An anti-up quark is an antiquark which corresponds to the up quark.

The up quark, the down quark, and the electron. Two up quarks and a down quark form a proton, and two down quarks and an up quark form a neutron.

-Down and anti-Down quark,

-Up and anti-Up quark or

-Strange and anti-Strange quark

The two are related, yes, but technically it would be more accurate to say it the other way around:

"Neutrons are heavier than protons because down quarks are heavier than up quarks"

Neutrons are composed of an up quark and two down quarks (udd). Protons are composed of two up quarks and one down quark (uud), so the difference in mass between a proton and neutron is (roughly) the same as the difference in mass between the neutron's down quark and the proton's matching up quark.

Because a down quark is heavier than an up quark, it is also possible for a down quark to decay into an up quark (releasing an electron in the process). This is how beta radiation occurs in atomic nuclei. One of the neutrons' down quarks decays into an up quark, changing that neutron into a proton, and releasing an electron (as radiation), so another way to look at it would be that a down quark is an up quark that has an electron trapped inside it (the mass of the electron, plus the energy required to "trap" it there, is what makes the down quark heavier).

The quark composition is different:

- proton: 2 up quarks + 1 down quark

- netron: 2 down quarks + 1 up quark

The down quark is heavier.