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Haploid organisms such as protists, fungi and algae have only one gene for a trait and will express the phenotypes of any kind of mutations because there is no hindrance of dominance. On the other hand, diploid organisms such as the Drosophila and pea have two alleles, one in each homologous chromosome.
The haploid organisms produce gametes which unite during fertilisation and form a zygote. The diploid zygote undergoes meiosis immediately to produce haploid organisms. This type of meiosis that occurs after fertilisation is called zygotic meiosis.
Neurospora crassa, a pink bread mould exists in two physiologically different types designated as the ‘+’ and the ‘-’ strains. The two strains unite during sexual reproduction and produce a diploid zygote, which divides by meiosis to produce a fruiting body called ascus. Each of the four haploid nuclei formed divides by mitosis into two nuclei producing a total of eight ascospores.
The ascospores are arranged linearly in a two-by-two arrangement, with a pair that is identical being placed close to each other. The ascospores can be separated and individually cultured to identify the parental and recombinant traits. This type of analysis of all the products of meiosis is called tetrad analysis (Fig. 19)
It is seen that the contrasting alleles segregate during meiosis and a phenotypic ratio of 1:1 is obtained, in contrast to the 3:1 Mendelian ratio. Similarly, a ratio of 1:1:1:1 is obtained when two pairs of genes are considered instead of the 9:3:3:1 ratio.
