bio genetic variation

chromosome

long molecule of DNA carrying genes

Gene

length of DNA that codes for a protein

Allele

alternate version of a gene

Locus

fixed position of a gene on its chromosome- the same gene is always in the same place

genotype

alleles an organism carries

phenotype

physical expression/characteristic that you can observe that is produced by the genotype and environment

homozygous

two alleles are the same

heterozygous

two alleles are different

dominant

allele that is always expressed when present- even when only one copy is present

recessive

allele that needs two copies in order to be expressed- allele is only expressed when both alleles are recessive

carrier

organism which is heterozygous for a recessive allele- carries allele but doesn’t show characteristic/phenotype

when writing phenotype and genotype ratios

phenotype- 1 Tall:3 Short, genotype- 1 TT: 2 Tt: 1 tt

population

all individuals of one species living in the same place at the same time

gene pool

all alleles present in a population

allele frequency

how common an allele is in the gene pool- written as percentage/decimal/fractions

Mutation

random permanent change in the base sequence of DNA and is the only original source of new alleles

mutagen

anything that increases the rate of mutations- UV radiation from sunlight, X-ray radiation and chemicals in tobacco

gametic mutation

mutation that occurs in cells which become gametes- passed/inherited to offspring at fertilisation as zygote divides by mitosis

somatic mutation

mutation that occurs in body cells which are only passed to cell’s descendants produced by mitosis- only affects patch of tissue not inherited

neutral/harmful/beneficial mutations

neutral- no effect on phenotype
harmful- protein the gene codes for works less well or not at all and organism is less likely to survive and reproduce
beneficial- protein the gene codes for works better within the current environment and organism is more likely to survive and reproduce

meiosis

cell division of 2 diploid cells into 4 daughter haploid cells that produces gametes by halving the chromosome number so that fertilisation restores rather than doubling it every generation, produces variation

homologus chromosomes

pair of chromosomes that carry the same genes but may have different alleles as each chromosome comes from each parent

process of meiosis

sperm and egg cells fuse during fertilisation to make 46 chromosomes and a zygote is formed.

segregation

homologus chromosomes are pulled apart, and two alleles of a gene end up in different gametes

independent assortment

the random way each pair of homologus chromosomes line up during meiosis, and therefore which member of which pair ends up in which gamete (23 pairs is 2 to the power of 23= over 8 million combos)

crossing over

exchange of matching segments between the chromatids of a homologus pair - chromatids touch at a point called chiasma, equal lengths are swapped resulting in a recombinant chromatid

random fertilisation

any one sperm can fuse with any one egg

why is crossing over the only process which can seperate linked genes?

crossing over changes which alleles share a chromatid

incomplete dominance

heterozygote shows blend- neither allele is fully dominant so neither allele fully masks the other - phenotype is inbetween the two homozygotes

co-dominance

both alleles are fully expressed in the heterozygote at the same time- shows both phenotypes together and is not a blend

why does incomplete/co-dominance have a phenotypic ratio of 1:2:1 instead of 3:1?

because it has an intermediate phenotype, as all phenotypes look different so a new catergory is created

+and- of complete dominance

+- recessive alleles that are useless remain in the gene pool as a backup for when the environment changes
— harmful recessive alleles remain in the gene pool for many generations, and less phenotypic variation as only two phenotypes possible

+and- of incomplete/co-dominance

+- more phenotypic variation, so more for NS to act on in changing conditions
— harmful recessive alleles cannot hide, so they are removed faster and carriers are affected

multiple alleles

a gene that carries more than two alleles in the population - each individual still holds two, but the gene pool is increased

lethal alleles

alleles that kill the organism when present in genotypes - not counted in ratios as the organism dies beforehand- but if recessive can be carried