- a gene is a sequence of bases on a dna molecule which codes for a protein (polypeptide) which results in a characteristic. - you can have one or more versions of the same gene. these different versions are called alleles . there can be many different alleles of a single gene, but most plants and animals, including humans, only carry two alleles of each gene, one from each parent. the order of bases in each allele is slightly different - that's because each allele codes for different versions of the same character. alleles are represented using letters.
- humans are diploid organisms . this means we have two copies of each chromosome - one from each parent. it's why we have two alleles of each gene. the allele of each gene is found at a fixed positions, called a locus , on each chromosome in a pair.
- the genotype of an organism is its genetic constitution, or put another way, the different alleles an organism has. this could be a list of all its alleles but usually its just the alleles for one characteristic at a time :
for example, one person may have the genotype BB for eye colour and another person Bb.
one pea plant might have the genotype RR for seed shape and another plant rr.
- the phenotype of an organism is 'the expression of the genetic constitution and its interaction with the environment'. this just means what characteristics an organism has as a result of both its genes and the effect the environment has on its genes:
for example, one person may have brown eyes and another may have blue eyes.
on pea plant may have round seeds and another may have wrinkled seeds.
dominant and recessive alleles
- a dominant allele is always expressed in the phenotype, even when there's only one copy of it. dominant alleles are represented by a capital letter. recessive alleles are those with characteristics that only appear in the phenotype if two copies are present. they're shown by lower case letters :
for example, the allele for brown eyes, B, is dominant, so if a person's genotype is Bb or BB they'll have brown eyes. the allele for blue eyes, b, us recessive, so a person will only have blue eyes if their genotype is bb.
the allele for round seed shape, R, is dominant, so if a pea plant's genotype is Rr or RR it will have round seeds. the allele for wrinkled seed shape, r, is recessive, so a pea plant will only have wrinkled seeds if its genotype is rr.
co dominant alleles
- some alleles are both expressed in the phenotype because neither one is recessive. they are said to be co dominant alleles : for example : horses can have alleles for white hair or coloured hair. neither allele is recessive, so a horse with one copy of each allele with have a roan coat - a coat with a mixture of white hairs and coloured hairs.
homozygous and heterozygous
- at each locus in a diploid organism, the genotype can be homozygous or heterozygous. if an organism carries two copies of the same allele, it's said to be homozygous at that locus. if an organism carries two different alleles for a gene, then it's heterozygous : for example, the genotypes BB and bb are homozygous and the genotype Bb is heterozygous.