Inheritance

Inheritance

1. Genetic Terms

   • Gene: Section of DNA coding from protein
   • Locus: Position of gene on chromosome
   • Allele: Alternative form of a gene
   • Dominant: Allele that's shown physically if homo/heterozygous
   • Recessive: Allele is shown only if homozygous (aa)
   • Genotype: The alleles an individual has
   • Phenotype: Physical expression of a gene
   • Homozygous: When alleles on chromosomes are the same (AA or aa)
   • Heterozygous: When alleles on chromosomes are different (Aa)
   • F2: Describes 2nd-gen offspring from a cross (Giving 3:1 or 9:3:3:1 ratio)
   • F1: Describes 1st-gen offspring from a cross
   • Autosomes: Chromosomes not involved in inheritance of gender
   • Sex chromosomes: Chromosomes involved in inheritance of gender
   • Degenerate: Amino acid has more than 1 code

2. Monohybrid Inheritance

   • Inheritance of 1 gene
     • Mendal used peas to show how characteristics are inherited
       • Checked if they were tall/dwarf or round/wrinkled
         • Perks of this choice
           • Easy to grow
           • Peas produce statistically meaningful offspring
           • Peas Self/Cross fertilise, Flower & fruit in same year
           • Controlled by single genes on different chromosomes
           • Easy to tell results
           • Show Discontinuous variation
   • In a monohybrid cross
     • 2 heterozygous individuals produce offspring
       • With a phenotypic ratio of 3 Dominant:1 Recessive

3. Test Cross/Back cross

   • Cross between an individual w/the phenotype of a dominant characteristic, but unknown genotype
     • w/an individual that's homozygous recessive for the gene in question
       • Back test: The test organism is crossed w/the homozygous recessive parent of the original cross that made it

4. When neither allele is dominant

   • Co-dominance
     • When both alleles in the heterozygote are individually expressed
       • E.g Blood grouping in humans (Groups A,B,AB,O) & Speckled chickens
         • Co= Allele Work together
           • Co-dominant are superscripted to a locus letter
   • Incomplete dominance
     • The dominant allele is not fully expressed when the recessive allele is present
   • Alleles wrote in exponent form (E.G. I^A)
   • If phenotypes of offspring are 1:2:1 ratio
     • Parents probs heterozygous for the gene that shows incomplete/co-dominance
     • When Mendal carried out experiements
       • & found 1:2:1 ratios, he developed his 1st law
         • Law of Segregation
           • The characteristics of an organism
             • Are determined by factors (Alleles) which occur in pairs
               • Only 1 allele of the pair is present in a gamete
           • Gametes only carry 1 allele for a gene

5. Independent assortment

   • Happens in meiosis (metaphase 1)
     • Shows that an allele can combine w/either allele in another pair
   • Law of independent assortment: (2nd law of inheritance)
     • States that the alleles of 1 gene sort into gametes
       • Independently of the alleles of another gene
         • I.e alleles are unlinked
   • LOOK Pg.165 & 168 Diagram

6. Dihybrid Inheritance

   • Simultaneous inheritance of 2 unlinked genes (I.e genes on different chromosomes)
     • Mendal tested this w/2 contrasting characteristics
       • Y = Yellow , y = Green
       • R= Round , r = Wrinkled
       • When F2 gen genotypes formed 9:3:3:1 ratio found
   • To calculate ratio of cross
     • Divide the No. of individuals for a phenotype
       • By the No. of homozygous recessive individuals

7. Linkage

   • Description of genes on the same chromosome
     • & therefore don't segregate independently @ meiosis
   • Linked genes are inherited together
     • Because they move together during meiosis & appear in the same gamete
   • In meiosis, however crossing over can link to the combination of linked genes
     • This is quite rare & can lead to odd ratios (non-mendelian)
       • Common explanation for rejecting null hypothesis
         • Crossing over value (COV) = No. of recombinants/ No. of progeny x 100

