IQ4: Genetic Variation

• Crossing over causes genetic material to be exchanged between the paternal and maternal chromosomes of the homologous pairs.
• Each pair of chromosomes then separate (anaphase I) and one entire chromosome moves into a daughter cell. This separation of maternal and paternal chromosomes leads to genetic variation depending on which chromosome of each pair ends up in which daughter cell also known as independent assortment. 
• • The process of fertilisation, which involves the random meeting of any two gametes, ensures further mixing of genetic material, producing variations in phenotype that may be acted on through natural selection in the process of evolution. 
• • Genotype: allele combination for a trait, homozygous or heterozygous
• Allele are the alternative forms of the same gene. An individual can only have two alleles - one from the mother and one from the father
• • Gregor Mendel’s model could predict the ratios of various types of offspring from any two specific parents.
  • It is not always possible to tell the genotype of an organism from its phenotype. 
• • Autosomal inheritance aka complete dominance have the following alleles:
  • A dominant allele is the trait that is expressed
  • A recessive allele is the one that is hidden
• • A homozygous dominant (TT) individual will have the same phenotype as a heterozygous (Tt) individual 
• Importance of Punnett Squares:

  • Table that can be used to predict the possible offspring genotype and phenotype outcomes for two individuals (parents) 
  • Allow you to predict the probability of offspring inheriting a condition
• Codominant Inheritance occurs when both alleles are equally expressed. This results in a blend of traits rather than just one being dominant over another. Neither allele is dominant or recessive
• Incomplete Dominance occurs when neither parent has a completely dominant allele. The resulting hybrid is intermediate between the two parents.
• Sex-linkage refers to genes that are located on the sex chromosomes

  • Most sex-linked conditions are X-linked (linked to the X chromosome)
  • This is because the Y chromosome is shorter and has less genes on it.
• • Most X-linked dominant traits are more common in females.
  • This is because either allele may be dominant for a trait.
• • X-linked recessive traits are more common in males.
  • This is because females can be a carrier without having the condition.
  • Males only have one X chromosome so only need one copy of the allele to have the condition.
• A carrier is a female that possesses a recessive allele for a condition/trait but does not display that condition herself. The recessive allele she carries is masked by a dominant allele (one allele on each X chromosome) A carrier female can still pass down the recessive allele for a trait.
• • Males will always inherit an X-linked recessive condition from their mother.
  • Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents
• Sex-linkage worked example

  • Haemophilia (a bleeding disorder) is an X-linked disorder.
  • Alleles for this gene occur on the X chromosome.
  • A male who inherits one copy of the mutant allele will suffer from the condition because he has no equivalent allele on the Y chromosome.
  • A female who inherits a mutant allele for haemophilia will be a carrier - she will not suffer from the disorder but can pass the allele down to her offspring.
  • A female who inherits two alleles for haemophilia will die as the condition is lethal.
• • Although individual humans can only have two alleles for a given gene, multiple alleles may exist in a population level
  • Different individuals in the population may have different pairs or combinations of these alleles.
  • Multiple alleles makes for many possible dominance relationships (one allele might be dominant over several other alleles, or maybe just dominant over one).
• • Polygenic inheritance refers to a single characteristic that is controlled by more than two genes (also called multifactorial inheritance)
  • Polygenic inheritance patterns normally follow a normal (bell-shaped) distribution curve - it shows continuous variation
  • By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous
• • Population genetics is the study of how the gene pool of a population changes over time, leading to a species evolving. 
• The gene pool is the sum total of all the genes and their alleles within a population.
• Genetic diversity is the total of all the genetic characteristics in the genetic makeup of a species.
• • Genetic variability is the tendency of individual genetic traits in a population to vary. 
• Frequency of allele G = Number of copies of allele (G) in the population/Total number of copies of the gene (G + g) in the population
• • Single Nucleotide Polymorphisms (SNPs) pronounced ‘snips’, are another way of examining genetic variation.
  • SNPs usually arise during DNA replication, where a single nucleotide is incorrectly inserted, creating an error in the DNA sequence at a particular location on a chromosome.
• • n non-coding regions (introns) a SNP does not lead to observable differences, however, are important as they can be used to identify disease susceptibility in individuals. 
  • In coding regions (exons), they can be the cause of a phenotypic change such as appearance or enzyme functioning. 
• Sickle cell anemia is a common genetic condition due to a haemoglobin disorder. This distribution reflects the fact that sickle-cell trait confers survival advantage against malaria and that selection pressure due to malaria has resulted in high frequencies of the mutant gene especially in areas of high malaria transmission. Although a single abnormal gene may protect against malaria, inheritance of two abnormal genes leads to sickle cell anaemia and confers no such protection, and malaria is a major cause of ill-health and death in children with sickle cell anaemia.
• Human Genome Project is the first human genome that sequenced DNA obtained from a number of individuals, with the resulting genome representing a mosaic rather than the genome of one person. 
• The Impacts of Discovering the Human Genome

  • The results of both projects have given scientists an improved understanding of many aspects of genetics, including genetic disorders, disease diagnosis and predisposition to disease. 
  • This knowledge provides scientists with the potential to individualise diagnosis and treatment of diseases. 
  • However, it also gives employers and health insurers information that could be used to discriminate against people on the basis of their future health.
• • Watson and Crick were able to determine the sequence of genes along DNA using techniques such as DNA sequencing and DNA profiling
  • In DNA sequencing, the precise order of nucleotides in a sample of DNA is determined. 
  •  In DNA profiling, an organism’s unique DNA profile is determined and represented as a distinct series of bands. 
• Fourth phaseof the Sangar Method

  Electrophoresis: DNA segments are placed in a gel and subjected to an electrical current which moves them. When subjected to an electrical current, the smaller nucleotides in the DNA move faster than the larger ones
• First phase of the Sangar Method

  DNA is removed from the cell. This can be done either mechanically or chemically
• Third phase of the Sangar Method

  The DNA clones are placed with a dye-labelled primer into a thermal cycler, a machine which automatically raises and lowers the temperature to catalyse replication
• Second phaseof the Sangar Method


  Breaking up the DNA and inserting its pieces into vectors, cell that indefinitely self-replicate, for cloning
• Sangar Method

  A method for DNA sequencing
• DNA profiling is the process where a specific DNA pattern (profile) is obtained from a person or sample of bodily tissue
• Polymorphic are specific regions that vary highly between people, and are inherited from multiple genes
• • DNA polymorphisms can be analysed to give a DNA profile
• Differences in polymorphic regions between people are known as polymorphisms
• Current technique for DNA profiling uses polymorphisms called short tandem repeats