What combines to form the first step of mammalian reproduction?

• A. Sperm and Egg
• B. Ovary and Testis
• C. Blastocyst and Placenta
• D. Zygote and Embryo

Answer: (A)

The first step of mammalian reproduction is the combination of sperm and egg. This process, known as fertilization, occurs when a sperm cell successfully penetrates an egg, leading to the formation of a zygote. The zygote then undergoes several divisions and developmental processes which eventually lead to the formation of an embryo.

In the context of mammalian reproductive biology, the sperm and egg represent the male and female gametes, respectively. Their fusion is essential for the initiation of a new organism's development, marking the essential point at which genetic material from both parents comes together to create a unique genetic identity.

The other options, while related to reproduction, do not represent the very first step. The ovary and testis are the organs that produce eggs and sperm, but their combination does not initiate reproduction. The blastocyst and placenta are stages and structures that occur after fertilization, involving the implantation of the embryo into the uterus. The zygote and embryo refer to different developmental stages that follow fertilization. Therefore, sperm and egg are indeed the correct answer as they are crucial in initiating the process of reproduction.