8. Chi test

   • Checks if the No. of different phenotypes is close enough to the predicted values
     • If number ain't close enough, test tells us it has arisen for another reason
   • Null Hypothesis
     • There is no significant difference between the expected ratio & the ratio of the actual results
       • Any difference is down to chance
     • Reason for this hypothesis
       • Mendelian inheritance is based on there being no difference in:
         • No. of different types of gametes
         • Probability of each gamete type fusing w/another type of gamete
         • Viabilty of the embryos, regardless of genotype
         • Genes aren't linked
   • Method
   • Degrees of freedom (df)
     • Used to find critical value
       • Do (No of phenotypes) -1
         • Giving value of df
   • Probablity
     • Statisticians say if Chi value is under 5%
       • Value can be accepted as any deviation is due to chance
         • Called critical value (Largest chi value can be to chance)
           • Any value above critical value
             • Null hypothesis is incorrect as results are affected by another factor
   • Conclusion
     • 1) Calculated value is more/less than critical value
       • 2) This is equivalent to a probability less/greater than 5%
         • 3) So null hypothesis is accepted/rejected
           • 4) @ 5% level of significance, inheritance is mendelian
             • 5a) Any deviation is down to chance (If value is lower than critical)
               • 5b) Another factor is involved (If value is bigger than critical)

9. Sex determination

   • Plants
     • Most angiosperms are hermaphrodites
     • Monoecious plants have separate male/female flowers on the same plant
     • Dioecious plants have separate male/female individuals
   • Animals
     • Male/female is controlled by many factors
       • Temp:
       • Sequential hermaphroditism: (Some fish)
         • Start out as 1 sex & change later on in life
       • Ploidy level
         • Haploid/diploid gametes/cells
       • Chromosome structure
         • In humans: XX = Female, XY = Male
           • In birds/moths: ZZ = Male, ZW = Female
   • Human Sex Determination
     • 22/23 chromosomes have identical genes (autosomes)
       • 23rd chromosome = Sex = Different sizes = Heterosomes
         • 2 regions on X & Y chromosome are homologous & pair at meiosis
           • They're called 'pseudoautosomal regions' (PAR1 & PAR2)
             • They reflect evolutionary loss of genes from Y chromosome
               • Leaving larger X chromosome
     • Inheritance of sex
       • Female gives 2 X in gametes (Homogametic)
         • Male gamete gives X/Y (Heterogametic)
           • At fertilisation there is a 50/50 chance of male/female
       • Maleness
         • Y chromosome contains 'sex-determining region' (SRY gene)
           • SRY gene ain't found on X chromosome
             • It seems to switch on genes in the autosome that give rise to the expression of male characteristics

10. Sex Linkage

    • A gene is carried by a sex chromosome so that a characteristic it encodes is seen predominantly in 1 sex
    • All alleles are superscripted to the chromosome they are found on
    • Sex-linked conditions
      • Haemophilia
        • Recessive allele on X chromosome
          • Blood doesn't clot
      • Colour Blindness
        • Recessive allele on X chromosome
      • Duchennes muscular dystrophy
        • Recessive allele on X chromosome
          • Muscles progressively degenerates
    • Pedigree charts/Family Trees
      • Can show the inheritance of a medical condition
        • Hence used to calculate probability of a child inheriting a condition

