Final answer:
Rudolf Jaenisch created the first transgenic mammal using mice, which are organisms that have been genetically modified to express certain genes for research or production of substances for medical use.
Step-by-step explanation:
Rudolf Jaenisch used mice to produce the first transgenic mammal. A transgenic organism contains a gene or genes which have been artificially inserted instead of the organism acquiring them through reproduction. In the context of Jaenisch's work, the mice were genetically modified to express and allow the study of the effects of recombinant genes and mutations. This pioneering work laid the groundwork for the creation of transgenic animals, which are used to produce substances such as human proteins for medical use, for instance, proteins expressed in milk of transgenic sheep and goats or in chicken eggs. The creation of such transgenic animals is part of biotechnology's advances, which also enabled the cloning of organisms like Dolly the sheep.