Reproduction and Inheritance: Essential Biology Concepts
Sexual Reproduction in Flowering Plants
1. What is Double Fertilization?
Answer: Double fertilization is a characteristic feature of angiosperms. In this process, two male gametes are involved in fertilization. One male gamete fuses with the egg cell to form a diploid zygote (syngamy). The other male gamete fuses with two polar nuclei to form a triploid primary endosperm nucleus (triple fusion). Thus, two fusions occur in the same embryo sac, so it is called double fertilization.
2. What is Triple Fusion?
Answer: Triple fusion is the fusion of one male gamete with two polar nuclei present in the central cell of the embryo sac. This results in the formation of a triploid primary endosperm nucleus which later develops into endosperm.
3. Write a Short Note on the Embryo Sac.
Answer: The embryo sac is the female gametophyte of angiosperms. It is present inside the ovule. A typical embryo sac is seven-celled and eight-nucleate in structure. It consists of one egg cell, two synergids, three antipodal cells, and two polar nuclei located in the central cell. The egg cell and synergids together form the egg apparatus. The embryo sac plays an important role in fertilization and seed formation.
4. What is Apomixis?
Answer: Apomixis is a type of asexual reproduction in which seeds are formed without fertilization. In this process, the embryo develops without the fusion of male and female gametes. The offspring produced are genetically identical to the parent plant.
5. What is Parthenocarpy? How can it be induced?
Answer: Parthenocarpy is the development of fruit without fertilization. Such fruits are seedless. It can be induced artificially by spraying growth hormones like auxins and gibberellins on flowers.
6. Explain Any Three Outbreeding Devices.
Answer: Outbreeding devices are special mechanisms developed by flowering plants to prevent self-pollination and encourage cross-pollination.
- Dichogamy: The condition in which the anther and stigma mature at different times, preventing self-pollination.
- Self-incompatibility: A genetic mechanism where pollen grains of the same plant are unable to fertilize the ovule.
- Unisexuality: The condition in which male and female flowers are present separately, reducing the chances of self-pollination.
7. What cells are present in a developed embryo sac?
Answer: A developed embryo sac contains seven cells and eight nuclei. These are one egg cell, two synergids, three antipodal cells, and one central cell containing two polar nuclei.
Human Reproduction
1. What are Leydig cells? State their function.
Answer: Leydig cells are interstitial cells present between the seminiferous tubules of the testes. They secrete the male sex hormone testosterone, which stimulates spermatogenesis and the development of secondary sexual characters in males.
2. Why do sperm contain mitochondria?
Answer: Sperm contain mitochondria in the middle piece to produce ATP, which provides energy for the movement of the tail and helps in the motility of the sperm.
3. Describe the formation of the placenta.
Answer: After fertilization, the zygote develops into a blastocyst which gets implanted in the endometrium of the uterus. The outer layer of the blastocyst, called the trophoblast, develops finger-like projections known as chorionic villi. These villi penetrate into the uterine endometrium and are surrounded by maternal blood spaces. The chorionic villi, together with the uterine tissue, form a temporary vascular organ called the placenta. The placenta acts as an interface between the mother and fetus and helps in the exchange of nutrients, oxygen, and carbon dioxide, and the removal of excretory wastes. It also functions as an endocrine gland and secretes hormones such as hCG, hPL, estrogen, and progesterone, which are essential for the maintenance of pregnancy.
4. How does implantation occur in the mother’s uterus?
Answer: Implantation is the process by which the blastocyst becomes attached to and embedded in the endometrium of the uterus. After reaching the uterus, the blastocyst loses its zona pellucida and the trophoblast cells come in direct contact with the uterine wall. The trophoblast secretes enzymes that digest the endometrial tissue and allow the blastocyst to sink into the uterine wall. The endometrium becomes thick and vascular under the influence of progesterone. The embedded blastocyst becomes firmly attached, and further development of the embryo begins.
5. Diagrammatic phases of spermatogenesis.
Answer: Spermatogonium (2n) → Primary spermatocyte (2n) → Meiosis I → Two secondary spermatocytes (n) → Meiosis II → Four spermatids (n) → Spermiogenesis → Four spermatozoa (n).
6. Changes during the follicular phase.
Answer: The follicular phase begins after the menstrual phase and lasts up to ovulation. During this phase, FSH secreted by the anterior pituitary stimulates the growth and maturation of ovarian follicles. One follicle becomes dominant and develops into a Graafian follicle. The growing follicle secretes increasing amounts of estrogen. Estrogen stimulates proliferation and thickening of the endometrium and increases vascularization of the uterine wall. Towards the end of this phase, high levels of estrogen induce a surge of LH, which leads to ovulation.
