In Parashat Vayeitze there is an episode of a dispute between Yakov Avinu and Lavan about Yakov’s share. Yakov came up with a proposal that certain type of sheep would be for him and the other for Lavan.  The deal in more details is as follows: 

Yakov said:

“אֶֽעֱבֹ֨ר בְּכל־צֹֽאנְךָ֜ הַיּ֗וֹם הָסֵ֨ר מִשָּׁ֜ם כּל־שֶׂ֣ה ׀ נָקֹ֣ד וְטָל֗וּא וְכל־שֶׂה־חוּם֙ בַּכְּשָׂבִ֔ים וְטָל֥וּא וְנָקֹ֖ד בָּעִזִּ֑ים וְהָיָ֖ה שְׂכָרִֽי” 

“let me pass through your whole flock today, removing from there every speckled and spotted animal—every dark-colored sheep and every spotted and speckled goat. Such shall be my wages.”

Further in the Parasha it explains how Yakov manage to multiply his flock over Lavan’s:

“Jacob then got fresh shoots of poplar, and of almond and plane, and peeled white stripes in them, laying bare the white of the shoots. The rods that he had peeled he set up in front of the goats, in the troughs, the water receptacles, that the goats came to drink from. Their mating occurred when they came to drink, and since the goats mated by the rods, the goats brought forth streaked, speckled, and spotted young. But Jacob dealt separately with the sheep; he made these animals face the streaked or wholly dark-colored animals in Laban’s flock. And so he produced special flocks for himself, which he did not put with Laban’s flocks.” (Genesis 30:38-40)*

The Arizal explains that it can be learned from the sheep that were focused on the sticks Yakov made, how it affected the genetics of their kids. The same way occurs by humans, at the moment of cohabitate with the spouse, both need to be concentrated on each other and on the moment they are experiencing.
There is a story brought by the Sages: 

” There was one time when there were a light skinned king and queen, they lived together and the baby they conceived was dark skinned. the king was sure his wife was cheating on him, so he went to ask a sage of his trust. The sage asked the king if in his chamber there were statues or images of dark skinned men, to what the king answered positively. The sage explained that the power of the mind and thoughts at the moment of procreating is so powerful that ir can visibly affect the conceivement of the baby. ” 

The Arizal explains that the moment of conceivement is one of the holiest moments a person experiences, so the intentions (kavanot) and thoughts should be focused on...