Length of Autosomal Segment Shared versus IBD probability

Watchers
Share
Return to The Tapestry Homepage
Enlarge
Return to The Tapestry Homepage
Return to Methods
Enlarge
Return to Methods

Contents

This is one of a series of articles on Genealogical Methods, prepared in association with The Tapestry. See Index for a list of related articles.
__________________________


Source

Intermediate:Robert Payne, 3 Nov 2013 to genealogy-dna at Rootsweb, mail list.
Original:Attributed to John Walden

Related

DNA. Genetic Length of Chromosomes

Background

Autosomal DNA (atDNA)is derived from both parents. The amount contributed by each is not necessarily equal. Their contributions to a childs DN will be passed on to succeeding generations, and with each generation the amount passed on is approximately, but not exactly one-half from each successive parent. The amount of DNA passed on is measured in 'centimograns' or cM. This measure adjusts for differences in the length of each chromosome. Onany given chromosome the size of a centimoregan is constant, but varies considerably between chromosomes.

With each succeeding generation the amount of atDNA passed on to a child on any given chromosome, from the ancestral parent, becomes increasingly divided.

GenerationAverage % passed on*
0100%
150%
225%
312.5%
46.25%
53.123%
61.7…%
70.85%
80.425%

*Assumes exactly half of the atDNA of each parent is passed on

Data

cM matching segment: IBD IBS
11+ >99% <1%
10 99% 1%
9 80% 20%
8 50% 50%
7 30% 70%
6 20% 80%
5 5% 95%
4 ca 1% ca 99%