| The graph shows
average progeny grades plotted against parental grades in the various types of matings.
The circles indicate the average progeny grade for each type of mating and the standard
error " bars above and below, joined by the vertical lines, represent the
"wobble" in terms of progeny with scores above and below the average. The
spacing of the bars (the length of the vertical line) is affected by the numbers of
progeny scored for each parental mating and the consistency of the scores within each
group. For example, the long span between the bars seen with 0 x 3 matings is in part due
to the small numbers of progeny from such matings, but also reflects the fact that some
progeny have high grades and others have low grades. This might be expected if a single
gene, or only very few genes are involved. At the other end of the scale, where there are
more progeny and the range is more limited, as seen with the 0 x 0 matings, the span is
small. The Grade 0/1 condition almost breeds true. It should be noted that when the bars/vertical lines for the progeny
scores with different parental mating types overlap, as for 1 x 1 and 0 x 2 matings, the
progeny scores are not significantly different from each other. However, where there is no
overlap, as between 0 x 0 and 1 x 1 matings, the progeny scores are significantly
different. The difference is real. |
The sloping line
indicates the straight line of best fit to all the data. That it clearly rises as parental
grades increase illustrate how progeny grades relate to parental grades; the worse the
parental grades, the worse the progeny grades, and the better the parental grades, the
better the progeny grades. This shift is statistically highly significant. The overall conclusion from this substantial body of
data is that aortic stenosis does have a genetic basis and that selection against the
condition based upon murmur grade does work.
What is quite remarkable is that these data
are based on data collected over a ten year period during which time the consistency of
the grading has been far from uniform. The grading has shifted each year such that progeny
almost invariably have been scored more rigorously than their parents. With the more
standardised grading that has now been finally achieved the relationship between progeny
and parental grade should become yet more obvious. |
