.document.form;
// convert parents to std units
fathDev = (1*doc.heightFather.value – 1*doc.meanFather.value)/doc.standardDeviation.value;
mothDev = (1*doc.heightMother.value – 1*doc.meanMother.value)/doc.standardDeviation.value;
midparent = (1*fathDev + 1*mothDev)/2;
regparam = 1*midparent*(1-1*doc.heritability.value);
finalExpDev = 1*midparent – 1*regparam;
convToUnitExp = 1*finalExpDev*doc.standardDeviation.value;
doc.meanMale.value = Math.round((1*convToUnitExp + 1*doc.meanFather.value)*100)/100
doc.meanFemale.value = Math.round((1*convToUnitExp + 1*doc.meanMother.value)*100)/100
doc.onesiglowmale.value = Math.round((1*doc.meanMale.value – 1*doc.standardDeviation.value)*100)/100
doc.onesighighmale.value = Math.round((1*doc.meanMale.value + 1*doc.standardDeviation.value)*100)/100
doc.onesiglowfemale.value = Math.round((1*doc.meanFemale.value – 1*doc.standardDeviation.value)*100)/100
doc.onesighighfemale.value = Math.round((1*doc.meanFemale.value + 1*doc.standardDeviation.value)*100)/100
doc.twosiglowmale.value = Math.round((1*doc.meanMale.value – 2*doc.standardDeviation.value)*100)/100
doc.twosighighmale.value = Math.round((1*doc.meanMale.value + 2*doc.standardDeviation.value)*100)/100
doc.twosiglowfemale.value = Math.round((1*doc.meanFemale.value – 2*doc.standardDeviation.value)*100)/100
doc.twosighighfemale.value = Math.round((1*doc.meanFemale.value + 2*doc.standardDeviation.value)*100)/100
document.getElementById(“calcOutput”).style.display = “block”;
}
onLoad=init();