State three causes of genetic variation
- Mutation
- Crossing over
- Independent segregation / assortment (of homologous chromosomes)
- Random fusion of gametes / fertilisation / mating
What is meant by a genome?
- (All) the DNA in a cell/organism;
- ‘(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’
- ‘the total number of DNA bases in a cell/organism’
What is a gene pool?
- All the alleles in a population;
How do multiple alleles of a gene arise?
- mutations;
- which are different / at different positions in the gene;
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
- Small sample size;
- Fusion/fertilisation of gametes is random;
- Linked Genes; Sex-linkage / crossing over;
- Epistasis;
- Lethal genotypes;
What is meant by a recessive allele?
- Only expressed in the homozygote / not expressed in the heterozygote / not expressed if dominant present;
What does Hardy Weinberg’s equation predict
- The frequency/proportion of alleles (of a particular gene);
- Will stay constant from one generation to the next/over generations / no genetic change over time;
- Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration;
Define gene linkage
- (Genes/loci) on same chromosome;
Define epistasis
- The allele of one gene affects or masks the expression of another in the phenotype;
Describe why observed phenotypes don’t match expected values
- Fertilisation is random
- OR
- Fusion of gametes is random;
- Small/not-large population/sample;
- Selection advantage/disadvantage/lethal alleles;
Define codominance
- Both alleles expressed in the phenotype;
Rules for recessive alleles
- Unaffected parents can have an affected offspring (if they are Heterozygous)
Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why.
(Recessive) allele is always expressed in males / males have one (recessive) allele;
* Females need two recessive alleles / females need to be homozygous recessive / females could have dominant and recessive alleles / be heterozygous;
What is meant by the term phenotype
- (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
- (Expression / appearance / characteristic due to) interaction with environment;
What is meant by the term phenotype
Explain how a single base substitution causes a change in the structure of a polypeptide
- Change in (sequence of) amino acid(s)/primary structure;
- Change in hydrogen/ionic/disulfide bonds;
- Alters tertiary/30 structure;
genotype defn
is the genetic constitution of an organism
