Unit 2: Variation And Inheritance From Achieve Flashcards
Define discrete and continuous variation
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.
Describe key genetic terms (gene, allele, phenotype, genotype, dominant, recessive, homozygous, heterozygous)
Key genetic terms include:
Gene - a small section of DNA that codes for 1 protein which controls a particular characteristic.
Allele - different forms (versions) of the same gene. e.g. Gene = eye colour, Allele = Blue, brown, green etc.
Phenotype is a ‘physical’ description of a characteristic. e.g. Blue eyes, Brown eyes.
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.
Carry out a monohybrid cross to F2
See diagram on achieve website
State reasons why predicted phenotype ratios are not achieved
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.
