Your research team suspects that lactose intolerance is a recessive trait. Which of the following observations would contradict your theory?

a) Two lactose intolerant parents always have children who are lactose intolerant

b) Genetic analysis shows that individuals must possess two copies of the "lactose intolerance gene" to show the lactose intolerant phenotype

c) Two parents, both lactose intolerant, have a child who is not lactose intolerant

d) Two parents, one able to tolerate lactose and the other intolerant, have a child who is able to tolerate lactose

e) Two parents, one able to tolerate lactose and the other intolerant, have a child who is not able to tolerate lactose

I would think the answer is C.

If lactos intolerance is recessive, then both parents, who are lactose intolerant, can only pass on the intolerance genes to the offspring. So, as far as I can tell, they could not have an offspring who can tolerate lactose.

To determine which observation would contradict the hypothesis that lactose intolerance is a recessive trait, we first need to understand the concept of recessive traits. In genetics, a recessive trait refers to a trait that only manifests when an individual receives two copies of the gene for that trait, one from each parent. This means that if an individual inherits one copy of the dominant gene for the trait, they will not exhibit the recessive trait.

Now let's analyze the given options:

a) Two lactose intolerant parents always have children who are lactose intolerant.
This observation supports the theory that lactose intolerance is a recessive trait. Since both parents are lactose intolerant, it is likely that they both carry two copies of the gene responsible for lactose intolerance. As a result, their children would inherit two copies of the gene and also be lactose intolerant. This does not contradict the theory.

b) Genetic analysis shows that individuals must possess two copies of the "lactose intolerance gene" to show the lactose intolerant phenotype.
This observation also supports the theory that lactose intolerance is a recessive trait. It confirms that individuals need to inherit two copies of the gene to exhibit the lactose intolerant phenotype. Therefore, this does not contradict the theory.

c) Two parents, both lactose intolerant, have a child who is not lactose intolerant.
This observation would contradict the theory that lactose intolerance is a recessive trait. If both parents are lactose intolerant, it suggests that they both carry two copies of the gene for lactose intolerance. Consequently, their child should inherit two copies of the gene and, therefore, also be lactose intolerant. If the child is not lactose intolerant, it indicates that the trait is not solely determined by genetics, and there might be another factor involved.

d) Two parents, one able to tolerate lactose and the other intolerant, have a child who is able to tolerate lactose.
This observation does not contradict the theory that lactose intolerance is a recessive trait. In this scenario, the parent who can tolerate lactose likely carries one copy of the gene for lactose intolerance and one copy of the dominant gene for lactose tolerance. The parent who is lactose intolerant carries two copies of the gene for lactose intolerance. When they have a child, it is possible for the child to inherit the dominant gene for lactose tolerance from the tolerant parent, making them lactose tolerant. This observation does not challenge the theory.

e) Two parents, one able to tolerate lactose and the other intolerant, have a child who is not able to tolerate lactose.
This observation would also contradict the theory that lactose intolerance is a recessive trait. Similar to option c, since one parent can tolerate lactose, they likely carry one copy of the gene for lactose tolerance. The intolerant parent carries two copies of the gene for lactose intolerance. As a result, the child should inherit at least one copy of the gene for lactose tolerance and, therefore, be able to tolerate lactose. If the child is not able to tolerate lactose, it suggests that the trait may not be solely determined by genetics.

Based on the explanations above, options c) and e) would both contradict the theory that lactose intolerance is a recessive trait.