Almost certainly too late to respond but will do so anyway incase there is anyone else interested.
Milk yield is an indicator of genetic gain but that itself is dependent on some other aspects namely:
- volume of the chest cavity - a broad rib cage means the cow can eat more kg of feed in a day and therefore has the potential to produce more milk
- height of the cow . As above; a larger animal can eat more in a day and if fed correctly, produce a greater volume of milk
-width of pins and rear legs. Ease of calving (the less traumatic the birth of the calf, the less likelihood of infections like metritis which can suppress milk yields). Rear leg width is more for ease of milking and also for reduced damage to the udder while walking and standing
-rump angle. You want a rump which angles downward toward the back of the animal so that the cow can clean sufficiently after calving. It also means she is less likely to introduce fecal matter into her reproductive tract creating suppressed reproductive performance through infections
- shape of udder and strength of udder attachments (ligaments)
a high (hock level or higher) udder with evenly spaced teats means all quarters are more likely to be milked out (teat size is also a factor in this) and less likey to get infections like mastitis which reduces milk yield
-temperament!! this is for the longevity of the cow in the herd (as are the factors above). Not often thought about in terms of genetic gain for a layman, but you dont want a herd full of cows who are going to kick the cups off and the person cupping them on!
For more information, look up somebody like ABS. They will give a comprehensive list of traits which can be improved upon.
Hope somebody finds this useful!!