Download Ch 3 Sec3

The Cell and Inheritance
• In 1903, an American geneticist, named
Walter Sutton studied the cells of
grasshoppers.
– Focused on the movement of chromosomes
during the formation of sex cells
– Discovered that grasshopper’s sex cells
have half the number of chromosomes as
their regular body cells.
– One chromosome in each pair came from
each parent.
Idea
• Sutton’s idea came to be
known as the Chromosome Theory of
Inheritance.
– Genes are carried from parent
to their offspring on
chromosomes.
Meiosis
• How do sex cells end up with half the
number of chromosomes as body cells?
– Meiosis
•Chromosome pairs separate
•They are distributed into 2 different
cells
•The resulting sex cells only have half
as many chromosomes.
Meiosis
• Before meiosis- every chromosome in
the parent cell is copied
• Meiosis I– chromosome pairs line up in the center
– The pairs separate and move to opposite
ends
– 2 cells form, each with half the number of
chromosomes, each chromosome still has
2 chromatids
Meiosis cont.
• Meiosis II– The chromosome with their 2 chromatids
move to the center of the cell
– Centromeres split and chromatids separate,
single chromosome move to opposite ends
of the cell
• End of meiosis- 4 sex cells have been
produced, each with only half the
number of chromosome as the parent
cell in the beginning
Line up of Genes
• The human body contains 23
chromosome pairs for a total of 46
chromosomes.
• Chromosomes are made up
of many genes, like beads on
a string.
• Those 23 pairs of chromosomes
contains 20,000 to 25,000 genes
– Each gene controls a trait.
Genetic Principles
1. Traits are passed from one
generation to the next
2. Traits are controlled by genes
3. Genes are inherited in pairs, 1
gene from each parent
4. Genes can be dominant or
recessive
5. Dominant genes “hide”
recessive genes
6. Some genes are neither
dominant nor recessive; they
show incomplete dominance