Wang Xiaomei, a classmate of Ji 'an County, is a professional cattle farmer. A black bull in her family paired with a black cow and gave birth to a brown calf. Please apply what you have learned.

Solution: (1) In organism cells, chromosomes exist in pairs. In the process of forming germ cells, pairs of chromosomes are separated, and one chromosome in each pair enters sperm or egg cells, so the number of chromosomes in germ cells is less than half that of somatic cells. If the chromosome number of bovine muscle cells is 30, then the chromosome number of bovine egg cells is 30.

(2), (2) and (4) It can be inferred from "two black cows mated to give birth to a brown calf" that brown is a recessive trait and black is a dominant trait; If A and A represent the dominant gene and recessive gene of cow coat color respectively, the genetic map of the pair of genes is as shown in the figure:

According to the genetic map, the genetic composition of the parent black bull is Aa, the genetic composition of the offspring black calf is Aa or AA, and the probability of giving birth to brown calf is 25%.

So the answer is: (1)30.

(2) Recessive

(3)Aa

(4)25%; Aa or AA