How can someone inherit a trait from their parent while the sibling does not please help me!!!

The organelle pictured is found in cells of and is theorized to have once been an independent organism.

The table below shows the number of foot bones in some horse fossils. horse fossil recordhorse fossil number of foot bonesp 19q 17r 24s 13ancient horses had more bones in their foot then present-day horses. the present-day horse has 11 foot bones. what is the correct order of evolution of the horse starting from the youngest fossil? a. q p r s b. s r p q c. s q p r d. r p q s

Ialready know the answers to these questions and they are on the attachment. but i just need to know why that answer is the correct one. plz asap! this is for a report that i need to email by 10!