1.3 Patterns of Inheritance

• Probability value table


  the likelihood that an observed result could have occurred by chance

  
• Degrees of freedom (df)


  (Number of phenotypes -1)

  
• chi squared example

  
• Chi-squared Example

  
• Chi-square test

  •A statistical test used to analyze results of genetic crosses.

  
  

  •To determine if observed results are similar to expected (theoretical) results.

  
  

  •It tests the null hypothesis (no significant difference between observed and expected data)

  
  

  •Formula:

  where

  x2= Chi- squared

  Z- the sum of

  O- observed values

  E- expected values

  
• Agouti cross Albino


  Let A represent agouti fur (dominant)

  Let a represent black fur (recessive)

  Let C represent coloured fur (dominant)

  Let c represent albino fur (recessive)

  
  

  Parental phenotypes: Agouti and Albino

  Parental genotypes: AaCc and Aacc

  
• hypostatic gene


  gene that is masked or suppressed by the action of a gene at a different locus,

  
  

  NB: The hypostatic is determined by the epistatic gene.

  
• epistatic gene


  gene that masks or suppresses the effect of a gene at a different locus

  
• Example of Epistasis


  Fur colour in mice

  -Controlled by a pair of genes occupying different loci.

  
  

  The epistatic gene determines the presence of fur colour.

  -Exists in two alleles :

  1.Coloured (dominant)- C

  2.Albino (recessive)- c

  
  

  The hypostatic gene determines the nature of the colour.

  1.Agouti /grey (dominant)- A

  2.Black (recessive)-a

  
  

  
  

  The albino mouse will have homozygous recessive alleles for fur colour

  even if alleles for agouti/ black fur are present.

  
  

  Three possible phenotypes: agouti, black, albino

  
• Epistasis


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

  
  

  Epistasis is the interaction of two or more gene loci in the control of phenotype.

  
  

  •An allele of one of them may have an overriding effect on the phenotype.

  
  

  •Epistatic genes are considered

  inhibiting genes.

  
  

  •They suppress the effect of a gene at another locus.

  
  

  •The other gene is described as the hypostatic gene.

  
  

  •Epistatic genes can produce ratios such as 9:4:3, 9:7, 12:4 (=3:1), 12:3:1, 9:6:1, or 15:1. You can see how these are all variations on the standard dihybrid result(9:3:3:1).
• Dihybrid cross seed shape and seed colour

  
• Dihybrid Crosses


  Cross or mating between organisms involving two pairs of contrasting traits.

  •Genetic crosses involving two pairs of alleles

  •Focuses on two specific traits. E.g. seed texture and colour

  
• Example of Pedigree chart for Hair colour


  Let B be the dominant allele for brown hair

  
  

  Let b be the recessive allele for black hair

  
  

  Brown hair - (BB/Bb)

  Black hair- (bb)

  
• Pedigree chart


  A diagram that shows the occurrence of a genetic trait in several generations of a family.

  
• Inheritance of Red/Green Colour Blindness

  •Let C represent dominant allele for normal vision.

  
  

  •Let c represent recessive allele for colour blindness.
• Inheritance of Haemophilia

  •Defective gene prevents blood clotting at wound.

  
  

  Let H represent dominant allele for normal blood clotting

  
  

  Let h represent recessive allele for haemophilia.

  
• Sex linkage

  •Genes carried only on the X chromosome are sex-linked genes.

  
  

  •Some genetic disorders are sex linked E.g. haemophilia and colour blindness.

  
  

  •Males are more likely to show sex-linked conditions.

  
  

  •Males only need to inherit an X chromosome with a recessive allele and they automatically become affected.

  
  

  •Only females can be carriers (Having one dominant and one recessive allele
• Sex Determination in Humans

  •Each cell has a pair of sex chromosomes

  •XX for females

  •XY for males

  •Since only the male can pass on

  the Y chromosome, he determines the sex of his offspring.
• incomplete dominance


  •Neither allele is dominant over the other.

  
  

  •Results in a mixing of traits

  Example: Flower Colour in

  Impatiens

  
  

  •Allele R produces red flowers

  •Allele W produces white flowers

  •R and W produces pink flowers

  
• ABO blood groups

  • Father's genotype: BO

  •Mother's genotype: AO

  
  

  •Children's genotypes:AB, BO, AO, OO.

  
  

  •Children's phenotypes:

  Type AB

  Type B

  Type A

  Type O

  
• Co-Dominance: ABO blood groups

  •One gene, three alleles:A,B,O

  
  

  •A and B are both dominant to O which is recessive

  
  

  •But there's no dominance between A and B "they coexist".

  
• Inheritance of Sickle Cell Anaemia

  •Let A represent dominant allele for normal haemoglobin

  
  

  •Let S represent recessive allele for (abnormal haemoglobin)

  sickle cell anaemia.

  
  

  If both parents are heterozygous (AS), they have the sickle cell trait.

  
  

  The phenotypes of the offspring are:

  1 normal haemoglobin (AA)

  2 sickle cell traits (AS)

  1 sickle cell anaemia (SS)

  
• Inheritance of Tongue Rolling

  •Let R represent dominant allele for ability to roll tongue

  
  

  •Let r represent recessive allele for non-tongue roller

  
  

  •If a tongue rolling father (RR) and non-tongue rolling mother

  (rr)have children:

  •All children are tongue rollers (Rr)

  
• If a heterozygous (Aa) and a homozygous recessive (aa) mate:

  
• If two heterozygous parents (Aa) mate:

  
• Cross between homozygous dominant(AA) & homozygous recessive (aa)

  
• Inheritance of Albinism (Recessive condition)

  Albinism- a condition in which pigmentation fails to develop.

  
  

  Two alleles control pigmentation:

  Let A represent dominant allele for normal pigmentation.

  
  

  Let a represent recessive allele for lack of pigmentation (albino condition).

  
  

  NB: A child will only be an albino (aa) if it inherits two

  recessive alleles from its parents.

  
• Homologous chromosomes showing

  three different genes

  
• Inheritance of characteristics

  loci (specific points) of

  homologous chromosomes.

  
  

  •A gene is represented by two alleles (A or a)

  •One paternal allele and one

  maternal allele

  •The combination of alleles

  controls a specific

  characteristic. E.g. eye colour
• Gene


  A basic unit of inheritance
• allele frequencies


  this changes within a population due to mutations and migration introducing new alleles into a population
• environment


  the ............................. has a greater effect on expression of polygenic characteristics than monogenic
• polygenic


  describes a characteristic that is influenced by many genes. Alleles of each gene contribute a small amount to the phenotype therefore each allele has an additive effect on the phenotype. Continuous variation
• categories - 1

  how do you work out degrees of freedom in chi squared?
• greater


  if the value of X2 is .............. than the critical value of chi squared at P=0.05, there is a significant different between observed and expected - reject null hypothesis
• chi squared test


  statistical test designed to find out if the difference between observed and expected data is significant or due to chance
• 9:7

  ratio for epistasis by complementary action in sweet pea plants
• enzyme


  example of epistasis by complementary action - colour of Sweet pea plants. One gene codes for a colourless substance, the second gene codes for an .................. that converts it to a coloured form.
• agouti

  example of epistasis by complementary action - coat colour in mice.

  C = gene for development of black

  A = gene which determines banding = forms ................. pattern

  Produces a 9:3:4 ratio
• complementary action


  type of epistasis where genes code for 2 enzymes which work in succession to catalyse steps in a metabolic pathway e.g. coat colour in mice