Biology Test 1

The entry of pollen tube into the embryo sac is under chemotrophic guidance. This happens as follows: Both pistil and pollen produces chemotrophically active substances. Chemotrophic substance of the pistil recognizes the chemotrophic substance in pollen forits acceptance or rejection. This interaction between the chemical components of pistil and pollen lead to the formation of pollen tube that enters into the embryo sac.

Ovules are of different types. In amphitrophous ovule, the embryo sac becomes horse-shoe shaped. Anatropous ovules are inverted during the development phase so that the micropyle lies very close to the hilum. In hemi-anatropous ovule, the body of the ovule lies at right angle to the funicle. And in circinotropous ovule, the funicle is extremely long which can encircle the ovule.

In entomophily, that is, insect pollination, the insect is attracted by factors like colour of flower, odour etc. They are also rewarded with nectar and pollen grains. Insects play a crucial role in pollination.

Double fertilization is the process in which the two sperm nuclei of a pollen grain unite with nuclei of the embryo sac of an angiosperm plant. One sperm nucleus unites with the egg to form the diploid zygote, from which the embryo develops.

In males, inhibin is secreted by Sertoli cells of seminiferous tubules. It is involved in the negative feedback control of sperm production. If spermatogenesis proceeds very rapidly, inhibin is released. Its target is the anterior pituitary gland where it reduces secretion of FSH. It may also have a slight effect on the hypothalamus reducing GnRH. 
In females, it is secreted by granulosa cells of ovarian follicles.

Testosterone- Testosterone stimulates spermatogenesis by endocrine and paracrine methods. Leydig cells synthesize testosterone which acts on Sertoli cells and peritubular cells as well. 
FSH- Follicle Stimulating hormone acta as an initiator of spermiogenesis along with testosterone in the absence of pituitary gland. It also stimulates the production of Androgen binding protein and formation of blood testes barrier.

Follicle stimulating hormone is secreted during the follicular phase along with luteinizing hormone. FSH stimulates maturation and helps in release of follicle from ovary. During maturation of follicle, it secretes the hormone estrogen. Estrogen helps in repairing the uterine wall. Progesterone is secreted in high conc. during the luteal phase and during pregnancy as well. Progesterone is secreted for about 14 days until corpus luteum turns into corpus albicans. However, progesterone is not usually present before ovulation.

One of the most effective method of birth control is the use of Intra Uterine Devices (IUDs). IUDs are available as: 

(i) Non-medicated IUDs- Lippes loop. 

(ii) Copper releasing IUDs- CuT, Cu7, Multiload 375, etc. 

(iii) Hormone releasing IUDs- Progestasert, Levonorgestrel, LNG-20, etc.

The genotype having a pair of different alleles for one character represents heterozygous condition. Therefore, Tt representing one dominant and one recessive allele for a single character shows heterozygous condition. TT, and RR represent homozygous condition i.e, same dominant alleles for one character. tt also represents homozygous condition, same recessive alleles for one character.

A dihybrid cross is wherein 2 characters are taken into consideration. Mendel took Round Yellow plants & Wrinkled Green plants, and crossed them. In the F1 generation, all plants obtained were round and yellow showing dominance over wrinkled green. These round yellow plants were bred and in the F2 generation, 4 types of combinations were obtained- Round Yellow, Wrinkled Yellow, Round Green and Wrinkled Green in the ratio of 9:3:3:1, the dihybrid ratio.