IGCSE Biology Paper-4: Specimen Questions with Answers 106 - 108 of 279

Explanation

Color blindness is an example of sex-linked character, which was first studied by Horner given in 1876. When a normal woman is married to a color blind man, their children will have normal color vision but when their daughters were married to normal man, 50 % of their sons are color blind and the remaining 50 % are normal, while the daughters were all normal.

In other case, when a colorblind woman marries a normal man, their daughters are normal but all sons are color-blind. When these F1 daughters are married to color blind men, colorblind sons and daughters are born in equal number.

Explanation

Bacterial-1. Chancroid (Haemophilus ducreyi)

2. Chlamydia (Chlamydia trachomatis)

Viral-1. Viral hepatitis (Hepatitis B virus)

Explanation

The sperms are microscopic and motile cells. They remain alive and remain their ability to fertilize an egg. It can be divided into three parts

1. Head
2. Neck
3. Middle piece
4. Tail

1-Head-

• Head is the anterior most part of the spermatozoon. It is often conical but its shape varies considerably
• Acrosome is formed from Golgi body of the spermatid
• It may be long and slender, spheroidal, rod-shaped or lance-shaped, spoon-shaped or short and flat, thin and , cork-screw-shaped or hooked

2-Neck-

• It is very short and present between head and middle piece
• It contains proximal centriole towards the nucleus which plays a role in first cleavage of zygote and distal centriole which gives rise to the axial filament of the sperm

3- Middle piece-

• Middle piece connect to the small neck of sperm. In neck of sperm 2 centrioles are present.
• Proximal centriole form the mitotic spindle after the fertilization process. This centriole arranged longitudinally to the main axis. Distal centriole form the axis of sperm.
• Axial filament of sperm structure as 9 + 2 manner. 2 central fibers and 9 peripheral fibers present

1. Tail

• It is a longest part of sperm. In the tail, the axial filament is covered by a sheet, which is made up of nine singlet fibers.
• Stored food materials absent in the sperm, ribosome endoplasmic reticulum, nucleolus and RNA is absent in the sperm.