AQA Biology Unit 2 Variation

• Mode- most frequent/common 
• Median- middle value 
• Mean- average value
• Bell shape is the usual curve on a graph. It shows even distribution of values above and below the mean.
• Thinner bell shape = smaller deviation value = less variation
• Standard error is ±1.96
• The standard error is how much of a sample mean is likely to vary
• Greater standard error = greater variation of mean
• Standard deviation is a measure of a spread of results about the mean of a normal distribution curve

Independent assortment of bivalents at the equator duting Anaphase 1

• Chromosomes at bivalents pulled to opposite poles randomly
• 2x no.of chromosomes = number of different combinations of chromosomes in 4 haploid cells produced

Crossing over between non-sister chromatids during prophase 1

• (tips of two chromatids swap)
• Homologous chromatids (equivalent pieces of genetic material) break and exchange opposite segments between maternal and paternal chromatids
• This results in a new combination of genes from the parents

Fertilisation or random fusion of gametes

• Genetic difference within zygote
• New combinations between alleles

Gene mutation and environmental factors

• Addition- at least one base added to DNA during replication
• Deletion- at least one base left uncopied (causes frameshift)
• Substitution- at least one base is copied wrongly
• Unusual base pairing- not A-T or C-G
• Degenerate code- different triplets code for same amino acids