7. Write the six functions of the placenta.
Answer: The placenta performs several important functions:
- Respiration: Supplying oxygen to the fetus and removing carbon dioxide.
- Nutrition: Transferring glucose, amino acids, vitamins, and minerals.
- Excretion: Removing excretory wastes such as urea from fetal blood.
- Protection: Acting as a barrier against certain harmful substances and pathogens.
- Endocrine Function: Secreting hormones like hCG, hPL, estrogen, and progesterone.
- Immunity: Transferring antibodies from the mother to the fetus to provide passive immunity.
Reproductive Health
Important Abbreviations
- ART – Assisted Reproductive Technologies
- IVF – In Vitro Fertilization
- ET – Embryo Transfer
- ZIFT – Zygote Intrafallopian Transfer
- GIFT – Gamete Intrafallopian Transfer
- ICSI – Intracytoplasmic Sperm Injection
- AI – Artificial Insemination
- AIH – Artificial Insemination by Husband
- AID – Artificial Insemination by Donor
- IUI – Intrauterine Insemination
- MTP – Medical Termination of Pregnancy
- STD – Sexually Transmitted Disease
- RTI – Reproductive Tract Infection
- HIV – Human Immunodeficiency Virus
- AIDS – Acquired Immuno Deficiency Syndrome
- WHO – World Health Organization
- CDRI – Central Drug Research Institute
- IUCD / IUD – Intrauterine Contraceptive Device
- Cu-T – Copper-T
- LNG-IUD – Levonorgestrel-releasing Intrauterine Device
- SAHELI – Non-hormonal oral contraceptive (Centchroman)
8. What are STDs and their preventive measures?
Answer: Sexually Transmitted Diseases (STDs) are infections transmitted through sexual contact, caused by bacteria, viruses, or other microorganisms. Preventive measures include: avoiding multiple sexual partners, maintaining mutual monogamy, using condoms, maintaining personal hygiene, early diagnosis, screening blood before transfusion, and using sterile needles.
9. Reproductive health problems and strategies.
Answer: Problems include early marriages, unwanted pregnancies, STDs, population explosion, and maternal/infant mortality. Strategies include: sex education in schools, awareness programs, promotion of small family norms, availability of contraceptives, maternal and child healthcare services, legal bans on female feticide, and safe MTP services.
10. Describe Assisted Reproductive Technologies (ART).
Answer: ART includes:
- IVF-ET: Fertilization outside the body followed by embryo transfer.
- ZIFT: Transfer of the zygote into the fallopian tube.
- GIFT: Transfer of ovum and sperm into the fallopian tube.
- ICSI: Direct injection of a sperm into the ovum.
- AI: Introduction of semen into the vagina or uterus.
11. What is AIDS? Discuss its prevention.
Answer: AIDS is caused by the Human Immunodeficiency Virus (HIV), which attacks the immune system. Symptoms include prolonged fever, weight loss, and chronic diarrhea. Prevention involves: avoiding multiple partners, using condoms, screening blood, using sterile needles, and creating awareness about safe practices.
12. What is MTP and its legal conditions?
Answer: MTP (Medical Termination of Pregnancy) is the intentional termination of pregnancy before full term. It is legally allowed if there is a risk to the mother’s life, pregnancy due to rape, failure of contraceptives, or serious fetal abnormalities. It must be performed by a registered medical practitioner.
Principles of Inheritance and Variation
Q. What is sex-linked inheritance? Give an example.
Answer: Sex-linked inheritance is the transmission of characters through genes located on sex chromosomes (usually the X chromosome). An example is hemophilia and color blindness in humans.
Q. Differences between sex chromosomes and autosomes.
Answer: Sex chromosomes (X and Y in humans) determine the sex of an individual and consist of one pair. Autosomes are the other 22 pairs of chromosomes that control general body characters.
Q. Differences between linkage and crossing over.
Answer: Linkage is the tendency of genes on the same chromosome to be inherited together, reducing variation. Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, increasing genetic variation.
Q. Incomplete dominance vs. Codominance.
Answer: In incomplete dominance, the heterozygote shows an intermediate phenotype (e.g., pink flowers in snapdragons). In codominance, both alleles express themselves equally (e.g., AB blood group).
Q. Klinefelter syndrome vs. Turner’s syndrome.
Answer: Klinefelter syndrome (47, XXY) affects males, resulting in sterility and feminine characteristics. Turner’s syndrome (45, XO) affects females, resulting in short stature and sterility.
Q. How is sex determined in human beings?
Answer: Sex is determined by the sperm. Females (XX) produce only X-containing ova. Males (XY) produce X and Y-containing sperm. If an X-sperm fertilizes the egg, the child is female (XX); if a Y-sperm fertilizes the egg, the child is male (XY). Thus, the father determines the sex of the child.