Answers to Hands-on Pg 63-64

Aim : To illustrate Mendel’s first law using coloured buttons

Colours of buttons	Number of pairs	Trait represented by buttons
Red and red	24	Tall
Red and white	53	Tall
White and white	23	Dwarf

Discussions:

1. Red
2. Gamete
3. Tall plant. Red button represent the gene for tallness which is dominant. Therefore when it is paired with a white button which is recessive, then a tall plant is produced.  A dominant gene expresses itself even when it is in the heterozygous condition.
4. The ratio of tall : dwarf is 3:1.  This is because an organism’s characteristic is determined by a pair of genes which will separate and be carried in a gamete and the dominant gene will express itself.

Conclusion :

Mendel’s first law is illustrated by separating the buttons before pairing them again which shows that a characteristic in an organism is represented by a pair of genes. The genes will separate and each gene is carried by a gamete.

Answer to Hands - on Pg 54-56

Objective Questions

1	B	11	D
2	D	12	C
3	B	13	D
4	B	14	D
5	C	15	A
6	D	16	B
7	B	17	C
8	B	18	B
9	C	19	A
10	B

1. (a) P : 2n           Q : n              R : n

(b) W : Mitosis

X : Meiosis I

Y : Meiosis II

(c) Spermatogonium →Primary spermatocyte→Secondary spermatocyte →Spermatid →

Spermatozoa

(d) (i) Mitochondrion

(ii) To produce energy through cellular respiration for the sperms to swim to the ovum

(iii) To propel the sperm forward when swimming towards the ovum

(e) Artificial insemination in which sperms from a donor can be obtained from a sperm bank and injected into the uterus of a woman during ovulation.

2. (a) (i) Root cap

(ii) Protects the root tip

(b) (i) Zone of cell division

(ii) Draw on your own

(c)(i) Draw on your own

(ii) Cell in zone Q absorbs water through osmosis. The vacuole expands and pushes

the cytoplasm against the cell wall. The cell elongates.

(d) Xylem and phloem

(e) Draw on your own

3. (a) Growth curve of a rat: Sigmoid curve

Growth curve of a cockroach: Series of steps

(b) (i) Early phase

(ii) Mainly cell division through mitosis where growth rate is slow

(c)(ii) In the exponential phase, growth speeds up exponentially. The number of cells increases and cells increase in size quickly. In the maturity phase, growth slows down as it reaches maturity.

(d) (i) X is a stage of ecdysis.

(ii) The cockroach takes in air to expand its body. The old exoskeleton is shed to expose the new and soft exoskeleton beneath it. Growth of organs and tissues takes place quickly before the new exoskeleton hardens.

Answers to Hands-on Pg 50-53

Aim : To study the growth pattern of maize plant (Zea mays)

Variables : Manipulated variable : Time of growth

                   Responding variable : Mass of seedlings

                   Fixed variable : Amount of water, fertilizer

Time (weeks)	Mass of 3 seedlings (g)		Mean mass of 1 seedling (g)
	Fresh	Dry	Fresh	Dry
0	0.90	0.60	0.30	0.20
1	0.60	0.45	0.20	0.15
2	0.90	0.60	0.30	0.20
3	1.20	0.90	0.40	0.30
4	2.40	1.80	0.80	0.60
5	4.20	3.15	1.40	1.05
6	6.00	5.25	2.00	1.75
7	8.85	7.50	2.95	2.50
8	11.40	10.20	3.80	3.40
9	14.10	13.20	4.70	4.40
10	15.30	14.70	5.10	4.90
11	15.90	15.00	5.30	5.00
12	16.20	15.30	5.40	5.10

Draw a graph based on the above table

Discussion :

1. Sigmoid growth curve
2. Week 5 to week 8
3. Growth rate is negative from the beginning of the activity ti week 1.  This is because the germinating seeds use food stored in the endosperm.  Week 1 to wek 4 shows slow growth rate because young leaves are starting to grow and photosynthesis has just begun.  Week 5 – 8 shows highest growth rate.  The growth rate slows down from week 8 onwards because the plant has reached maturity.

Conclusion :

The growth pattern of the maize seedling represented by a graph of the mass of maize seedlings against time is a sigmoid curve. The hypothesis is accepted.

Answers to Hands-on Pg 47-49

Aim : To observe the germination of pollen grains

Observation :

Pollen grain

Germinating pollen grain

Discussion

1. The slide with the sucrose solution.
2. To stain the male nuclei and the tube nuclei.
3. The surface of the pollen grain is coarse and has germinating pores.
4. The sucrose solution secreted by the stigma cells stimulates the growth of a pollen tube which grows into the ovule to fertilise the eg cell and polar nuclei in the embryo sac

Genetics and Variations

Presentation by 5C students

 

 

 

GENETIC DISEASES

What is Klinefelter Syndrome ?

Klinefelter syndrome is the condition where a male has an additional X chromosome in his cells. Under normal conditions, a male has XY chromosome pattern whereas for males with Klinefelter syndrome the pattern differs to XXY (44 + XXY).

This extra X chromosome leads to development of various abnormalities like :

·      small testes,

·      enlarged breasts,

·      infertility

·      long legs,

·      mental retardation

·      and reduced sperm production.

What is Turner syndrome?

Turner syndrome is a condition in which a female does not have the usual pair of two X chromosomes. (44 + XO)

Possible symptoms in young infants include:

·                     Swollen hands and feet

·                     Wide and webbed neck

A combination of the following symptoms may be seen in older females:

·                     Absent or incomplete development at puberty, including sparse pubic hair and

            small breasts

·                     Broad, flat chest shaped like a shield

·                     Drooping eyelids

·                     Dry eyes

·                     Infertility

·                     No periods (absent menstruation)

·                     Short height

What is Down’s syndrome?

Down's syndrome, also known as trisomy 21, is a chromosomal condition caused by the presence of three chromosome 21.

Common physical signs include:

• Flattened nose
• Upward slanting eyes
• Single crease in the palm of the hand
• Small ears
• Small mouth
• Wide, short hands with short fingers

Physical development is often slower than normal. Most children with Down syndrome never reach their average adult height.

Children may also have delayed mental and social development. Common problems may include:

• Impulsive behavior
• Poor judgment
• Short attention span
• Slow learning