Please email your answers to this Year 10 Preparation worksheet to widmerjo@werribeesc.vic.edu.au
Chromosomes!
Long ago the Ancient Egyptians recognised that the
racing characteristics of greyhounds were inherited. It
is believed that they selected parent dogs that were
strong and fast runners, and bred them to produce
young dogs with these characteristics. By selectively
breeding they produced dogs that were highly valued
in Egyptian society.
Greyhounds reproduce sexually. That is, the male
produces sperm and the female produces eggs. A new
individual forms when the nuclei of the sperm and egg
join together. After fertilisation, the egg divides and the
new organism gradually grows in size.
It is reasonable to assume that the sperm and the egg must contain all the instructions for the
cells to make the new organism.
Chromosomes and inheritance
At the turn of the 20th century, many scientists wanted to find out what part of an organism’s
cell was responsible for passing on characteristics from one generation to another. An
American biologist, Walter Sutton, provided one of the earliest clues to the problem. He
wondered about the importance of sausage-shaped objects that had been observed in cells
undergoing division. He noticed that the rounded nucleus in a cell seemed to change into
these sausage-shaped objects just prior to cell division,
and each of the new cells contained these objects. These
objects we now call chromosomes.
Sutton also observed that a particular species of
organism always had the same number of chromosomes,
and that the number of chromosomes was different for
different organisms.
Sutton chose fruit flies for his experiments because
they had only 8 chromosomes in their cells and they bred
very quickly. On the basis of his experiments, Sutton
inferred that the chromosomes carry the inherited
characteristics and that each organism has a particular number of chromosomes.
Chromosomes can be seen only during cell division, and the two new cells that are formed
each get one of every chromosome. This means that the new cells contain exactly the same
number of chromosomes as the original cell.
Chromosomes are usually in pairs. This is why the numbers in the table here are even
numbers. For example, humans have 23 pairs, a fruit fly has 4 pairs and corn has 10 pairs.
Questions
1. What did the Egyptians learn about breeding dogs?
2. What is reasonable to assume about the genes carried in the sperm and the egg of breeding
dogs?
3. What did the scientist, Walter Sutton, notice about the nucleus of the dividing cell?
4. Why did he choose to breed fruit fly for his experiments?
5. How may chromosomes do humans carry?
6. How many pairs of chromosomes does a chicken have?
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