▁sexual ▁reprodu ction , ▁a ▁gene ▁has ▁a ▁single ▁chain ▁of ▁ancest ors ▁and ▁a ▁tree ▁of ▁descend ants . ▁ ▁Given ▁any ▁gene ▁in ▁the ▁body ▁of ▁an ▁organ ism , ▁we ▁can ▁trace ▁a ▁single ▁chain ▁of ▁ancest or ▁organ isms ▁back ▁in ▁time , ▁following ▁the ▁line age ▁of ▁this ▁one ▁gene , ▁as ▁stated ▁in ▁the ▁co ales cent ▁theory . ▁Because ▁a ▁typical ▁organ ism ▁is ▁built ▁from ▁tens ▁of ▁thousands ▁of ▁gen es , ▁there ▁are ▁numerous ▁ways ▁to ▁trace ▁the ▁ancest ry ▁of ▁organ isms ▁using ▁this ▁mechanism . ▁But ▁all ▁these ▁inheritance ▁path ways ▁share ▁one ▁common ▁feature . ▁If ▁we ▁start ▁with ▁all ▁humans ▁alive ▁in ▁ 1 9 9 5 ▁and ▁trace ▁their ▁ancest ry ▁by ▁one ▁particular ▁gene ▁( act ually ▁a ▁loc us ), ▁we ▁find ▁that ▁the ▁farther ▁we ▁move ▁back ▁in ▁time , ▁the ▁smaller ▁the ▁number ▁of ▁ancest ors ▁become . ▁The ▁pool ▁of ▁ancest ors ▁continues ▁to ▁shr ink ▁until ▁we ▁find ▁the ▁most ▁recent ▁common ▁ancest or ▁( MR CA ) ▁of ▁all ▁humans ▁alive ▁in ▁ 1 9 9 5 ▁via ▁this ▁particular ▁gene ▁path way . ▁ ▁In ▁theory , ▁one ▁can ▁also ▁trace ▁human ▁ancest ry ▁via ▁a ▁single ▁chrom os ome , ▁as ▁a ▁chrom os ome ▁contains ▁a ▁set ▁of ▁gen es ▁and ▁is ▁passed ▁down ▁from ▁parents ▁to ▁children ▁via ▁independent ▁ass ort ment ▁from ▁only ▁one ▁of ▁the ▁two ▁parents . ▁But ▁gen etic ▁recom bin ation ▁( chrom