  Recently V.H.L\'opez Sol\'is and I.Shestakov solved an old problem by
N.Jacobson on describing of unital alternative algebras containing the $2\times
2$ matrix algebra $M_2$ as a unital subalgebra. Here we give another
description of $M_2$-algebras via the 6-dimensional alternative superalgebra
$B(4,2)$ and an auxiliar $\mathbf {\mit Z}_2$-graded algebra $\mit\Gamma$. It
occurs that the category of alternative $M_2$-algebras is isomorphic to the
category of $\mit\Gamma$-algebras. For any associative and commutative algebra
$A$, we give a construction of a ${\mit \Gamma}$-algebra $\mit\Gamma(A)$, which
turns to be a Jordan superalgebra; if $A$ is a domain then $\mit\Gamma(A)$ is a
prime superalgebra. We describe also the free $\mit\Gamma$-algebras and
construct their bases.
