Biology 9 Lesson 12: Mechanism of sex determination
1. Theoretical Summary
1.1. Sex Chromosomes
Sex chromosomes are chromosomes that contain sex-determining genes and other genes.
In diploid (2n) cells:
- There are normal pairs of chromosomes.
- 1 pair of sex chromosomes XX (homologous) and XY (non-homologous).
Each sex chromosome has 2 segments:
- The non-homologous segment contains genes specific to each chromosome
- The homologous segment contains the same gene loci.
- In humans and mammals, fruit flies …. XX in the female, XY in the male.
- In birds, frogs, reptiles, butterflies… XX is in the male and XY is in the female.
- The sex chromosomes carry genes for male, female, and sex-related traits.
1.2. Mechanism of sex determination
– In most species, sex is determined during fertilization.
– The segregation and combination of sex chromosomes in meiosis and fertilization is the mechanism of sex determination in organisms.
- For example, the mechanism of sex determination in humans.
– The self-replication, segregation, and assembly of chromosomes during meiosis and fertilization are the sex-determining mechanisms.
- In the father: The segregation of the XY sex chromosomes gives two types of sperm X and Y in equal proportions.
- In the mother: The cleavage of the XX sex chromosome produces only one type of X egg.
+ Through fertilization: Create 2 types of zygotes XX develops into a girl and XY develops into a boy, with equal numbers and vitality. Therefore, the male to female ratio is usually approximately 1:1
\(P : XY x XX\)
\(Gp: X, Y; X\)
\(F1: 1XY; 1XX (1 Boy; 1 Girl)\)
1.3. Factors that affect sex differentiation?
– Sex hormones:
+ Disruption of sex hormone secretion will change the sex, but the sex chromosome pair will not change.
- Using Methyl testosterone (sex hormone) on female goldfish can make fish turn into male fish.
- Turtle eggs incubated at temperatures below 28oC will hatch into males over 32oC then hatches into a female.
+ Temperature, light … also change sex
- Castor plants grown in low light have a reduced number of male flowers
2. Illustrated exercise
What is the main difference between sex chromosomes and autosomes?
3.1. Essay exercises
Question 1: Why can one adjust the male:female ratio in livestock. What does that mean in practice?
Verse 2: Describe the mechanism of male and female birth in humans. Is the notion that the mother decides whether to have a son or a daughter is true or false?
Question 3: Why in the population structure the ratio of male: female is approximately equal?
Question 4: In species where the male sex is heterogametic, which of the following would ensure a male:female ratio of approximately 1:1?
a) The number of male gametes is equal to the number of female gametes.
b) Two types of gametes carry the same number of X and Y chromosomes.
c) The number of males and females in the species is inherently equal.
d) Fertilization of two male gametophytes carrying X and Y chromosomes with eggs of equal numbers.
3.2. Multiple choice exercises
Question 1: The characteristics of sex chromosomes are:
A. There are many pairs in somatic cells
B. There are 1 to 2 pairs in the cell
C. The number of pairs in a cell varies by species
D. There is always only one pair in a vegetative cell
Verse 2: In the vegetative cells of each organism, the sex chromosomes are:
A. Always a similar pair
B. Always a dissimilar pair
C. Is a homologous or dissimilar pair depending on gender
D. There are many pairs, they are not similar
Question 3: In human 2n cells, the symbol for the sex chromosome pair is:
A. XX in female and XY in male
B. XX in men and XY in women
C. In both males and females, there are XX homologous pairs
D. in both males and females there is an XY . non-homologous pair
Question 4: The following species have a pair of sex chromosomes XX in the male and XY in the female:
A. Fruit flies
B. Animals of the class Chim
Question 5: In humans, the expression ‘homozygous sex’ refers to:
C. Both male and female
D. Male entering puberty
After completing this lesson, you should:
- State some characteristics of sex chromosomes and their role in sex determination.
- Explain the mechanism of sex chromosome determination and the ratio of male: female in each species is 1: 1.
- State the factors of the internal and external environment that affect sex differentiation.