Why is the law of uniformity observed only in the first generation when crossing clean lines?

Because pure line organisms are a group of organisms that have some traits that are completely transmitted to descendants due to genetic homogeneity. When individuals are crossed in the second generation, splitting occurs (2 Mendel law)
For example, consider this process on cats.
We will cross two cats homozygous for the color genes – black and chocolate,
B – allele responsible for the synthesis of black pigment;
b – allele responsible for the synthesis of chocolate pigment.
The black cat has the BB genotype; the chocolate cat has bb. Parents in genetics are designated by the Latin letter P (from the Latin parenta – “parents”). Spermatozoa will carry one B allele, and the egg cells will carry the b allele.
As a result of fertilization, zygotes are formed containing a diploid set of chromosomes (a characteristic set of a milking fertilized egg and all somatic cells of the gene) and bearing Bb alleles. Hybrids of the first generation, which in genetics is usually designated F1, will be heterozygous for this locus – Bb. Allele B completely dominates the b allele, so all kittens received will be black. Sometimes the dominance of one allele over another is denoted as follows: B> b.
When crossing homozygous cats, the offspring of the same phenotype is obtained. These results illustrate Mendel’s T law.
I analyze this crossing, we are talking only about one sign – black and chocolate color. The whole variety of features that determine both the similarity and the difference in parents is not of interest to us at the moment. This type of cross is called monohybrid,

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