Connect!
•Think of 3 traits you inherited.
•Why do you think you inherited these
traits?
•Why aren’t we identical to our parents?
•Why is each person a unique individual?
•What is DNA and why is it important?
•Explain why ducks produce ducklings
and not piglets when they reproduce.
The Gene-Chromosome Theory
GENES – Characteristics of
organisms are inherited as a result of
transmission of hereditary factors
called genes
The Gene-Chromosome Theory
• Genes exist at definite locations in a
row on chromosomes.
• Chromosomes
are found in the
nucleus of
cells.
Chunk Time!
• What are the hereditary factors that
transmit traits called?
• Explain the Gene-Chromosome
Theory.
• Where are chromosomes found?
• Organisms inherit 2 alleles for
each trait: one from Mom and
one from Dad.
A human cell nucleus contains 23 pairs of chromosomes.
They are all well known and numbered.
We all have two ch. 19s, two ch. 12s, two ch. 11s, etc…
Chunk Time!
• What is an allele?
• How many alleles do you receive for
one trait?
• How many pairs of chromosomes are
there in a human body cell nucleus?
• Think…how many total chromosomes
in a human body cell nucleus?
Homologous Chromosomes
• are the two matching chromosomes –
one from Mom, one from Dad
• the same genes in the same order are
on each chromosome of the pair
• however, the alleles might be
different… one might be for blue eyes,
the other might be for brown eyes
When a cell is getting ready to
divide, each single homologous
chromosome is replicated.
Chunk time!
• What are homologous chromosomes?
• Describe the genes on homologous
chromosomes.
• Are the genetic messages identical on
homologous chromosomes?
• What happens to each chromosome when
a cell is getting ready to divide?
One arm of the duplicated
chromosome is called a
chromatid.
The centromere holds the
two sister chromatids
together, and is where
spindle fibers will attach
during cell division to pull
the sister chromatids
apart.
DNA molecule
The spiral staircase shape is called a
double helix.
The DNA is coiled very tightly to form the
arms of the chromosome.
Chunk Time!
•
•
•
•
•
What is a chromatid?
Why are they called “sister chromatids”?
What is a centromere?
What is the function of the centromere?
What are the arms of the chromosome
made of?
• What is the shape of the DNA molecule?
Can
you
believe
this
?!!!