Answer:
Perder un testículo usualmente no tiene ningún efecto en la capacidad de un hombre de lograr una erección y tener sexo. No obstante, si se extirpan ambos testículos, no se pueden producir espermatozoides y el hombre queda estéril.
Explanation:
 
        
             
        
        
        
Answer:
No, CREs usually don't account for differences in gene expression levels.
Explanation:
<em>Cis</em>-regulatory elements (CREs) are non-coding DNA regions that regulate adjacent gene <em>transcription</em>. They usually do it by binding to transcription factors. They are in charge of influencing the rate of which transcription occurs, and less with how much a gene is expressed. On the other hand, trans-regulatory elements are DNA sequences that encode for upstream regulators that can modify or regulate <em>gene expression levels</em>.
Therefore, cis elements are not really in charge of expression levels, mostly transcription rates of single genes, rather. Trans elements are more involved in expression levels.
 
        
             
        
        
        
I think the hydra lacks the specialized cells for touch and chemical detection. On the other hand earthworms have touch, light, vibration and chemical receptors along the entire body surface. Both of them are invertebrates (those animals without a backbone), they include insects, worms, jelly fish, spiders among other animals. 
        
             
        
        
        
Answer: 
Valley lizard = Oviparous
Mountain lizard = Viviparous
Explanation:
In Darwin's theory survival of the fittest, he believed species adapt or change within their environment. So I believe lizards living in a valley would be more safe to lay their eggs but lizards living in the mountains would be viviparous because of the non-predicted and unsafe environment which could harm the better chances of development of the offspring. 
 
        
                    
             
        
        
        
In contrast to the stratified squamous epithelium of the skin, the intestinal barrier is composed of a single layer of columnar epithelial cells However, this single layer of intestinal epithelial cells is made of diverse cell types with absorptive, secretory, and immune function