Applications of Mendelian Genetics — Cross Calculations and Clinical Use — Summary

Blood group O (ii) = universal donor in packed RBC transfusion $\frac{no A}{B antigens on surface}$
Blood group AB ( $I^A$ $I^B$ ) = universal recipient (no anti-A or anti-B antibodies in plasma)
Clinical rule: transfuse compatible — do not give O patients anything but O; AB patients can receive anything
Forensic and paternity testing uses ABO impossibility: two AB parents cannot have an O child; one O parent cannot contribute $I^A$ or $I^B$ to child

Monohybrid carrier detection: if one parent is Tt (carrier) and the other is tt (affected), each child has 50% chance of being affected
Dihybrid risk: for two independent recessive diseases, probability of child affected by BOTH = $\frac{1}{4}$ × $\frac{1}{4}$ = 1/16
Test cross result counselling: 1:1 ratio confirms heterozygous parent; all dominant confirms homozygous

Cross	Offspring ratio	Notes
$I^A$ i × $I^B$ i	1 AB : 1 A : 1 B : 1 O	All four blood groups possible
$I^A$ $I^B$ × ii	1 A : 1 B	Only A and B; no AB, no O
TtRr × ttrr	1:1:1:1	Confirms independent assortment
AaBb × AaBb	9:3:3:1	Standard dihybrid
Rr × Rr (snapdragons)	1 Red : 2 Pink : 1 White	Incomplete dominance 1:2:1

Pure-breeding lines (homozygous) created by repeated self-pollination
Hybrid vigour (heterosis): F1 heterozygotes often outperform both parents — Mendel's framework predicts heterozygous F1 genotypes
Test cross identifies heterozygous carriers of undesired recessive traits in breeding programmes
Genetic maps (from recombination frequencies) guide marker-assisted selection in modern crop breeding