Answer any two questions (restrict each answer within 200 words):
Write short notes on any two of the following:
(a) Menopause
(b) Parturition
(c) Types of sex-linked genes
(d) Codominance

Explanation

It is the phase in a woman.s life when ovulation and menstruation stop. It occurs between 45-55 years of age. Some women have irregular cycles for months or years prior to menopause. Others simply stop menstruating abruptly.
Cause One theory is that menopause is a result of changes in the secretions of pituitary gland and the nearby hypothalamus. Another theory suggests that menopause may begin when no follicles are left in the ovaries. In fact decline in oestrogen and progesterone level leads to menopause.
Symptoms The uterus, and the vagina gradually become atrophic (decreased fertility). Women may suffer temporarily depression, hot flashes, and other physiological and psychological problems during menopause. Most of these symptoms can be relieved by taking hormones prescribed by a physician. But this replacement of hormones is not encouraged due to some side effects.
(b) Parturition is the act of expelling the full term young one from the mother.s uterus at the end of gestation period. It is induced by both nervous system and hormones secreted by the endocrine glands of the mother. The signals for childbirth (parturition) originates from the fully matured foetus and placenta which induce mild uterine contractions called foetal-ejection reflex. This causes quick release of oxytocin from maternal posterior lobe of pituitary. The amount of oxytocin is increased just before and during labour pains which promote contractions of the uterine muscles. Relaxin increases the flexibility of the pubis symphysis and helps to dilate the uterine cervix.
(c) The sex-linked genes are of the following types:
(i} X-linked genes These are sex-linked genes which lie on X-chromosomes, e.g. genes for colour blindness and haemophilia. These X-linked traits have a unique mode of inheritance as females have two pairs ofX-linked genes, while males have only one. Thus, males are hemizygous for X-linked traits and they possess only half the number of X-chromosomes that a female possesses. An X-linked gene can be dominant or recessive due to which a female, can be a heterozygous carrier of X-linked trait.
(ii) Y-Iinked genes These are sex-linked genes, which are inherited straight from father to son or male to male, e.g. genes for hypertrichosis. Any gene which occurs exclusively on Y-chromosome is said to be holandric and it shows holandric inheritance pattern.
(iii} Pseudoautosomal genes These are the genes located on homologous parts of both X and Y-chromosomes.
(4) Codominance is the inheritance of a trait where two different dominant alleles of a trait express themselves simultaneously in the progeny. For example, ABO blood groups in human population.
(a) Gene for blood group occurs in three diallelic forms, viz `I^(A), I^(B), and i`.
(b) `I^(A) and I^(B)` produce RBC surface antigen A and B respectively, whereas i does not produce any antigen.
(c ) `I^(A) and I^(B)` both are dominant alleles, whereas .i." is the recessive allele.
(d) In case `I^(A) and I^(B)` are present together, both expresses themselves equally and produce both surface antigen A and B.