The document discusses chromosomes and their role in determining sex. It notes that body cells contain 23 chromosome pairs while sex cells contain 23 single chromosomes. Amniocentesis allows doctors to check the fetus' chromosome number. Females have two X chromosomes while males have an X and a Y chromosome. The father determines the child's sex by providing either an X or Y chromosome.

1. Chapter 27:1
The Role of Chromosomes
• Objectives:
• 1. Compare the chromosomes number in
body cells and sex cells
• 2. Describe methods that doctors use to
study chromosomes of a fetus
• 3. Compare the chromosomes of males and
females

2. Chromosome Number
• Each human sperm or egg has 23 chromosomes
• Each human body cell has 23 PAIRS of
chromosomes. There are a total of 46
chromosomes in each human body cell
• Different organism have different a number of
chromosomes
• The chromosomes in most living things are paired

3. Chromosome Numbers
Animal or Plant Number of Number of
Chromosomes Chromosomes
in Body Cells in Sex Cells
Pea 12 6
Onion 14 7
Cat 38 19
Rabbit 44 22
Human 46 23
Chicken 78 39

4. A Way to tell
Chromosome Number
• Amniocentesis is a way for doctors to tell if a fetus
has the correct chromosomes before it is born.
• A needle is stuck into a woman's abdomen and a
little of the amniotic fluid is removed
• The fluid is grown in the lab for about 10 days and
they are able to count the number of chromosomes
• They are able to study the chromosomes and
determine if there are the correct number or if
there is anything missing or anything extra on the
chromosome.

5. Sex- A Genetic Trait
23 pairs of human
chromosomes
The sex chromosomes XX
indicates a female
The XY indicates a male
The Y chromosome is
only found in males
Each sex cell would have
only one type of sex
chromosomes, either a XX
for a female or an XY for
a male.

6. This combination of
chromosomes would indicate a
male

7. • Chromosomes numbered 1-22 are called
autosomes. They are the same in a male or
female.
• The only chromosome that is different is the sex
chromosome. It is either an XX or XY.
• Since females have only one sex chromosome,
they make only one type egg cell. It has the X
chromosome only.
• Males have the X and the Y chromosome so they
can make two types of sperm cell. One containing
the X chromosome and one containing the Y
chromosome.

8. • The father determines the sex of the child.
• If the child receives the X chromosomes
from the father , it will be a male.
• If the child receives the Y chromosomes
from the father , it will be a female.