variation and inheritance

• Discrete variation is caused by inheritance due to a single gene. Measurements of the characteristic fall into distinct groups.e.g. blood groups
• 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. skin colour, leaf area, height.
• • Gene - a small section of DNA that codes for 1 protein which controls a particular characteristic.
• • Phenotype is a ‘physical’ description of a characteristic. e.g. Blue eyes, Brown eyes.
• • Allele - different forms (versions) of the same gene. e.g. Gene = eye colour, Allele = Blue, brown, green etc.
• • Genotype is a description of the genes using letters to represent the alleles.
• 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 for a characteristic.
• • Heterozygous - If you inherit 2 different alleles i.e. 1 dominant and 1 recessive then you are said to be heterozygous for a particular characteristic.
• 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.