How many Test Crosses are needed

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Vetrock

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I can't find anyone to give me a straight answer on this. If I wanted to prove a bull was homozygous for a certain gene (for instance the black gene in an Angus bull), how many test crosses are required to prove such? Does anyone have a number?
 
I don't know if there is a straight answer to 100% certainty.
It's possible 1 cross can prove a bull is not.
My estimate is 16 would put you in the range of 94% certainty and it would take many more to reach 99.4%
DNA test would tell you heterozygous or homozygous to 100% certainty and you would know much faster.
 
Breeding to what cows? a set of blacks? reds? whites? chocolates? Then you could have diluter genes? Like SoB said DNA would be the easiest to know for certain.
 
I believe the number they used to use for genetic testing before DNA was breeding to 20 daughters. I might be wrong on this it may have been 30 but pretty sure it's one of those two.
 
Son of Butch":3dembz98 said:
I don't know if there is a straight answer to 100% certainty.
It's possible 1 cross can prove a bull is not.
My estimate is 16 would put you in the range of 94% certainty and it would take many more to reach 99.4%
DNA test would tell you heterozygous or homozygous to 100% certainty and you would know much faster.
Click to expand...

A lot depends on the breed of cow as well as several breeds don't play by the rules.
 
I don;t know how many is required but I know 16 isn;t enough. Granny was a black Angus red carrier. Bred to either a Polled Hereford, red Fleck or a Red Angus the first year she calved for us she had a red heifer. The next 16 calves were all black.
 
dun":3h9ohmmt said:
I don;t know how many is required but I know 16 isn;t enough. Granny was a black Angus red carrier.
Bred to either a Polled Hereford, red Fleck or a Red Angus the first year she calved for us she had a red heifer.
The next 16 calves were all black.
Click to expand...
She was proven heterozygous black by having a red calf in her first 16 tries... so in her case it was enough.
Just because her 17th calf was black doesn't mean the next 4 wouldn't all have been red.
Besides 93.75% (aka about 94%) is not 100%

16 crosses = 93.75% and 16 is the lowest number I would feel comfortable at calling him proven
20 = 95%
25 = 96%
30 = 96.666%
50 = 98%
To reach 99.4% certainty would take 167 crosses and yet there is still a minute chance the 168th calf would be red.
Without DNA testing you just have to pick the most reasonable number at which you feel comfortable.

Trivia:
97.37% probability of picking the wrong number on one spin of a Roulette Wheel
 
Son of Butch":35c80g22 said:
dun":35c80g22 said:
I don;t know how many is required but I know 16 isn;t enough. Granny was a black Angus red carrier.
Bred to either a Polled Hereford, red Fleck or a Red Angus the first year she calved for us she had a red heifer.
The next 16 calves were all black.
Click to expand...
She was proven heterozygous black by having a red calf in her first 16 tries... so in her case it was enough.
Just because her 17th calf was black doesn't mean the next 4 wouldn't all have been red.
Besides 93.75% (aka about 94%) is not 100%

16 crosses = 93.75% and 16 is the lowest number I would feel comfortable at calling him proven
20 = 95%
25 = 96%
30 = 96.666%
50 = 98%
To reach 99.4% certainty would take 167 crosses and yet there is still a minute chance the 168th calf would be red.
Without DNA testing you just have to pick the most reasonable number at which you feel comfortable.

Trivia:
97.37% probability of picking the wrong number on one spin of a Roulette Wheel
Click to expand...
The point was that even 16 may not mean anything. In her case 17 would have been enough
 
I remember reading somewhere that if you wanted to check a bull for genetic defects you had to breed him to 13 of his daughters. Not sure if that is the same.
 
In the case of homozygous polled, breeding to 10 horned cows and getting 10 polled calves, 99.9% accuracy. I would think breeding to 10 red cows and 10 black calves would be the same accuracy. It would be quicker to have him tested if it is important to you.
 
CSM":3fzrnun6 said:
In the case of homozygous polled, breeding to 10 horned cows and getting 10 polled calves, 99.9% accuracy.
Click to expand...
Correct for polled gene when both breeds are British. (any zebu blood screws everything up)
2 of 2 = 75% probability of being homozygous
4 of 4 = 93.75%
6 of 6 = 98.44%
10 of 10 = 99.90%
14 of 14 = 99.99%


Gene for scurs is another matter as it's transmitted separately with no relationship to whether horned or polled.
Not all horned cattle carry the scur gene and not all polled cattle are free of it.
 
Dependant on exact trait you are trying to prove is homozygous?

You know his lineage at least 2 generations both parents ?

Color is always tricky lots of purebred and hybrid cows that are "refined" or a bit steer looking are homozygous also.

So in that you get luck of the draw normally when you breed homo to homo you get a 40/20 split as opposed to a 30/30
 
Son of Butch":2nacdvsi said:
I don't know if there is a straight answer to 100% certainty.
It's possible 1 cross can prove a bull is not.
My estimate is 16 would put you in the range of 94% certainty and it would take many more to reach 99.4%
DNA test would tell you heterozygous or homozygous to 100% certainty and you would know much faster.
Click to expand...

If I remember correctly after that it had to breed true for four generations as well was in there somewhere in my college biology.
Now that was a long long time ago.
 

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