Download Our Genes - 10Mackillop

Our Genes:
The drawing below shows three generations of the McFlea family. Mr. and Mrs. McFlea
are shown with four children. Mr McFlea’s parents are shown at the rear left and Mrs.
McFlea’s parents are at the rear right.
1. Which characteristics give the McFlea’s grandchildren their ‘family resemblance’?
2. Which of the characteristics have come from Mr. McFlea’s side of the family? Which
have come from Mrs. McFlea?
3. Suggest why baby McFlea has red hair when neither of his parents have red hair.
Genetics:
Genetics is the branch of biology that deals with the study of inherited characteristics and
the expression of these traits. People who study Genetics are called Geneticists. They
study the characteristics passed on from one generation to the next.
Heredity:
The tendency for offspring to be like their parents and resemble each other is called
Heredity.
The environment however, can play a major part in altering or distorting some of the
characteristics we inherit.
Where are all these characteristics found?
Scientists have discovered that thread-like bodies called chromosomes carry all the
genetic information that are required to make ‘you’.
Chromosomes are found in the nucleus of the cell. Along the length of these
chromosomes are the instructions for controlling the cell. Each of these instructions are
called genes.
Chromosomes:
Genes and Chromosomes:
It is known that individuals inherit characteristics
from their parents. 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 nuclei of the sperm and the egg must contain all the
instructions for the cells to make the new organism. We call these instructions genes, and
they tell the cells how to grow and when to grow. They determine the shape of the body,
the colour of the eyes, hair and skin, and many other characteristics.
At the turn of the 20th century, many scientists became interested in the ‘new’ science of
genetics - the biological study of inheritance. They 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. On the basis of many experiments, Sutton inferred that the chromosomes
carry the inherited characteristics and that each organism has a particular number of
chromosomes. For example, each cell in the human body contains 46 chromosomes.
The nuclei of sex cells (sperm and eggs)
contain only as many chromosomes as other
cells. The formation of sperm and ova occurs
by a special type of cell division during
which the chromosome pairs separate. This
means that each sex cell receives 23 single
chromosomes. When the nuclei of the sperm
and egg join during fertilisation, the new cell
then contains 23 pairs of chromosomes.
Organism
human
mouse
fox
fruit fly
pea
corn
Number of chromosomes
46
40
34
8
14
20
Chromosomes are always in pairs. This is why
the numbers in the table above are even
numbers.
For example, humans have 23 pairs, a fruit fly
has 4 pairs and corn has 10 pairs.
In the diagram below the 46 chromosomes in a
human have been placed in pairs from largest to
smallest (except the X chromosomes). Notice
that each chromosome is doubled and is joined
to form roughly an H shape.
Each characteristic of an organism is
controlled by a pair of genes - one on each
chromosome in a pair. And each gene is found
at the same location on each of the two
chromosomes. In the diagram below the gene
for hair shape is found at a particular location
on a chromosome. In this case the person
carries a gene for wavy hair on one
chromosome and a gene for straight hair on
the other.
Inherited characteristics:
The photograph below shows some of the characteristics children inherit from their
parents. The development of these characteristics is controlled by chromosomes.
Acquired characteristics:
You may know how to swim, or rollerskate, or speak French. Or you may have a scar
from a cut. These are acquired characteristics. You pick them up (acquire them) as you
go through life. You are not born with them, and you cannot pass them on to your
children.
Variation:
No two people are exactly the same. Even identical twins are different in some ways.
People are different heights and weights. Their hair and eyes are different colours and
their faces have different shapes. Their eye-colour and hair-colour show variation.
 Continuous variations:
These people are arranged in a line from the shortest to the tallest. Their height
shows continuous variation. It varies from short to tall with many small
differences in between. Intelligence also shows continuous variation. Can you
think of other variations which are continuous?
 Discontinuous variation:
People can either roll their tongues or they cannot. This is an example of
discontinuous variation. You can either do it or you can’t. There is no ‘in between’
state. Blood groups show discontinuous variation. You belong to only one group:
A, B, AB, or 0. Find out what blood group each member of your class Tongue
rolling. Either you can... or you can’t belongs to. Can you think of any other
variations that are discontinuous?
Questions:
1. Write down eight inherited characteristics.
2. Write down six acquired characteristics.
3. Intelligence shows continuous variation. What does this mean?
4. Colour blindness shows discontinuous variation. What does this mean?
5. Write down four of your own acquired characteristics.