Usually, sex is determined by an organism's genes. With genetic sex determination, most alleles or genes that influence sexual development are on the same chromosome. That chromosome is then called the sex chromosome. Because genetic sex determination is controlled by a pair of sex chromosomes (or if of one of the chromosomes is there or not), there are usually the same number of male and female offspring. In humans, for instance, sperms carry either an X or a Y chromosome, and they occur in roughly equal numbers.
For some species sex is not determined by inheritance, but instead by environmental factors experienced during development or later in life. Many reptiles have temperature-dependent sex determination: the temperature embryos experience during their development determines the sex of the organism. In some turtles, for example, males are produced at lower incubation temperatures than females; this difference in critical temperatures can be as little as 1-2 °C.
Some fish change sex over the course of their life. This phenomenon is called "sequential hermaphroditism". In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. In many wrasse the opposite is true—most fish are female at birth and become male when they reach a certain size. Sequential hermaphrodites may produce both types of gametes over the course of their lifetime, but at any given point they are either female or male.
In some ferns the default sex is hermaphrodite. Ferns which grow in soil which has previously supported hermaphrodites are influenced by hormones remaining to develop as male.
Hacksaw
A hacksaw is a tool for sawing. A hacksaw is designed for cutting materials like very thin metals and plastic. It has a handle on one end, and can be either pushed or pulled across the object being cut. There are manual and electric hacksaws.
Spade
In gardening, a spade is a tool for digging. It is similar to a shovel. It has a straight handle. For some purposes the difference between a spade and a shovel is important. It is hard to move gravel with a spade but a proper shovel can do it.
In games, a spade is one of the four shapes or "suits" on playing cards.
Hoe
A hoe is a tool in farming and gardening. It has a blade, usually metal, attached to a long handle, usually wood. There are many types of blade and several uses, probably the most common of which is weeding. Other uses include shaping the soil and harvesting root crops such as potatoes. Along with the spade and fork, the hoe is considered a basic, essential hand farming implement.
Shovel
A shovel is a tool used for moving loose material. Shovels have a flat bottom head which is angled slightly from the handle to enable scooping and moving of material.
Although a shovel can be used for digging, it is much less efficient than a spade. Spades are straight, and transmit force straight downwards. Shovels move loose material, such as coal, snow, gravel etc. They are usually made of metal at the bottom, and wood is sometimes used for the handle.
Plunger
A plunger is a tool. It is used to get rid of things that are blocking pipes. The plunger is placed over one end of the pipe, and the handle is pushed up and down. Doing this creates a sucking force, which makes the thing blocking the pipe move and unclog.
Ladder
A ladder is a tool which is made for people to climb up or down. Ladders have two vertical supports that go along the whole length of the ladder. Between these supports are the horizontal rungs. The rung is what the climber can put his foot on. The climber can use his hands to hold on to the rungs above him, or to the supports at the side.
Ladders can be made of metal, wood or even rope. There are different types of ladders.
Some ladders are made in one piece. They can be carried around and made to lean against something such as the wall of a house. There should be a good distance between the bottom of the ladder and the wall so that the ladder is not too vertical, otherwise the ladder might fall backwards and the climber would fall off. Sometimes these ladders are extension ladders. Extension ladders are made in two or more sections so that the ladder can fold up to make it easier to carry about and to store. To open up the ladder, each section slides up almost to the top of the next section.
Step ladders are useful in the home or the garden to reach things that are not too high, e.g. ceilings or the tops of hedges. They have two parts which are joined together by a hinge, so that they are shaped like an upside down V. There is usually a platform at the top to stand on, but this can be dangerous unless great care is taken that the ladder will not wobble or fall down.
Some ladders are vertical (they go straight up or down). These are fixed onto something. Examples are: ladders in a swimming pool for climbing in and out of the water, small ladders to climb up to a top bunk bed, ladders at the side of a big boat or at the side of a canal lock or of any other high or low construction where workmen may need to get up or down to do repairs.
Fire engines always have ladders. They often have a turntable ladder which makes it possible for the ladder to be facing in any direction.
Rope ladders can be folded away easily. They may be used for climbing trees or for rock climbing or in caves.
Oral history
Oral history is history that is told rather than written down. It is given through talking rather than reading a book. It is usually passed from generation to generation in order to explain events that have occurred.
Though today oral history is not as popular as written history, it is still used in many places as a way of transferring knowledge about the past.
Generation
A generation is used to mean the average amount of time between the birth of parents and the birth of their children. It is also used to mean all the people on one level of a family tree. For example, your brothers, sisters and cousins are in the same generation as you. It can also be used to mean a series of something, like a generation of a car model (2nd generation, etc.).
Gene
Genes are parts of DNA. DNA is a molecule inside a cell that carries the instructions for making the proteins the cell will need. Each gene contains a single set of instructions. These instructions usually code for a particular protein. Humans have about 20,000 genes that code proteins and many more that are non-coding. Half of a person's genes come from the mother. The other half come from the father.
Originally: a hereditary unit which occupies a specific position ("locus") on a chromosome. Other definitions are ways the gene showed itself:
Modern definitions must take note of later discoveries. There are now two classes of genes:
Genes are passed on from parent to child and are an important part of what decides how children look and act (their biological properties). Genes affect the way our bodies work, including how we look. Our eye, hair and skin color are decided by genes. It is said that genes cause "genetic" effects in our bodies.
A gene may be "dominant" or "recessive". These terms refer to the effect a gene has on the offspring who carry it in their genome.
For example, let's say a mother only has genes for brown hair and a father only has genes for red hair. The child will inherit – receive – genes for red hair (from her father) and brown hair (from her mother). The brown hair gene is 'dominant' to the red hair gene. This means the child will have brown hair even though she has genes for both red and brown hair. This means only one dominant gene is needed for the child to receive that particular trait, while two recessive genes are needed for one.