What is a gene?
- section of DNA which codes for a specific polypeptide
What is a genotype?
- the genetic constitution of an organism, all the alleles an organism processes
What is a genepool?
- all the alleles within an interbreeding population
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as expected ratios.
Suggest two reasons why.
- small sample size
- fertilisation of gametes is random
- sex-linkage
- epistasis
- selection advantage
What is a phenotype?
- is the expression of its genetic constitution and its interaction with the environment
Define gene linkage
- genes on the same chromosome
What is a multiple allele?
- more than 2 alleles for the same gene
Define locus
- different alleles for the same gene are found at same position of a homologous chromosome
What is meant by the term homozygous?
if two copies of a gene are the same allele
What is meant by the term heterozygous?
if two copies of a gene are different alleles
What is a dominant allele?
- it will always be expressed in the phenotype regardless whether the organisms genotype is homozygous or heterozygous
What is a recessive allele?
- it will only be expressed in the phenotype if the genotype is homozygous
What is meant by codominant?
equally expressed within phenotype
State three causes of genetic variation
- mutation
- crossing over
- independent segregation
- random mating
how do multiple alleles of a gene arise?
- mutations
- which are at different positions on the gene
What does the hardy weinberg principle predict?
‘The allelic frequencies of a particular gene from one generation to the next will remain constant’
if there is no:
- migration
- gene mutations
- selection for or against a particular allele
ALSO should be a large population and mating should be random
Define epistasis
- the allele of one gene affects or masks the expression of another in phenotype
What are the rules for dominant alleles
- affected offspring must have at least one affected parent
- unaffected parents only have unaffected offspring
- if both parents are affected and have an unaffected offspring, both parents must be homozygous
What are the rules for recessive alleles
- unaffected parents can have an affected off spring if they are heterozygous
Male off spring are more likely than females to show recessive sex-linked characteristics. Explain why.
- recessive allele is always expressed in males
- females need two recessive alleles
Explain how a single base substitution causes a change in the structure of a polypeptide
- change in primary structure
- change in hydrogen/ionic/disulphide bonds
- alters tertiary structure
