Genetic Basis for Variation

Question 1

Test cross

Answer 1

an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with homozygous recessive genotype

Question 2

Epistasis

Answer 2

A type of gene interaction in which phenotypic expression of one gene alters the phenotypic effects of another gene

Question 2

Pure breeding organism

Answer 2

a group of identical individuals that is homozygous for every trait, producing offspring with the same phenotype

Question 3

Why is there a greater number than expected of the parental phenotypes

Answer 3

• gene for two traits are linked/on the same chromosome
• alleles tend to be inherited together thus parental gametes occur at higher frequency
• when parental gametes fuse with ab in test cross, this gives rise to more parental phenotypes
• if the two genes are unlinked, 4 different types of gametes will be found in equal numbers to form 4 different phenotypes in equal proportion

Question 4

How can different characteristics be inherited independently in dihybrid inheritance

Answer 4

• inheritance of the two characteristics is controlled by alleles of two genes found on different chromosomes
• during metaphase I of meiosis, pairs of homologous chromosomes align along the metaphase plate random to other bivalents, followed by the independent segregation of chromosomes to opposite poles of the cell
• different combinations of the alleles are found in the gametes

Question 5

Why are mutations usually recessive

Answer 5

• mutations result in loss of function in the proteins/enzyme coded for
• since there are a pair of alleles for each gene, the normal dominant allele, which will result in the synthesis of sufficient quantities of functional protein, masks the effect of the mutated recessive allele

Question 6

Why carry out chi-squared test

Answer 6

• a chi-square is used to determine if the difference between observed and expected results is significant or due to chance
• if probability is less than 5%/0.05, the difference between expected and observed results is significant and not due to chance

Question 7

Locus

Answer 7

locus is the specific position of a gene along the length of a chromosome; homologous chromosomes have the same gene at the same locus

Question 8

Why is there a range of phenotypes for continuous variation

Answer 8

• trait is controlled by many genes
• each gene has little overall effect
• addictive effects of each gene gives rise to the range of phenotypes
• phenotypic expression is also affected by environmental factors