11. Mutations

    • A change in the amount, arrangement or structure in the hereditary material of an organism
      • Can be:
        • Spontaneous
          • Happen without an apparent cause
        • Random
          • Appear to have equal probability anywhere in the genome of diploid organism
    • Occur in all cells, mostly harmful
      • Some = Good but rare, gives selective advantage
        • Can cause variation > Natural Selection > Evolution
          • Haploid Organism > Mutation > Expressed
            • Diploid Organism > Dominant mutations shown
              • Most are recessive & happen in cells w/Dominant alleles thus not expressed
                • Mutation in diploid organisms have less impact on evolution
          • Source of variation
    • Mutation Rate
      • Mutations occur in DNA Replication
        • Organisms w/short life cycles have more frequent meiosis
          • Hence bigger mutation rate
      • Normally low but affected by
        • Ionising Radiation <260nm
          • Radiation joins adjacent pyrimidine bases in a DNA strand
            • Thus DNA polymerase may add wrong nucleotide
        • Mutagenic chemicals
          • Polycyclic hydrocarbons, Mustard gas
            • Slide between bases, preventing DNA polymerase from adding the right nucleotide at replication
    • Gene (Point) Mutations
      • DNA polymerase makes a mistake, resulting in a gene mutation
      • Addition
        • A base is added
      • Duplication
        • Same base is incorporated twice
      • Subtraction
        • Base is deleted
      • Substitution
        • A different base is added
      • Inversion
        • Adjacent bases on the same DNA strand exchange position
      • Changes allele > Changes mRNA bases > Polypeptide change > Effecting phenotype
        • Depending of nature of mutation
          • Could code for same amino acid 'Silent Mutation'
            • Amino acid could be replaced for similar chemical
              • Could change important place on protein
      • E.g Sickle cell Anaemia
        • Muatation in beta polypeptide
          • Glutamic acid > Valine
            • Side chain: Large, hydrophilic > Small, hydrophobic
              • Haemoglobin aggregates in low O2 conc
                • Joint pain but protected from malaria
      • Insertion/deletion lead to frame shift
        • Frame Shift: mRNA reading frame shifts & codes for different amino acids
    • Chromosome mutation
      • Changes in structure/No. of chromosomes
        • Changes in structure
          • During crossing over at Chiasmata, Chromosomes don't accurately join together
            • Leading to mutation, After gamete fusion no more meiosis will take place
        • No. of chromosomes
          • Happen during anaphase 1/2
            • Called Non-disjunction
              • Faulty spindle in meiosis resulting in 1 daughter cell getting 2 copies of a chromosome
                • & other receives none
        • Due to damage > Can repair > But sometimes not correctly
      • Down syndrome
        • 2o oocyte gets either 0/2 copies of 21st chromosome
          • 0 copies doesn't make a viable embryo
            • So embryo has 3 copies of 21st chromosome
              • Called Trisomy 21
        • Translocation Down's
          • Part of chromosome 21 attaches to chromosomes 14
    • Changes in No. of sets of Chromosomes
      • Euploid: Cells w/complete sets of chromosomes
        • Aneuploid: too many/too few chromosomes
          • Polyploid: Having 2+ sets of chromosomes
            • Arises from:
              • Non-disjunction
              • 2 diploid gametes fuse together (tetraploid 4n)
              • Endomitosis: Replication of chromosomes not followed by cytokinesis
                • Can happen in tetra/tri-ploid cells
                  • Infertile Triploid > Fertile Hexaploid
            • More common in flowering plants
              • Because
                • Reproduce assexually
                • Hermaphrodite so don't use chromosomes to determine sex
    • Carcinogens
      • Agent that causes cancer
      • Mutagens
        • Agents that causes mutations
      • Tumour suppressor genes regulate mitosis & prevent rapid cell division
        • Mutation can affect a gene's regulatory function > causing it to go through repeated mitosis
          • If undetected by immune system it can make a tumour
            • That can spread around the body into other tissues
              • Forming secondary tumours/metastases (Malignant tumour)
    • Oncogenes
      • Proto-oncogene codes for a cell division protein
        • Mutations can permanently switch on gene
          • Making excess protein, causing rapid, repeated mitosis
            • Oncogene: When a proto-oncogene mutates such that it causes cancer
              • Caused By:
                • Mutation causing chromosomes to rearrange
                • Extra copy of proto-oncogene

12. Control of gene expression

    • Epigenetics
      • Control of gene expression by modifying Histones/DNA
        • But not by affecting the DNA nucleotide structure
      • DNA methylation:
        • Methyl group can be added to Cytosine
          • If regions are highly-methylated, less likely to be transcribed
      • Histone modification
        • If DNA is wound strongly around histone
          • Becomes harder for enzymes to attach, reducing transcription
        • E.g Happens when stem cells differentiate
    • Consequences of epigenetic change
      • Genomic imprinting
        • If genes are deactivated in gametes, they are passed onto next gen
      • Can inactivate whole chromosomes (X inactivation)

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