Understanding each statement

• A. “Markers that are not linked to target traits cannot be used to test accumulation of the desired recurrent parent’s genetic background in a backcross breeding program.” – Incorrect

  • In marker‑assisted backcrossing, markers unlinked to the target locus but distributed genome‑wide are used for background selection, checking how much of the recurrent parent genome has been recovered.

  • Therefore, unlinked markers are exactly what you use to test background recovery. So A is false.

• B. “Co-dominant markers are more effective for marker assisted backcrossing since they facilitate selection of heterozygous progeny.” – Correct

  • Co-dominant markers (e.g., SSRs, SNPs) distinguish AA, Aa, and aa genotypes.

  • In backcrossing, the desired donor allele is kept in heterozygous state for several generations, so being able to select heterozygotes is crucial. Hence B is true.

• C. “MAS can be used to facilitate backcrossing of recessive genes that influence traits of interest.” – Correct

  • Recessive alleles are hard to select phenotypically in heterozygotes, but markers tightly linked to the recessive gene allow selection of carriers during backcross generations.

  • MAS is therefore particularly helpful for recessive traits, so C is true.

• D. “Use of tightly linked flanking makers for a trait of interest increases linkage drag in a breeding program.” – Incorrect

  • Linkage drag is the co‑introgression of unwanted donor segments linked to the target gene.

  • Using tightly linked flanking markers on both sides allows breeders to select recombinants with the smallest possible donor segment, thereby reducing linkage drag, not increasing it. So D is false.

Summary of correctness

• A – incorrect

• B – correct

• C – correct

• D – incorrect

Thus, the combination of all incorrect statements is A and D → option (3).