Genes & Alleles
Function
Genes
A gene is a basic unit of heredity, a strand of DNA which codes for a polypeptide via an RNA chain. These chains determine traits an individual receives, such as eye colour and blood type.
Alleles
An allele is an alternative form of a gene, alleles lead to the encoding of different RNA chains hence, different traits. For instance, the gene for eye colour has an allele for blue eyes and an allele for brown.
Dominant & RecessiveAlleles are either dominant or recessive. A dominant allele is an allele that is always expressed if present. Whereas a recessive allele is only expressed if both alleles are the same recessive allele.
Homozygous & HeterozygousHomozygous dominant occurs when the organism receives two of the same dominant alleles of a certain gene and displays the dominant allele. Heterozygous occurs when the organism receives two different alleles, in this case if one is dominant and the other is recessive the organism will normally display the dominate allele, however if the organism receives two different dominant alleles it will show a combination of the two traits. Homozygous recessive occurs when the organism receives two of the same recessive alleles of a certain gene and displays the recessive allele.
|
Brown (Dominant) and Blue (Recessive) eyes
Example of Dominant and Recessive Alleles Source: http://www.diffen.com/difference/Image:eye-color-alleles.png |
Genotype & Phenotype
Genotypes are the alleles carried by an organism, this means the genotype also shows the alleles that did not influence the specific trait they code for. Whereas a phenotype is the expression of the genes or the traits that can be observed due to their genetic makeup.
Blood GroupIn terms of the blood types gene in humans there are three alleles; IA,IB and IO which have four possible genotypes; AA,AO,BB,BO,AB and OO which result in four possible phenotypes: A produced by AA homozygous and AO heterozygous genotypes, B produced by BB homozygous and BO heterozygous genotypes, AB heterozygotes, and O homozygotes. However it isn’t that simple, within each A,B and O Allele there are a class of multiple alleles with different DNA sequences that produce proteins with identical properties. There are more than 70 alleles are known in the ABO locus, this means an individual with blood type A may be an AA homozygote, AO heterozygote, or an A’A heterozygote with two different A alleles.
|
Example of Genotypes and Phenotypes
Source: http://study.com/academy/lesson/dihybrid-cross-definition-example-quiz.html |