In some instances, an ovary releases more than one egg at one time, or both ovaries release an egg simultaneously. Each egg has the potential to be fertilized, resulting in multiple pregnancies. Since each conception originates c»o from a separate egg and sperm, individuals created in this way are as different as those conceived as separate births. These are referred to as fraternal twins. In rare instances, cells from the same embryo separate to create distinct embryos that are genetically identical and are referred to as monozygotic, or identical, twins.

The exact details of fertilization vary from animal to animal. Fertilization does not always take place inside an animal. For example, sea urchins, spiny animals attached to rocks on the ocean floor, release their eggs and sperm directly into the water. Large numbers of each (millions of eggs and billions of sperm) are necessary to ensure that enough eggs will be fertilized to maintain the population. Many other ocean creatures also release egg and sperm cells into the water. However, the eggs are fertilized only by sperm of the same species because of unique proteins on the surface of the egg. As in humans, fertilization immediately triggers a change in the surface of the egg, protecting it from penetration by other sperm, even sperm of the same species.

Reproductive technology has introduced further variations in how eggs may become fertilized, permitting the process to occur outside the fallop-in vitro "in glass"; in ian tubes. One of the most common is in vitro fertilization, in which eggs and artificially capacitated sperm are combined in a glass dish and the dividing embryos are later transplanted into the uterus. see also Meiosis; Reproductive Technology; Twins.

Susan E. Estabrooks lab apparatus, rather than within a living organism


Primakoff, Paul, and Diana G. Myles. "Penetration, Adhesion, and Fusion in Mammalian Sperm-Egg Interaction." Science (Jun. 21, 2002): 2183-2185.

Tobin, Allan, and Jennie Dusheck. Asking about Life, 2nd ed. Orlando, FL: Harcourt, 2001.

Wasserman, Paul. "The Biology and Chemistry of Fertilization." Science 235 no. 4788 (1987): 553-560.

Forensics See DNA Profiling alleles particular forms of genes

Founder Effect

The term "founder effect" refers to the observation that when a small group of individuals breaks off from a larger population and establishes a new population, chance plays a large role in determining which alleles are represented in the new population. The particular alleles may not be representative of the larger population. As the new population grows, the allele frequencies will usually continue to reflect the original small group.

genetic drift evolutionary mechanism, involving random change in gene frequencies

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