Chapter 27:1The 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

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

Chromosome NumbersAnimal or Plant Number of

Chromosomes in Body Cells

Number of Chromosomes in Sex Cells

Pea 12 6

Onion 14 7

Cat 38 19

Rabbit 44 22

Human 46 23

Chicken 78 39

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.

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.

This combination of chromosomes would indicate a

male

• 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.

• 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.