3.12 and 3.13 Alleles and Basic Definitions
Definitions
Recessive allele
Alleles
Homozygous
Dominant allele
Heterozygous
Alleles
Genotype
Gene
Phenotype
Chromosome
Zygote
Gamete
An organism’s reproductive cell (egg in female and sperm in males),
which has half the number of chromosomes (23)
A structure found in the nucleus which is made up of a long strand of
DNA
A short section of DNA that codes for a protein, and therefore contribute to a characteristic
Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans
However, most characteristics are the result of many different genes interacting.
The different forms of the gene - humans have two alleles for each
gene as they inherit one from each parent
Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed
Two copies are needed for it to be expressed and for the corresponding the phenotype to be observed
When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
When one of the inherited alleles is dominant and the other is recessive
The combination of alleles an individual has, e.g. Aa
The physical characteristics that are observed in the individual, e.g. eye colour
The stage of development immediately after fertilisation - a diploid (2n) cell formed from the fusion of two haploid gametes
This is because different alleles code for different forms of the same protein - an allele that codes for a damaged form of a protein can cause illness
For example, in a condition called Huntington’s Disease, an allele of the gene that codes for a particular protein is different
Alleles (different forms of the same gene) lead to differences in inherited characteristics
This leads to the protein becoming folded incorrectly and causing the condition