Inheritance and Variation of Traits
How organisms reproduce
Tell the asexual modes apart by the organism that uses each, follow pollen from stigma to seed and an egg from ovary to implantation, and say why variation is worth the cost of two parents.
Reproduction is the one life process an individual can do without. Nutrition, respiration and excretion keep it alive; making copies of itself costs energy and buys the individual nothing. What it buys is a population. And because the chemistry that copies DNA is good but not perfect, the copies come out slightly different from each other — which is the whole of the reason a species can outlive a change in its surroundings.
Why two parents, then?
Variation arrives slowly if you wait for copying errors. Two individuals in a population have each been accumulating different variations, and since both are alive, none of those variations is disastrous. Combine them and you get a fresh mixture in one step. The cost is arithmetic: two lots of DNA would double the amount each generation, so germ-cells are made by meiosis with half the usual chromosomes, and the full number is restored when they join.
Worked example
A gardener wants a hundred plants of one particular rose, each bearing the same flower. Why take cuttings rather than sow seed?
A cutting grows a whole plant from a piece of stem — vegetative propagation.
Root, stem and leaf of many plants will develop into a new plant under the right conditions, which is what layering and grafting make use of.
Try it together
A grain of pollen lands on the sticky top of a pistil.
Follow it to the seed.
1.The female germ-cells are down in the ovary and the pollen is at the top. What grows out of the pollen grain to bridge the gap?
Have a go
Have a go on your own. Papaya flowers carry either stamens or a pistil, but not both. What are such flowers called?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.