and may use mitochondrial DNA (mtDNA), the non-recombining portion of the Y chromosome (NRY) or autosomal DNA. MtDNA and NRY DNA share some similar features which have made them particularly useful in genetic anthropology. These properties include the direct, unaltered inheritance of mtDNA and NRY DNA from mother to offspring and father to son, respectively, without the 'scrambling' effects of genetic recombination. We also presume that these genetic loci are not affected by natural selection and that the major process responsible for changes in base pairs has been mutation (which can be calculated). Whereas Y-DNA and mtDNA haplogroups represent but a small component of a person's DNA pool, autosomal DNA has the advantage of containing hundreds and thousands of examinable genetic loci, thus giving a more complete picture of genetic composition. Descent relationships can only to be determined on a statistical basis, because autosomal DNA undergoes recombination. A single chromosome can record a history for each gene. Autosomal studies are much more reliable for showing the relationships between existing populations but do not offer the possibilities for unraveling their histories in the same way as mtDNA and NRY DNA studies promise, despite their many complications. Analyses of nuclear and ancient DNA Nuclear DNA analysis shows that Spanish and Portuguese populations are most closely related to other populations of western Europe. There is an axis of significant genetic differentiation along the east–west direction, in contrast to remarkable genetic similarity in the north–south direction. North African admixture, associated with the Islamic conquest, can be dated to the period between c. AD 860–1120. A study published in 2019 using samples of 271 iberians spanning prehistoric and historic times proposes the following inflexion points in Iberian genomic history: Mesolithic: hunter-gatherers from the European Steppes of Western Russia, Georgia and Ukraine are the first humans to settle the northwest of the Iberian Peninsula. Neolithic: neolithic farmers settle the entire Iberian Peninsula from Anatolia. Chalcolithic: Inflow of Central European hunter-gatherers and some gene inflow from sporadic contact with North Africa. Bronze Age: Steppe inflow from Central Europe. Iron Age: Additional Steppe gene flow from Central Europe, - the genetic pool of the Basque people remains mostly intact from this point on. Roman period: genetic inflow from Central and Eastern Mediterranean. Some additional inflow of North African genes detected in Southern Iberia. Visigothic period: no detectable inflows. Muslim period: Inflow from Northern Africa. Following the Reconquista, there