Module 6.1.2- Patterns of inheritance

• Chlorosis

  the yellowing seen on the leaves of plants when they cannot make chlorophyll due to a lack of magnesium ions
• Environmental factors for chlorosis

  lack of light

  - plants turn off chlorophyll production (etiolation)

  mineral deficiencies

  - ions needed as cofactors for enzymes producing chlorophyll

  virus infections
• Animal body mass

  Can be determined by both environmental and genetic factors including diet, exercise, disease or the genetic makeup of the organism
• Genotype


  The combination of alleles an organism inherits for a characteristic
• Phenotype


  An organism's physical appearance, or visible traits.
• Ways genetic variation occurs

  -crossing over

  -independent assortment

  -random fusion of gametes in fertilisation

  
• Homozygous


  they have two identical alleles for a characteristic

  organism could be homozygous recessive/dominant
• Heterozygous


  they have two different alleles for a characteristic
• Dominant


  the version of a gene that will always be expressed
• Recessive


  the trait of an organism that can be masked by the dominant form of a trait. it is only expressed when both copies are recessive
• Continuous variation


  a characteristic that can take any value within a range, e.g. height.

  controlled by a number of genes (polygenes)
• Discontinuous variation


  a characteristic that can only appear in specific(discrete)values

  controlled by 1 or 2 genes
• Monogenic inheritance


  a characteristic inherited on a single gene
• Performing a genetic cross steps

  - state phenotype of both parents

  - state genotype of both parents

  - state gametes of each parents and circle them

  - use Punnett square to show results of random fusion of gametes

  -label gametes on the edges of the square

  
• Difference between genotype and phenotype in a Punnett square


  A genotype is the letters expressed in a Punnett square, while a phenotype is the physical trait expressed from the alleles.
• Homozygous genetic cross have offsprings that are all...

  heterozygous
• Codominance


  A condition in which neither of two alleles of a gene is dominant or recessive and so both alleles for a gene are fully expressed
• Showing codominance in a genetic cross


  A letter is chosen to represent the gene

  Since neither allele is dominant or recessive, the different alleles are represented using a superscript with a second letter.

  
  

  For example

  
  

  C^w and C^r

  
• Multiple alleles

  three or more forms of a gene that code for a single trait for example blood type

  
• Blood type (multiple alleles)

  IA IA or IA IO = Blood group A

  IB IB or IB IO = Blood group B

  IA IB = Blood group AB(they are codominant)

  IO IO = Blood group O

  IO is recessive to both IA and IB

  
• Difference in sex chromosomes between males and females


  Females have 2 X chromosomes and Males have 1 X chromosome and 1 Y chromosome
• Difference between a Y chromosome and an X chromosome

  The X chromosome is large and contains many genes not involved in sexual development

  
  

  The Y chromosome is small, containing almost no genetic information but a gene that causes an embryo to develop into a male

  
• Sex linkage

  an association between genes in sex chromosomes that makes some characteristics appear more frequently in one sex than in the other
• Sex inheritance genetic diagram


  50 50

  
• Why do men suffer from X-linked disorders more?

  because they only have one X, so one bad copy (recessive) leads to the disorder, but females can have a good X (dominant) counteract the bad X
• Haemophilia


  a sex-linked inheritable disease which have blood that clots extremely slow

  Females who are heterozygous to this gene are carriers however males who have only one recessive allele for this develop the condition since they do not have a dominant allele on the Y chromosome to counteract it
• Sex inheritance genetic diagram for

  haemophilia XH represents the healthy allele

  
  

  Xh represents the recessive allele for haemophilia

  
  

  Y chromosome with no allele attached :(

  
• Dihybrid inheritance


  The inheritance of two characteristics which are controlled by two different genes
• Dihybrid cross

  9:3:3:1 ratio

  
• Autosomal linkage


  When two genes are located on the same autosome and are inherited by the offspring together.
• Linked genes


  Genes located on the same chromosome that tend to be inherited together in genetic crosses.
• Recombinant offspring


  Offspring with different gene combinations than the parents
• Recombinant frequency


  the proportion of offspring of a genetic cross that have phenotypes different from the parental phenotypes due to crossing over between linked genes during gamete formation

  
  

  50% indicates no linkage and genes are on separate chromosomes.

  <50% indicates there is gene linkage and random process of independent assortment has been hindered

  
• Chi-squared test

  A statistical test that measures the difference between an observed distribution of frequency (count) data and a distribution of these data that would be expected under particular conditions (by default, the expected

  distribution would be due to the conditions of chance).

  
• Calculating the degrees of freedom in a Chi-squared test

  n-1

  
  

  where n is the number of categories or possible outcomes (e.g. phenotypes)
• Exam technique for Chi-squared test results if value is less

  If calculated value is less than the critical value, you accept the null hypothesis(not enough evidence to reject) since there is no significant difference between what we observed and what we expected
• If Chi-squared calculated value is more than the critical value, you ________ the null hypothesis (not enough evidence to reject) since there is _______ significant difference between what we observed and what we expected

  If Chi-squared calculated value is more than the critical value, you reject the null hypothesis (not enough evidence to reject) since there is a significant difference between what we observed and what we expected
• Epistasis


  A type of gene interaction at different loci in which one gene alters the phenotypic effects of another gene that is independently inherited.

  for example Gene regulation with the lac operon
• Epistasis in biochemical pathways

  genetic block in the previous step, cannot produce phenotype

  
  

  For example in the image, if gene C does not/cannot synthesise an enzyme it will prevent any reaction going forward

  (In a way it is like a chain reaction, each one is needed for each other)

  
• Dominant and recessive epistasis

  when either having a dominant allele at one locus or being homozygous recessive at the other locus produces the same phenotype.

  
  

  e.g. in the first gene locus if there is a dominant allele that results in a gene having another effect on another gene, but then the next enzyme in the pathway would then lack a suitable substrate, the pigment would not be produced essentially meaning the dominant allele had no effect wherever it was because it was cancelled out by the recessive in the end. All of the genes in the sequence would be effectively 'masked'.