(Plink) Sex as a covariate: What's the coding for male/female?

Question

[Armaboo111]

What is the code for male and female in plink covariate files? Is it (male = 1, female = 0), or ('1' = male, '2' = female, '0' = unknown)? Is the code for gender different in covariate and fam. files?

I'm wondering about the inconsistent statements in the the manual of plink 1.9, as showed below. Thank you in advance.

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As mentioned in the manual of plink 1.9 which discusses the coding for male/female of covariate files: "By default, when at least one male and one female is present, sex (male = 1, female = 0) is automatically added as a covariate on X chromosome SNPs, and nowhere else."

As mentioned in the manual of plink 1.9 which discusses the coding for male/female of .fam files: Sex code ('1' = male, '2' = female, '0' = unknown).

Answer 1

[chrchang523]

The expected coding in the .fam file is male='1', female='2'; this is then coded by plink 1.x as male=1, female=0 during the --linear/--logistic regression. In other words, you would get the same results (outside of chrX) with --linear sex as you would with --linear combined with a covariate file with a male=1, female=0 sex covariate added.

Yes, this is counterintuitive, so I got rid of this discrepancy in plink 2.0. From its --glm documentation: "Note that PLINK 2.0 encodes the .fam/.psam sex covariate as male = 1, female = 2, to match the actual numbers in the input file. This is a minor change from PLINK 1.x." So with PLINK 2.0, if you use male=1, female=2 coding in both file types, you don't have to worry about the sign of the sex beta coefficient changing on you with .fam vs. --covar.

(With that said, even with PLINK 1.x, you don't have to worry about any p-values being affected by the 1/2 vs. 1/0 coding.)

[Armaboo111]

Thank you so much, chrchang523!