Many a good cow hath had a bad calf.

A parent's good qualities do not guarantee that a child will share them. The contrast between cow and calf makes that difference plain.

The proverb also warns against judging a parent solely by a child's behavior. A child may act badly despite having a parent who sets a better example.

Use it when someone assumes that a child's poor conduct must reflect poorly on an otherwise respected parent. It is a blunt saying, so it may be unkind if directed at the child or family involved.