Q.Distinguish between heterogametic and homogametic sex determination systems.

About 94% of flowering plants are sexually monomorphic (every plant bears both stamens and carpels), while about 6% are dimorphic (separate male and female plants); C.E. Allen (1917) pioneered the genetic study of this dimorphism, and plant sex determination is now understood to depend on genes, environment and hormones acting together, not a single simple switch. Three plant systems illustrate different mechanisms: in papaya (Carica papaya, 2n=36), males are XY and females XX on a sex-chromosome pair that cytologically resembles the autosomes (having evolved from one), with sex fine-tuned by a further three-allele series (m, M1, M2) at one locus, where mm gives female, M1m gives male, M2m gives rare bisexual plants, and any two-dose-dominant combination is inviable/sterile; in the liverwort Sphaerocarpos donnellii, the haploid gametophyte generation itself is heteromorphic, with a large X chromosome producing female gametophytes and a small Y chromosome producing male gametophytes, while the diploid sporophyte is always heterogameti …