This document discusses human sex determination and sex chromosomes. It notes that males have XY sex chromosomes while females have XX. It explains that sex is determined at conception - if a sperm carrying a Y chromosome fertilizes an egg, the offspring will be male, while an X chromosome will result in a female offspring. There is a 50% chance of producing a male or female child. The presence of a Y chromosome determines maleness.

1. Sex Chromosome
and
Sex Determination

2. How many chromosomes do humans have?
46 Chromosomes
How many pairs of chromosomes do each individual
have?
23 pairs of chromosomes for both males and females
What do you call the 22 pairs of chromosomes?
Somatic chromosomes
How about the 23rd chromosome?
Sex chromosome

3. •Human males and other male
organisms, such as other
mammals and fruit lies, have non
identical sex chromosomes (XY).
•Females have identical (XX) sex
chromosomes.

4. How is sex determined
and inherited?

5. Figure 3. Gamete formation involving only sex
chromosomes
Sex
chromosomes
Meiosis
Gametes

6. Figure 4: Sex determination

7. If an egg is fertilized by a sperm with a Y
chromosome, as shown in Figure 4, the
offspring is male. When an egg is fertilized by
a sperm carrying an X chromosome, the
offspring is female. Note that there is a 50
percent chance of having a male or female
offspring. The greater the number of
offspring, the greater is the chance of getting
the expected 1:1 ratio of male and female.

8. Activity 4: Girl or Boy ?
Objective:
Discuss how sex in humans is determined
Materials:
Activity sheets
Pen
paper

9. Procedure:
1. Draw a Punnett square which shows the inheritance
of the sex chromosomes. Represent the female sex
chromosomes with XX and the male sex chromosomes
with XY.

10. Guide Questions:
Q 13. What will be the sex of a child produced when an egg
is fertilized by a sperm that has a Y chromosome?
_________________________
Q 14. What type of sperm must fertilize an egg to result in
a female child?____________________
Q 15. Based on this Punnett Square, what percent of
children would you expect to be
male?___________________

11. Q 16. Which sex chromosome is present in both
male and female?__________________________
Q 17. Infer which sex chromosomes determines a
person’s sex._________________________
18. What are the other factors that may influence
the expression of human
sexuality?._________________

12. Answers:Q 13. Male
Q 14. X bearing sperm
Q15. 50%
Q16. X chromosome
Q17. Y chromosome
Q18. Environmental factors, such as age of mother that may lead to
nondisjunction (Trisomy 21), Aneuploidy where there is an excess or
lack of X or Y chromosome and genital development during
conception, and the social interactions after birth may contribute to
a certain degree to the expression of human sexuality.

13. Key Concepts
•Males have 44 body chromosomes and two sex
chromosomes X and Y. The males determine the
sex of their children.
•Females have 44 body chromosomes and two
sex chromosomes, both X. The total number in
each cell of an individual is 46. These
chromosomes contain the genes, which are the
factors of heredity.