Outbreeding Works in a Single Generation

As I have mentioned before, cousin marriages were fairly common among my family. My parents are first cousins. So are my father’s parents. My mother’s parents are second cousins once removed. So instead of 32 great-great-great grandparents, I have only about 18.

Since my wife and I are not related, I wondered how my inbred genome had transmitted to our daughter.

Using David Pike’s ROH utility, I computed the regions of homozygosity for my parents, me, my wife, and my daughter, all tested by 23andme.

I used the default settings for the utility. The total Mb gives the total size in megabases of the long autosomal regions where both alleles are the same. The longest ROH gives the size of the longest such region. Percent Homozygous is the percentage of the genome where the two alleles are the same.

I included the worst chromosome column because of my chromosome 9, which is beyond crazy. This column gives the percent homozygosity of the worst chromosome.

Person Total Mb Longest ROH (Mb) % Homozygous Worst chromosome (%)
Dad 297.45 57.4 72.498% 76.921%
Mom 112.13 22.99 70.662% 79.802%
Me 402.78 71.38 73.588% 93.542%
Wife 37.33 9.64 70.003% 72.411%
Daughter 42.40 8.82 69.936% 71.759%

As you can see, my Dad has higher levels of homozygosity than my Mom as expected and I have the highest levels. My wife is not inbred at all and our daughter has ROH results about the same as my wife. So one generation of marrying someone unrelated, even if from the same/similar ethnicity, has removed all the long runs of homozygosity bred over generations. Good news!