How does the motif of blood/milk relate to the theme of gender roles ?

In the play, Macbeth, the motif of milk signifies femininity and, by extension, weakness.  On the other hand, the motif of blood signifies masculinity, which is denoted through violence and cruelty.  For example, Lady Macbeth, in discussing Macbeth's willingness to commit Duncan's murder, says that she fears that Macbeth is "too full o'the milk of human kindness" (Act 1).  Moreover, later she also asks that the spirits "take my milk for gall" (Act 1) and...

In the play, Macbeth, the motif of milk signifies femininity and, by extension, weakness.  On the other hand, the motif of blood signifies masculinity, which is denoted through violence and cruelty.  For example, Lady Macbeth, in discussing Macbeth's willingness to commit Duncan's murder, says that she fears that Macbeth is "too full o'the milk of human kindness" (Act 1).  Moreover, later she also asks that the spirits "take my milk for gall" (Act 1) and says that she would gladly bash out a babe's brains while feeding it at her breast (Act 2).  All of these references connect the idea of milk to feminine nurturing and the normal human emotions that come along with motherhood.  However, these traits are deemed undesirable by Lady Macbeth because they make one weak and ineffective.  Here, Lady Macbeth demonstrates that feminine qualities make one incapable of pursuing one's ambitions. In contrast, blood is perceived to denote masculinity in the text, as when Lady Macbeth says "make my blood thick" (Act 1).  Here we see Lady Macbeth again asking for masculine qualities that might make her capable of the violence required to murder Duncan.  

It is also worth noting that Lady Macbeth consistently uses this language of femininity and masculinity to manipulate Macbeth into doing what she'd like him to do.  She consistently questions his manhood in order to push him to follow her ambitious plans for him.