|
Site author Richard Steane
|
The BioTopics website gives access to interactive resource material, developed to support the
learning and teaching of Biology at a variety of levels.
|
|
|
|
GENETICS
Genes are the individual units in which the inherited information mentioned in the previous topic are passed from one generation to the next. The hereditary information they contain ("genetic blueprint") is used to make different (protein) substances, or to switch on or off cell activities, which are responsible for the different characteristics. We believe that genes are actually composed of a substance
called DNA. Genes are too small to be seen, even with a microscope, but they are parts of chromosomes, which are contained within the nucleus, and these can be seen. The way in which chromosomes move during the events of reproduction confirms our understanding of how characteristics are passed from one generation to the next - genetics.
Fertilisation and Development - from a genetic viewpoint
Although this account refers to Man, a practically identical process occurs in other (sexually reproducing) animals, and a broadly similar process occurs in flowering plants.
Sperms and eggs are special cells called gametes, which have nuclei containing half the normal number of genes. When a sperm and an egg join together, and their nuclei fuse, the normal number of genes are once more present in the single cell. Of course, half the genes come from the male, via the sperm, and half comes from the female, via the egg.
In Man, each sperm carries 23 chromosomes, and each egg also carries 23 chromosomes. In this way, each child inherits an equal amount of genetic information from its father and mother. After fertilisation, a single cell called a zygote is produced, containing 46 chromosomes. As this "new" cell eventually develops into a new individual, more cells are produced, each one containing a nucleus with an identical genetic make-up, due to the process of mitosis .
This means that for any given characteristic each body cell contains a pair of chromosomes, and therefore a pair of genes.
It is essential to understand that in ordinary body cells there are 2 genes for each characteristic, and in gametes there is only 1 such gene.
How genes work in combination
There are usually at least two forms (called alleles) of most genes, and the effect they have when expressed in the individual, i.e. the characteristic they produce, varies according to the form of the other
gene they are combined with in each body cell.
Dominant alleles show their effect whether there are one or two of them in a pair.
Recessive alleles show their effect only if both genes are of this type.
Conventions and terms used in explaining genetic problems
It is usual to use a single letter to represent a gene, and a combination of (usually 2) letters to describe the genetic make-up of different cells and organisms, and to ignore the other genes and chromosomes they also contain.
Any letter will do, but usually it has some connection with the characteristic being described:
e.g. H (for hair colour?), in the examples chosen.
Dominant alleles are written as a capital letter (H), and recessive alleles as a small (lower case) version (h) of the same letter.
An organism with both genes of the same type is called homozygous (a homozygote) - also called pure-breeding or pure-bred:
e.g. HH or hh
whereas one with two different forms (Hh) is called heterozygous (a heterozygote) - also called a hybrid.
The terms genotype and phenotype are different sorts of descriptions for organisms in genetics.
A genotype describes an organism in terms of its combination of genes.
e.g. HH, Hh and hh are all shorthand descriptions for genotypes.
An organism's phenotype is a description based on its observable characteristics,
i.e. what an organism looks like, as a result of its genes, interacting with its environment.
e.g. black hair, or blond hair.
"Phenotype = genotype + environment"
How to explain inheritance of characteristics
It is important to remember that:
- each parent's normal body cells contain 2 genes (2 letters)
- due to meiosis, each gamete contains only 1 of these genes
- assuming fertilisation is a random process, different combinations of (2) genes may be produced in the offspring.
Phenotypes of the resulting offspring can be expressed either as ratios (i.e. proportions of the "total" possible), or as a chance that any one individual will have the characteristic stated.
The combinations of offspring from a "cross" may be set out in a matrix of boxes :
(although there is an alternative method which simply involves setting out the same stages in order down the page).
What will the offspring look like?(phenotypes)
Hh - all dark
Is there any difference amongst the offspring, in respect of the hair colour gene? no
Actually in these sorts of crosses there are several possible combinations. It is fairly easy to forecast what will be the result in most cases, but the matrix system is especially useful in the more complicated cases.
It is worth noting that the same system can be used to explain and forecast the likelihood of passing on genetic conditions.
The following diagram shows the situation when 2 heterozygotes (hybrids) have children.
What will the offspring look like?
(phenotypes)
HH - dark
Hh - dark
hH - dark
hh - blond
What is the ratio of offspring showing the different characteristics?
> 3(dark):1 (blond)
How does HH differ from Hh, phenotypically?
> not at all- (both dark)
How does hH differ from Hh, phenotypically?
> not at all- (no genotypic differences, either)
How does HH differ from hh, phenotypically?
> HH looks dark -
hh looks blond
How does HH differ from Hh, genotypically?
>HH is HOMOZYGOUS - 2 dominant alleles
Hh is HETEROZYGOUS - only 1 dominant allele
How does HH differ from hh, genotypically?
> these are 2 different homozygotes
>(dark vs. blond is a phenotypic difference)
As an exercise, complete the box to show the genotypes and phenotypes of the offspring in the following case, where a dark haired (but heterozygous) person and a blond haired person have children
(Use the mouse, or tap the screen) :
What will the offspring look like?
(phenotypes)
Hh - dark
hh - blond
What is the ratio of offspring showing the different characteristics?
>
1:1Sex determination
The sex of an individual is determined by a combination of chromosomes. In the normal set of human chromosomes (genome) there are 44 (22 pairs) of "ordinary" chromosomes, and 2 sex chromosomes. Females have two (more or less) identical chromosomes - called "X", whereas males have one "X" and one "Y".
If we extend the matrix system to include sex chromosomes:
- XX and XY are different genotypes, and
- "femaleness" and "maleness" are phenotypes.
(Use the mouse, or tap the screen)
What sex will the offspring be?
(phenotype)
>XX - female
>XY - male
This explains why male and female children are produced in approximately equal numbers, or, put another way, there is a 50% chance of any child being male (or female!).
Sex linked conditions
The genes for certain conditions (e.g. haemophilia, red-green colour blindness, and Duchene muscular dystrophy) are known to be carried on the X chromosome. In these cases, males are especially at risk, because any defective genes they inherit will not be masked (dominated) by a partner gene because there is only one X chromosome. On the other hand, females are much less likely to suffer from these conditions because the effect of one defective gene may be masked by the other X chromosome. Instead, female heterozygotes may act as carriers.
Co-dominance
In some crosses, there may be an intermediate form showing a characteristic between the two parental
types, so there is (old term - incomplete, or) no dominance. In these cases, it is best to use two different
letters (or, preferably, superscripts).
- For instance, white snapdragons [flowers] crossed with red ones give pink offspring. These do not
"breed true", and always produce 25% red, 50% pink and 25% white.
- Sickle cell anaemia is an inherited condition (problems with red blood cells and oxygen transport)
caused by a double dose of a mutant gene. A person with only one such gene (heterozygote) may suffer
from a less severe condition (sickle cell trait).
- In human blood groups, there are 3 possible alleles, producing blood of type A, B or O. A and B may be present together, to give blood type AB. O is recessive to A and B, so people with type O blood are homozygous, whereas people with blood groups A and B might be homozygous or heterozygous.
Home