Chapter 2 - Heredity

• Living organisms grow, develop, respire, feed, reproduce, excrete and eventually age and die. For continuity of life, the most critical characteristic of living organisms is reproduction.
• Organisms must be able to reproduce to pass on their genes, whether by replicating themselves or by mating with another individual to produce fertile offspring.
• Offspring carry the same genetic traits or a mix of traits from their parents into the next generation, ensuring that, even though individuals die, the gene pool and the species continue.
• Asexual reproduction


  Involves only one parent and gives rise to offspring that are genetically identical to each other and to the original parent
• Sexual reproduction


  Usually involves two parents who produce offspring that have a mix of the parents' genes and therefore differ from each other and from the parents
• The greatest advantage that sexual reproduction is thought to provide, in terms of continuity of life at the species level, is genetic diversity.
• Some offspring may possess random variations that make them better suited to new and changing environmental conditions. They may out-compete their parents and/or other individuals in the population, thereby gaining a selective advantage.
• Disadvantage of sexual reproduction

  It demands a greater expenditure of time and energy, involving processes such as finding a mate, courtship behaviour, gamete production and mating, before the production of young. These processes may also make organisms more vulnerable to predators.
• Diploid


  Containing two sets of chromosomes
• Haploid


  Containing one set of chromosomes
• Gametes


  Sex cells that carry genetic information from both parents to the offspring
• Somatic cells


  Body or non-reproductive cells
• Paternal chromosomes


  Chromosomes inherited from the father
• Maternal chromosomes


  Chromosomes inherited from the mother
• Meiosis


  A type of cell division that takes place in the reproductive organs of plants and animals to maintain the characteristic chromosome number during sexual reproduction
• Fertilisation and meiosis are reciprocal processes – that is, one is a fusion from haploid to diploid, and the other is a reduction from diploid to haploid.
• External fertilisation


  • Better suited to organisms that reproduce in an aquatic environment or a very moist environment
• Internal fertilisation


  • Typical of many terrestrial organisms
• Vertebrate sexual reproduction is thought to have started in the ocean (fish) and freshwater environments (amphibians) and then evolved once vertebrates such as reptiles, birds and mammals colonised the land and the air.
• The chances of successful external fertilisation are increased by synchronisation of reproductive cycles, mating behaviours and the release of gametes.
• When fertilisation and development of the young take place externally, there is little or no parental care. This means that less time and energy are required of the parents, but a larger number of gametes must be produced to ensure that some young survive.
• The advantage of external fertilisation is the wide dispersal of young. Some marine animals release their gametes into the sea, and fertilised eggs are carried away to settle in an area far from their parents.
• Staghorn coral


  • A colony of invertebrate marine animals (polyps) that achieve fertilisation by simply shedding millions of gametes into the sea
• Environmental cues, such as water temperature, tides and day length, help synchronise the reproductive cycle of staghorn coral.
• When polyps in one coral colony start to spawn, pheromones released along with gametes stimulate nearby individuals to spawn, resulting in coordinated spawning over a wide area.
• Staghorn coral


  Colony of invertebrate marine animals (polyps) that achieve fertilisation by simply shedding millions of gametes into the sea
• Staghorn coral


  • Environmental cues (water temperature, tides, day length) help synchronise the reproductive cycle
  • When polyps in one coral colony start to spawn, pheromones released along with gametes stimulate nearby individuals to spawn, resulting in coordinated spawning over a wide area
• Pheromones


  Chemical substances released by one organism that have an effect on another organism
• During the mass spawnings of coral on Australia's Great Barrier Reef, the number of gametes shed is so great that, for a time, the sea turns milky
• Fertilisation of staghorn coral

  1. Fertilised eggs develop into swimming larvae

  2. Larvae descend to find a suitable site to form a new colony

  3. Almost all larvae are eaten by predators

  4. Only a tiny proportion reach adulthood
• Bony fish


  • Females produce eggs in large batches and release them into the water, where they fuse with sperm outside the body of the female
  • Release of gametes is restricted to a few brief and clearly determined periods
  • Many of the resulting offspring succumb to microbial infections or predation, and few survive to maturity
• Amphibians


  • Gametes from both males and females are released in fresh water
  • In frog and toad copulation, the male grasps the female and discharges fluid containing sperm onto the eggs as they are released by the female into the water
  • An enormous number of gametes are produced to ensure many undergo fertilisation and to ensure the production of a large number of offspring
  • Most amphibians provide no parental care, so the young tadpoles are easy prey and not many survive to reproductive age
• Southern gastric brooding frog

  • Eggs were fertilised externally in a watery environment, after which the female would swallow the eggs
  • Eggs settled into the stomach wall, where they were protected and absorbed nutrients from the mother for about 6-7 weeks
  • Young frogs were then regurgitated through the mouth
• Internal fertilisation


  Protects gametes from dehydration and loss to external elements, and protects the fertilised eggs and developing young from immediate predation
• Mammals


  • Fertilised egg becomes an embryo that is nurtured inside the female parent's body, obtaining nutrients through a placenta, and is born alive (viviparous development)
  • Placental mammals produce one to a few young at a given time and they invest a large amount of energy in parental care, increasing the chance of survival of the young
• Subclasses of mammals


  • Monotremes
  • Marsupials
  • Eutherians (placental mammals)
• Monotremes


  • Lay eggs that develop outside the mother's body
  • Young hatchlings (puggles) obtain milk from their mother's mammary glands by licking her abdominal skin
• Marsupials


  • Offspring are born at a very young age and crawl up the mother's abdomen to the pouch
  • Can have three offspring at different stages of development at any one time
  • Employ embryonic diapause to delay development of the new embryo until the older one is no longer suckling
• Eutherians (placental mammals)


  • Following internal fertilisation, the young completes its embryonic development inside the body of the mother in a special organ, the uterus
  • Once one or more fertilised eggs implant into the uterine wall, a placenta develops, connecting the young to a supply of nutrients and oxygen that passes from the bloodstream of the mother to the developing young
• Comparison of internal and external fertilisation

  • Gametes
  • Union
  • Conception mechanism
  • Chance of fertilisation
  • Environment for zygote
  • Number of offspring/zygotes
  • Breeding frequency
  • Parental investment