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The outcome are as follows: Top-left cell: paternal grandfather’s chromosome maternal grandfather’s chromosome Top-right cell: paternal grandfather’s chromosome maternal grandmother’s chromosome Bottom-left cell: paternal grandmother’s chromosome maternal grandfather’s chromosome Bottom-right cell: paternal grandmother’s chromosome maternal grandfather’s chromosome As an example, if on chromosome 15 two siblings were characterized by the top-left cell, we might say that they were 100% “identical-by-descent” (IBD).This just means that their genes came down from the exact same ancestors.So “paternal” and “maternal” here is in reference from the parental generation, so there are two of each.The ones inherited from the parental mother I’ve italicized.But, the relationship between grandparent and grandchild is not deterministic at any given locus. To give a concrete example, consider an individual who has four grandparents, three of whom are Chinese, one of whom is Swedish. One can assume reasonably then on the locus which controls blue vs.non-blue eye color difference one of the grandparents is homozygous for the “blue eye” allele, while the other grandparents are homozygous for the “brown eye” alleles.
Finally, I explain how this clarification allows us to potentially understand with greater precision the nature of inheritance of complex traits which vary within families, and across the whole population. We have two copies of each gene, inherited from each parent (the exception here is for males, who have only one X chromosome inherited from the mother, and lack many compensatory genes on the Y chromosome inherited from the father).
The chromosomal numbers also correspond roughly to a rank order of size.
To give you a sense of the gap, chromosome 1 has 250,000,000 bases and 4,200 genes, while chromosome 22 has 1,100 genes and 50,000,000 bases (the Y chromosome has a paltry 450 genes, as opposed to the 1,800 on the X).
Rather, during meoisis, an individual’s chromosomes often “mix & match” their strands so that new mosaics are formed.
So instead of inheriting homologous chromosomes which resemble exactly those carried by their grandparents, individuals often have chromosomes which are a mosaic of maternal and paternal due to the two meoisis events which intervened (one during the formation of the gametes which led to one’s parents, and another during the formation of the gametes of their parents’).