Patterns of Inheritance

• Define variation

  The differences in characteristics between organisms
• Define interspecific variation

  Variation between different species
• Define intraspecific variation

  variation within a species
• Name and describe the two causes of variation

  An organisms genetic material- differences in the genetic material an organism inherits from its parents lead to genetic variationThe environment which the organism lives- causes environmental variation
• Describe 5 causes of genetic variation between individuals within a population.

  Alleles- genes have different alleles Mutations- changes to the DNA sequenceMeiosis- Independent assortment and crossing overSexual reproduction- each individual differs from the parents as only receives half genetic info from each parentChance- Individuals differ from siblings produced
• Define the terms "phenotype" and "genotype"

  Phenotype- observable characteristics of an organismGenotype- genetic makeup of an organism
• Describe how etiolation and chlorosis in plants, and body mass and height in animals are examples of phenotypes influenced by both genetic and environmental factors

  They are characteristics that are observable in each organism. Body mass and height are affected by the genetic makeup of an organism but also depends on environmental factors such as food supply, nutrition, sleep etc. Etiolation and chlorosis is affected by lack of sunlight which is an environmental factor.
• Define alleles

  different versions of the same gene
• Define Dominant Allele

  version of the gene that will be expressed if present
• Define Recessive allele

  Version of a gene that that will only be expressed if two copies of this allele are present
• Define homozygous

  having two identical alleles for a characteristic
• Define heterozygous

  having two different alleles for a characteristic
• Define carrier

  a person who has one copy of a recessive allele coding for a genetically inherited condition
• Define monogenic inheritance

  A characteristic inherited on a single gene
• Define dihybrid inheritance

  A characteristic inherited on two genes
• Define autosomal linkage

  Genes present on the same non-sex chromosome
• Define sex linkage

  genes carried on the same sex chromosomes
• Define codominance

  when different alleles of a gene are equally dominant and both are expressed in the phenotype
• Define epistasis

  the effect of one gene on the expression of another gene
• Describe the 6 steps for drawing a genetic cross diagram

  1- State the phenotypes2- State the genotypes3- State the gametes4- Use a Punnett square to show the results of the random fusion of gametes 5- State the proportion of each genotype6- State the corresponding phenotype
• Draw a diagram to show a simple genetic cross between a homozygous dominant individual and a homozygous recessive individual.

  
• Draw a diagram to show a simple genetic cross between two heterozygous individuals.

  
• Describe how to present the outcomes for genetic crosses as probabilities, ratios and percentages.

  Direct proportionHalf the offspring have the combination Aa and half have aa.orOut of 4 offspring 2 have the combination Aa and 2 have aa.ProbabilityThe probability of the offspring being Aa is 50%.The probability of the offspring being aa is 50%.RatioThe ratio of Aa to aa is 1:1.
• Draw a genetic cross diagram to show an example of codominance.

  
• Draw a genetic cross diagram to show an example of multiple alleles.

  
• Describe how sex is determined in humans.

  Sex is genetically determined. Females have 2 X chromosomes, men have 1 X and 1 Y.
• Draw a genetic cross diagram to show an example of sex linkage.

  
• Draw a genetic cross diagram to show an example of dihybrid inheritance.

  
• State the expected phenotypic ratio for dihybrid crosses involving: i) two double heterozygotes, ii) one double heterozygote and one double homozygous recessive.

  i) 3:1ii) 1:1
• Explain why expected phenotypic ratios may not occur if there is linkage between two genes.

  Linkage means that the genes are located on the same chromosome. Linked genes are inherited as one unit so no independent assortment.
• Explain why crossing over disrupts autosomal linkage, but only occasionally

  Autosomal linkage is when the genes that are linked are found on other pairs of chromosomes. The closer the genes are on a chromosome the less likely they are to be separated during crossing over. If the linkage is above the point of break there will not be a disruption however if the linkage is below the point of break the gene loci will be exchanged.
• Define the term "recombinant offspring" and "recombination frequency"

  Recombinant offspring- different combinations of alleles to either parentRecombinant frequency- the measure of the amount of crossing over that has happened in meiosis
• Describe how epistasis can occur.

  Many genes interact epistatically. Multiple genes need to be expressed. to produce enzymes. If one of the genes is not expressed then one step is missing. The lack of the enzyme normally produced when the gene is expressed means the intermediate molecules necessary for the next reaction is not produced. This results in a lack of substrate for the next enzyme in the pathway and so the expression of this gene will not be expressed in the phenotype. The gene is effectively 'masked'.
• Define the terms "hypostatic gene" and "epistatic gene"

  Hypostatic gene- A gene that is affected by another geneEpistatic gene- A gene that affects the expression of another gene
• Define the terms "dominant epistasis" and "recessive epistasis"


  Dominant epistasis- If a dominant allele results in a gene having an effect on another geneRecessive epistasis- Two recessive alleles at a gene locus lead to the lack of an enzyme
• Describe when a chi-squared test would be used to analyse data.

  To determine whether differences in the expected and observed results are significant or not.
• Describe the meaning of each of the symbols in the equation for calculating the x2-value (the test statistic) from a chi-squared (x2) test.

  x2= test statisticE= expected frequenciesO= observed frequencies
• Describe how to conduct a chi-squared test to determine whether data from a genetic cross supports the hypothesis about how the characteristic(s) is/are inherited.

  Step 1- Calculate the probability of getting each expected phenotypeStep 2- Calculate the expected number of each phenotype based on probabilities and total number of offspringStep 3- Put your calculated and observed results into a tableStep 4- Calculate the chi squared value using the equationStep 5- The chi squared value is determined from the chi squared distribution table at the 0.05 probability level for the correct degrees of freedomStep 6- Accept or Reject the null hypothesis
• Define the terms "continuous variation" and "discontinuous variation".

  Continuous variation- a characteristic that can take any value within a range eg. heightDiscontinuous variation- a characteristic that can only result in certain discrete values eg. blood type
• Describe the causes of variation that result in discontinuous variation.

  mostly genetic as only involves one or two genes