Variation and inheritance

• Discrete variation is caused by inheritance due to a single gene. Measurements of the characteristic fall into distinct groups. e.g. eye colour
• Continuous variation is caused by polygenic inheritance (characteristics affected by more than one gene). Measurements of continuous variation occur over a range of values between a minimum and a maximum. e.g: height
• • Gene - a small section of DNA that codes for 1 protein which controls a particular characteristic.
• Allele - different versions of the same gene.
• Phenotype: the appearance of an organism
• Genotype: genes present in an organism written as code
• • Dominant - some alleles are dominant over other alleles. You only need to inherit one copy of a dominant allele to express the characteristic. Dominant alleles are always represented using capital letters.
• • Recessive alleles - These alleles are often masked by dominant alleles and you have to inherit 2 copies to express this characteristic. These alleles are represented by using lower case letters of the dominant characteristic.
• • Homozygous - If you inherit 2 of the same alleles, you are homozygous.
• • Heterozygous - If you inherit 2 different alleles i.e. 1 dominant and 1 recessive then you are heterozygous.
• predicted Phenotypic ratios are not always achieved because fertilisation is a random process and the ratio is also dependent on the sample size. If the sample size is too small, then the expected phenotypic ratio may not be shown.