genetics page 4

Individuals and team players

Whereas some of our traits are controlled by just one pair of alleles, the majority of our visible traits are controlled by two or more pairs.

How can we tell?

Traits controlled by one pair of alleles tend to show "either/or variation". That is, you either see one phenotype or another.
This is also called discrete or discontinuous variation.

Examples: Human

Freckles or not,
dimples or not,
free earlobes or attached earlobes
Kevin C. Hartzog's WEB site gives several simple examples of human facial traits.

If you pass the mouse arrow over Dom here, you will see his dominant traits turn into recessive traits.

Examples: Other species

Hairless cat or furry cat. (To read about cat genetics go to Cat World.)
Tall growing or dwarf pea plant
Round pea seeds or wrinkled pea seeds
For a list of 7 contrasting traits in garden peas go to Pink Monkey.
"Silvering" of horse coat colour. (For other genes affecting horse coat colour go to table 2.

Traits controlled by several pairs of alleles at different places on the chromosomes show a range in the phenotype from one extreme to another.
This is called continuous variation.

Most of the individuals in a population are near the average phenotype with only a few individuals at each extreme.

A good example of this is height in humans. There are a few very tall people and a few very short people, but the majority are in between and most are around average.
If you graph the heights of a large group of people, the graph usually has a bell shaped curve. This is typical for traits controlled by several genes.

Skin colour in humans is also controlled by a number of genes, but it is common for some populations of people to all be the same colour - e.g. black, white or brown. However, in places where there has been mixing between these populations, there is a whole range of shades.

Not all variation between individuals is caused by different genes or alleles. Some of the other factors that affect phenotype include:

Effects of the environment such as wind damage to trees or sunburn in humans.
Other physical effects that cause harm or injury such as scars cause by fights or accidents.
Nutrition - different nutrients or foods being taken in. Lack of food or nutrients can cause deficiency diseases such as yellow leaves in plants or stunted growth in animals. Chemicals taken in by a pregnant mother can cause changes to her growing child that result in birth defects.
Diseases can cause short-term or long-term changes to appearance and functioning of organisms. For example chicken pox causes spots when you have it but may leave permanent pock marks if you scratch too much.
Deliberate changes made by people. For example using hair dye to change hair colour, or docking the tails or horns of sheep or cattle.

Phenocopies

If a plant or animal has a phenotype that appears to be caused by genes but is actually caused by something environmental, it is said to be a phenocopy. Examples include:

A shrub may grow along the ground (= be prostrate) because it has genes for that growth form, but in an area of strong winds, shrubs that have the genes for upright growth may show a phenocopy of prostrate plants.
Some cattle are naturally "polled" = don't grow horns, but people often dock horns of cattle, so make a phenocopy.
Some people have naturally blonde hair while others dye theirs and so are a phenocopy.

Pedigree Diagrams

A pedigree diagram shows a family tree in which the inheritance of a certain characteristic is followed through two or more generations.

It is common to symbolise females using circles and males with squares. Individuals with the trait under study are shown with shaded shapes. (Put the mouse arrow on the diagram below for more information)

Example: For the trait of being shortsighted in humans.

Gwen, Fred and Guy are shortsighted. (Put the mouse arrow on the diagram below to see genotypes).

If N is the allele for normal sight and n is the allele for shortsighted, we know for sure that Gwen, Fred and Guy are all homozygous recessive (nn) as they have the trait.
We also know that Bill and Millie, and Laura and Todd must be heterozygous. They have normal vision but they must have had hidden alleles that they passed on to some of their offspring.
Shelly has normal vision but she might have received a recessive allele from either parent. Her genotype could be NN or Nn. We can use N- to show that we don't know the other allele.