How many Test Crosses are needed

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Vetrock
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How many Test Crosses are needed

Postby Vetrock » Thu Oct 05, 2017 9:48 am

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?
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Son of Butch
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Re: How many Test Crosses are needed

Postby Son of Butch » Thu Oct 05, 2017 11:33 am

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.
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Re: How many Test Crosses are needed

Postby Franke » Thu Oct 05, 2017 12:45 pm

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.
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Re: How many Test Crosses are needed

Postby Jake » Thu Oct 05, 2017 3:18 pm

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.
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Caustic Burno
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Re: How many Test Crosses are needed

Postby Caustic Burno » Thu Oct 05, 2017 3:55 pm

Son of Butch wrote: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.


A lot depends on the breed of cow as well as several breeds don't play by the rules.
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Re: How many Test Crosses are needed

Postby dun » Thu Oct 05, 2017 7:15 pm

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.
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Son of Butch
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Re: How many Test Crosses are needed

Postby Son of Butch » Fri Oct 06, 2017 2:34 am

dun wrote: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.

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
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dun
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Re: How many Test Crosses are needed

Postby dun » Fri Oct 06, 2017 6:20 am

Son of Butch wrote:
dun wrote: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.

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

The point was that even 16 may not mean anything. In her case 17 would have been enough
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Re: How many Test Crosses are needed

Postby Air gator » Fri Oct 06, 2017 7:12 pm

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.
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Re: How many Test Crosses are needed

Postby Chocolate Cow » Fri Oct 06, 2017 9:23 pm

http://onpasture.com/2014/10/20/breedin ... -breeding/
Before the advent of genetic testing for recessive traits the only way to statistically ensure genetic “purity” of a bull/ram/buck etc. was to breed that bull to 35 of his own daughters concurrently. If no genetic defects show up in any of the offspring, the bull is 99.7% likely to be genetic defect free.
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Re: How many Test Crosses are needed

Postby CSM » Fri Oct 13, 2017 6:56 pm

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.
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Re: How many Test Crosses are needed

Postby Son of Butch » Fri Oct 13, 2017 7:35 pm

CSM wrote:In the case of homozygous polled, breeding to 10 horned cows and getting 10 polled calves, 99.9% accuracy.

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.
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