Dynamic genomes, transposons and complex traits

Question 1

What are transposons

Answer 1

• a transposon or transposable element is a discrete DNA sequence element that can move from one location in the genome to another often in control of a transposase encoded by the element itself
• transposable elements are flanked by short direct repeat sequences
• DNA transposons move by either replicable or conservative transposition
• 2.8% of the human genome consists of DNA transposons
• natural transposons can be exploited to generate insertional mutants (transposon mutagenesis)

Question 2

What did McClinton discover

Answer 2

• demonstrated transposons can cause chromosome breaks in recombination events
• found Ac (acceptor) elements have encoded transposase, an enzyme allowing fragments of the genome to be cut and reinverted somewhere else , this leads to transposition
• Ds (disassociator) elements couldn’t produce transposons, they contained deleted transposons
• Ds elements can move if Ac element in the same genome
• recombination between pairs of transposons can result in deletion of segments of the genome or chromosome breaks

Question 3

What did McClintock work with in her experiments

Answer 3

C’ - dominant allele preventing pigment being expressed in aleurone layer
C - recessive allele that leads to pigments in aleurone layer
Bz - dominant allele producing purple aleurone pigment
bz - recessive allele producing dark brown alurone pigment

McClintock selected homozygous dominant males and homozygous recessive females, this meant alleles in triploid endosperm are known

• found no coloured sectors, bronze sectors and NO purple sectors
• this concluded that a chromosome break between Bz and centromere caused bronze sectors

then a new cross was found where ONLY purple sectors formed
- if Ds transposes to a new location when it is crossed in genetic background as Ac, the position of chromosome break will move to Ds location

Question 4

What is a complex trait

Answer 4

• a trait influenced by multiple genes eg. height, high blood pressure, obesity, diabetes, depression
• phenotype is determined by a measurement
• called quantitative traits as measured along a continuum with small intervals between individuals
• traits are influenced by the environment
• seen in a normal distribution (bell shaped curve)

Question 5

How are complex traits similar to Mendel’s law

Answer 5

• complex traits are subjected to the same laws as single-gene traits
• shown in a 64 square punnett square
• follows Mendel’s law as phenotypes in progeny show normal distribution
• expression of complex traits is susceptible to lifestyle choices and other environmental factors
  eg. inadequate nutrition may slow growth and development

Question 6

What is plasticity

Answer 6

• the effect of a genotype is sometimes specified by the environment, other times not

Francis Galton studied complex traits such as height, strength and physical characteristics and found
- regression towards the mean eg. parents below average height have children taller than them but they are still shorted than average

conclusion
- during meiotic division, segregation and recombination break up combinations of genes contributing to complex traits, this means extreme phenotypes from parents are less likely in offspring
- phenotype of parents results not only genes but from environment

Question 7

What is heritability

Answer 7

• the proportion of the total variation due to genetic differences among individuals
• for a complex trait, heritability determines how closely the mean values of the progeny resembles that of the parents
• multiple genetic and environmental factors affecting complex traits implies different people have the same disease for different reasons. personalised medicine will identify how the medicine will work on the individual and what potential side effects can be avoided