Multiple crosses were made between true-breeding lines of black and yellow Labrador retrievers. All the F1 progeny were black. When these progeny were intercrossed, they produced an F2 consisting of 91 black, 39 yellow, and 30 chocolate. (a) Propose an explanation for the inheritance of coat color in Labrador retrievers. (b) Propose a bio- chemical pathway for coat color determination and indi- cate how the relevant genes control coat coloration.

a. The observed frequencies (91 black, 39 yellow, and 30 chocolate) are similar to the expected values for a dihybrid cross, which has a phenotype ratio of 9:3:3:1  

b. Coat color is a trait characterized by the presence of melanin, thereby genes that encodes for enzymes involved in the biosynthesis of melanin are expected to be involved in the expression of this trait

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation: