Sex Chromosomes

Sex Chromosomes
1 Each human cell contains 46 chromosomes or 23 pairs of chromosomes. 44 or 22 pairs of these chromosomes are known as autosomes.
2 The 23rd pair or the last two chromosomes are known as sex chromosomes.
3 There are two types of sex chromosomes which are the X chromosome or Y chromosome. These chromosomes determine the sex of a person. A human male will have one X and one Y chromosome, whereas a human female will have two X chromosomes.
4 The X chromosome is longer than the Y chromosome.

Sex Determination
1 The human male will have a genetic makeup (genotype) of 44 + XY, that is 22 pairs of autosomes and two types of sex chromosomes, X and Y.
2 The human female will have a genetic makeup of 44 + XX, that is 22 pairs of autosomes and one type of sex chromosome, XX.
3 Only the X and Y chromosomes play a role in determining the gender of the individual. The autosomes do not play any role in gender determination.


4 The sperm (male gamete) will carry either the Y chromosome or the X chromosome.
5 The ovum (female gamete) will carry only the X chromosome.
6 Figure 3.6 shows the production of male gametes or sperms in the testes through meiosis.

7 Figure 3.7 shows the production of female gametes or ova in the ovaries through meiosis.


8 During fertilization, a sperm will fertilize one ovum to produce a zygote.
9 If the sperm that carries the Y chromosome fuses with an ovum, the zygote formed will be a male.
10 However, if the sperm that carries the X chromosome fuses with the ovum, the zygote formed will be a female.
11 Figure 3.8 examines both of these possibilities.


12 The sex chromosome of the sperm of the father is responsible for determining the gender of the child.
13 Theoretically, the chance of getting a male child is 50% and a female child is also 50%.

Evaporation of water is affected by the surface area of the water.

Hypothesis: As the surface area of water increases, the rate of water evaporation also increases.

(i) Aim: To study the effect of surface area on the rate of water evaporation.

(ii) Variables:
manipulated variable - surface area of water
responding variable - rate of water evaporation
constant variable - water volume

(iii) Apparatus:
Three containers with different diameters, measuring cylinder

(iv) Procedure:
• 100 cm3 of water is measured with a measuring cylinder and is poured into the three containers with different diameters.
• All the three containers are left in the laboratory for one day.
• The next day, the water left in the containers is poured into the measuring cylinder and their volumes are measured respectively

(v) Results:

(vi) Conclusion:
The rate of water evaporation increases as the surface area of water increases.

To study the effects of nitrogen, phosphorus and potassium (NPK) on plants.

Aim:To study the effects of nitrogen, phosphorus and potassium (NPK) on plants.
Hypothesis:The macronutrients nitrogen, phosphorus and potassium are important for the healthy and normal growth of plants.

Variables:
(a) manipulated variable : contents of the culture solution
(b) responding variables : growth of plant and colour and size of leaves
(c) fixed variables : size and type of seedlings, volume of culture solution

Materials:
Cotton wool, black paper, glass tube, rubber tube, distilled water, maize seedling, calcium nitrate, potassium nitrate, potassium dihydrogen phosphate, magnesium sulphate, ferum phosphate, calcium chloride, potassium chloride, sodium nitrate and calcium dihydrogen phosphate.

Apparatus:
Bottle, measuring cylinder and weighing machine.



Procedure :
1 Four bottles are labelled A to D.
2 Each bottle is filled with a culture solution as shown in the table below.



3 Each bottle is fitted with a cork carrying a maize seedling and a glass tube as shown in Figure 2.10.
4 Each bottle is wrapped with a piece of black paper and placed near a window in the science laboratory where there is sunlight for 4-5 weeks.
5 The bottles are aerated each day.
6 The culture solution in each bottle is replaced each week.
7 The seedlings are observed each week and the observations are recorded.

Results :



Discussion:
1 Bottle A contains a complete culture solution with all the macronutrients and micronutrients required. The seedling shows normal and healthy growth.
2 Bottle A is set up as a control for comparison.
3 The bottles are covered with black paper to prevent sunlight from reaching the solution because sunlight encourages the growth of algae. The algae would absorb the nutrients, thus affecting the experiment.
4 The seedlings used must be of the same size (same age).
5 The bottles are aerated by blowing air through the rubber and glass tubes to supply oxygen for respiration.
6 The solutions are replaced each week to replace the minerals absorbed by the seedlings.

Conclusion:
The seedling in the complete culture solution grows into a healthy and normal plant.
The lack of a particular nutrient affects the growth of a plant as well as the colour and size of its leaves.
Thus, the hypothesis can be accepted.

Effects of the deficiency of nitrogen, phosphorus and potassium on plants ;