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CELL DIVISION AND REPRODUCTION

trades places with its other chromosome copy. Crossing over allows for
genetic variation in the organism because it results in new and unique
chromosome sequences.
In the next phase of meiosis, genetic information in each chromosome
of each cell splits in half. The result is four daughter cells that have only
one set of chromosomes. These are haploid cells. During fertilization,
the haploid sperm and egg cells join, and the chromosomes of each
come together in the daughter cell nucleus to create a diploid cell. These
new diploid cells will then undergo mitosis to begin growing into a
human embryo.
Errors that occur during mitosis or meiosis may lead to chromosome
abnormalities such as trisomy 21 (also known as Down syndrome).
Researchers can detect these with a karyotype test. They stain white
blood cells to see the shape of a patient’s chromosomes under a
microscope. They can then identify any abnormalities, such as extra or
missing pieces of chromosomes.

he called pangenesis. He believed that gemmules—his term for
minute particles of inheritance thrown off by all cells of the
body—collected in the parents’ reproductive organs and combined
to form an embryo. Darwin said gemmules were responsible
for the transmission of human characteristics to children. He
also suggested that an organism’s external environment could
alter gemmules in the body and that parents could pass down
modified gemmules to their offspring (although he later rescinded
this idea). Although Darwin’s insights advanced the field of
biology, they did not fully explain the mechanism of inheritance.
Scientists continued to search for the substance that actually

Genetic Concepts Then and Now
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