Genes and Alleles

what is an allele?

different forms of the same gene

homozygous

• a genotype in which the two alleles, for the characteristic, are identical.

• eg. TT of tt

genotype

• the alleles that an organism has for a particular characteristic.

• usually written as letters.

phenotype

the visible characteristics of an organism which occurs as a result of its genes.

dominant and recessive

alleles can be either dominant, or recessive

dominant

• the ‘yes’ gene

• capital letter.

• a dominant allele is always expressed, even if one copy is present.

recessive

• the ‘no’ gene

• lower case letter

• only expressed if the individual has 2 copies, and doesn’t have the dominant allele of that gene.

homozygous alleles

homozygous alleles are both identical, for the same characteristic.

• eg. BB or bb

heterozygous alleles

heterozygous alleles are both different, for the same characteristic.

• eg. Bb

evolution

a change in the inherited characteristic of a population over time through the process of natural selection.

can result in the formation of a new species.

process of natural selection

• meiosis and mutations introduce variation into a species.

• this produces variations in how well adapted organisms are according to their environment.

• the best adapted are more likely to survive and reproduce.

• the best adapted genes are therefore more likely to be passed on.

• this is repeated over many generations, many times.

• tiny changes in the organisms phenotype caused by a build up of mutations is seen over time.

the process of antibiotic resistance

• due to random mutations, populations of bacteria show variation, (including in their resistance to antibiotics.)

• many bacteria are killed by antibiotics, but ones with the most resistance are more likely to survive.

• genes that code for increased resistance are passed on to the next generation.

• as these genes are copied, more random mutations occur, which increase levels of resistance.

• therefore, the next generation has a higher average resistance than the previous one.

• process repeated until bacteria is completely resistant to the antibiotic.

why is it essential that patients finish an entire course of antibiotic tablets?

This ensures all bacteria are killed, and so none survive which can mutate and produce resistant strains

monohybrid inheritance

Monohybrid inheritance is the inheritance of characteristics controlled by a single gene