Topic 42

Question 1

Discontinuous hybridization

Answer 1

two, three or four breed cross; makes maximum use of heterosis and breed complementary, It may utilise 2, 3, or 4 breeds

Question 2

What happens in terminal crossing?

Answer 2

All of the crossbred offspring are sold and replacement females must be purchased or produced by mating a proportion of the stock to males of the same breed

Question 3

Three breed crossing

Answer 3

Crossing two breed to produce a crossbreed (F1); F1 crossed by a 3rd breed; F2 is end product; Cattle and swine breeding

Question 4

Four breed crossing

Answer 4

Double crossing; Use 4 breeds; Mate A and B breeds to produce F1; Mate C and D breeds to produce F1; Mate F1 and F1 to produce F2 (ABCD); Swine breeding

Question 5

Heterosis

Answer 5

hybrid vigour; result of crossbred offspring; Mathematically heterosis is the difference between the crossbred and the average performance of the purebred parent; groe faster than purebreds

Question 6

Effects of heterosis

Answer 6

tend to be large for traits that are lowly heritable (e.g. reproduction) and small for traits that are highly heritablke (growth, carcass, wool)

Question 7

Effects of heterosis are

Answer 7

cumulative

Question 8

reciprocal recurrent selection

Answer 8

The selection of breeding animals in two populations based on the performance of their offspring after animals from two populations are crossed.

when choosing an animal to use for breeding we look at its offspring with the other population