- Genotype is the genetic constitution (make-up) of an organism
- Phenotype is the observable characteristics that results from the expression of the genotype and the environment
- A gene is a length of DNA that codes for a polypeptide chain
- An allele is one of the different forms of a gene
- The locus is the position of a gene on a chromosome.
NOTE:
Genes can exist in two or more forms - these are the different alleles of a gene. Some genes have more than two allelic forms. If this is the case the organism is said to have multiple alleles for the character but since there are only two chromosomes in a homologous pair it follows that a maximum of two of the alleles in existence can be present in the organism (it can be one allele if it is homozygous, see below). Examples of this include the human ABO blood system. Instead of using lower and uppercase letters to represent recessive and dominant alleles of these genes, each allele is linked to a letter that represents the gene. For example, in the human blood system there are three alleles associated with the gene I. These are represented as I^A, I^B, and I^O.
Genes can exist in two or more forms - these are the different alleles of a gene. Some genes have more than two allelic forms. If this is the case the organism is said to have multiple alleles for the character but since there are only two chromosomes in a homologous pair it follows that a maximum of two of the alleles in existence can be present in the organism (it can be one allele if it is homozygous, see below). Examples of this include the human ABO blood system. Instead of using lower and uppercase letters to represent recessive and dominant alleles of these genes, each allele is linked to a letter that represents the gene. For example, in the human blood system there are three alleles associated with the gene I. These are represented as I^A, I^B, and I^O.
There can only be one allele of a gene at each locus of any chromosome. In homologous chromosomes (these occur in diploid organisms) there are two loci each carrying one allele of a gene (one from mummy and one from daddy). If the alleles are the same (e.g blue eyes) then the organism is homozygous blue eyed. If each allele is different (e.g one for blue eyes one for brown eyes) then the organism is heterozygous for the characteristic that is expressed. Which characteristic is expressed depends on which is dominant - in this case, brown eyes are dominant over blue so the individual would be heterozygous brown eyed. If both alleles are dominant they can both be expressed in the phenotype. This is known as codominance. For example, white and red splodgy petals for a plant with an allele for red pigment and an allele for white pigment - both alleles contribute to the phenotype.
NOTE: If the allele is not dominant, it is recessive.
Genetic diagrams
Genetic crosses
These represent all the possible allele combination of the offspring - they can therefore also be used to determine the percentage chance of obtaining each outcome. However, it is very rare that the actual results of genetic crosses are the same as predicted results. These discrepancies are due to statistical error. It is pure chance that determines which gamete fuses with which gamete.
Monohybrid crosses represent only one genotype and therefore feature - e.g the alleles for eye colour. This shows how a single characteristic is passed on from parent to child. In practice many characteristics are actually inherited together. Dihybrid inheritance refers to the inheritance of two characteristics by two different genes located on different chromosomes. E.g a dihybrid cross might represent the alleles for both eye and hair colour. Since they are different genes they would have different codes - e.g the letter 'B/b' for eye colour alleles and 'E/e' for hair colour alleles. The offspring from the first generation will produce up to four types of gamete because the genes for hair colour and eye colour are situated on different chromosomes so, as the chromosomes arrange themselves at random along the equator during meiosis, any two alleles can combine with any other two alleles.
NOTE: It is important to realise in breeding, the first generation is known as the first filial (F1) whilst the second generation is known as the second filial (F2)
Sex-linkage
Females have two X sex chromosomes at pair 23, males have one X and one Y sex chromosome. Females produce the same gametes (in that they all contain an X chromosome), males produce different gametes (as in half have an X chromosome and half have a Y chromosome).
All genes carried on either X or Y chromosomes are said to be sex-linked. it is important to realise that the Y chromosome is much shorter than the X chromosome, and the X chromosome has no equivalent portion of the Y chromosome (past of it basically just isn't there). Characteristics controlled by recessive alleles on the non-homologous part of the 23rd pair chromosomes will ore frequently appear in men because there is no part on the Y chromosome that might have a dominant allele - so it must be expressed.
Specifically, an X-linked disorder is a disorder cased by a defective gene on the X chromosome. An example is haemophilia. It is can be caused by a recessive allele with an altered DNA base sequence that codes for a faulty protein. This protein does not function but is required in the clotting process - therefore the individual cannot clot blood.
Autosomal linkage
Any two genes that occur on the same chromosome are linked - all the genes on a singe chromosome form a linkage group. The 22 chromosomes that are not sex chromosomes are known as autosomes. When there are two or more genes carried on the same autosome, it is known as an autosomal linkage.
If two genes are on different alleles, there are four possible combinations of the alleles in the gametes. If the two genes are linked there are only two possible combinations of the alleles in the gametes (provided there is no crossing over).
Epistasis
Epistasis arises when the allele of one gene affects or masks the expression of another in the phenotype. For example, say that gene 'B/b' cannot be expressed without allele A. Therefore if the organism is homozygous for ‘aa’, 'B/b' cannot be expressed.
The chi-squared test
This is used to test the null hypothesis - the null hypothesis is based on the assumption that there will be no statistically significant difference between sets of observations and any difference is due to chance alone. The chi-squared test is a means of determining whether any deviation between the observed and expected numbers in an investigation are significant or not. There are criteria:
- sample size must be relatively large (over 20)
- data must fall into discrete categories - e.g heads and tails
- only raw counts can be used - no percentages/rates etc
- is it used to compare experimental results with theoretical ones.
The formula is as follows...
Next we read the obtained value off the chi-squared distribution table to determine whether or not any deviation (from the expected values) is significant. we need to know the number of degrees of freedom to do this. This is the number of classes minus 1.
In the chi-squared test the critical value is p=0.05 (5%). Read off from the degree of freedom to the critical value. If your figure is equal to or greater than p=0.05 then the deviation is not significant and the null hypothesis is accepted and we can assume that the results are due to chance. If the deviation is less than p=0.05 the null hypothesis is rejected as there is another factor other than chance that is affecting the results basically.
The chi-squared test can be used in genetics to determine whether the difference in expected and obtained values for ratios of gametes of offspring were due to chance or another factor. The null hypothesis will state that there is no difference between the observed and expected results.
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