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Elementary Science Education

Sexual Reproduction

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      Bernard Nebel


      The life cycle of every species includes sexual reproduction: production of eggs and sperm and subsequent fertilization, that is the fusion of an egg and sperm into a single cell called the zygote. By subsequent growth, division, and differentiation, the zygote develops into an individual of the next generation.

      Sea urchins are a “lab animal” commonly used to demonstrate fertilization and the early development of the zygote into an embryo as seen in the following video:

      Fundamentals of Sexual Reproduction

      Emphasize that each of the adult’s cells has two sets of chromosomes (like  each shoe in your closet has an opposite “mate”). Likewise, emphasize the each chromosome is a thread of DNA coding for specific genes. The genes on a given pair of chromosomes code for the same basic things or functions, for example, both will have a gene for eye color. However, these two genes (alleles) may be exactly the same, e.g. both for brown or both for blue. Or, they may be somewhat different, one for blue and one for brown eyes.

      Now, the cell divisions leading to eggs and sperm (meiosis) are such that each egg and each sperm has just one set of chromosomes (one shoe of each pair). Revisit meiosis, Lesson B-27. Also model meiosis using strips of paper or other material of different colors or markings.

      Emphasize how the segregation of pairs of chromosomes that occurs in meiosis is a random process. That is, using our shoes analogy again, suppose the mother and father both started with two pairs of chromosomes (pairs of shoes, each a right and a left). After meiosis, a  given egg might have two right shoes, two left shoes, a left and right, or a right and left. The same would occur in the case of sperm. Then, in fertilization which sperm combines with which egg is also a random process. Hence, the resulting zygote might have two pairs of right shoes, two pairs of left shoes, or various other combinations of rights and lefts. The bottom line is VARIATION AMONG OFFSPRING.

      (This example used just two pairs of chromosomes. Humans have 23 pairs. How many variants might there be then?

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