gametes remain pure they either carry dominant or recessive genes
Mendel’s principle of segregation is inherent in the results of Mendel earlier described in this section. It is obvious that though in F1 the dominant phenotype appears, the recessive phenotype is not lost but reappears in F2.
This suggested that there is no blending of Mendelian factors in F1, but that they stay together and only one is expressed. At the time of the formation of gametes, these two factors obviously separate or segregate, otherwise recessive type will not appear in F2.
The gametes which are formed are always pure for a particular character. A gamete may carry either the dominant or the recessive factor but not both as we find in F1 individuals. This is why it is called either as ‘principle of segregation’ or as ‘law of purity of gametes’.
This concept of segregation is often called Mendel’s first principle. While a 3 : 1 ratio in F2 generation of a monohybrid cross suggested that segregation of alleles does take place.source:http://www.biocyclopedia.com/