6.1.2 Patterns of inheritance

    Cards (54)

    • Causes of genetic variation
      1) Mutation

      2) Crossing over

      3) Random fertilisation

      4) Independent assortment
    • Explain mutations
      - a gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait


      - can lead to a different sequence of amino acids being translated

      - changes in secondary and tertiary structures

      - different shape of protein being formed = can affect phenotype
    • Explain what independent assortment
      homologous chromosomes line up in metaphase I

      their orientation towards the opposing poles is randon

      total number of combinations that can occur is 2^n

      (n = number of chromosomes)
    • Explain what is Crossing over
      exchange of segments of DNA between homologous chromosomes during prophase I

      exchange of genetic material occurs between non-sister chromatids
      = chiasmata
    • What is random fertilisation
      As meiosis results in genetically distinct gametes

      random fertilisation by egg and sperm will always generate different zygote
    • What is continuous variation
      A characteristic that can take any value within a range
    • What are properties of continuous variation
      - polygenic (more than one gene involved)

      - environment has a large effect

      e.g animal mass, chlorsis

      - can be represented using a histogram(bar graph but with no betweens) and a line graph
    • What is chlorosis?

      - pale/yellow leaves

      a lack of chlorophyll, which is responsible for the green color of leaves.


      It can be caused by a number of factors:

      > lack of light
      > mineral deficiency
      > viral infection
    • What is discontinuous variation
      a characteristic that can only appear in specific values
    • Properties of discontinuous variation
      - monogenic (1 or 2 genes involved)

      - environment has little, if any, effect

      e.g blood type, wrinkled pea shape

      - usually represented in a bar chart/pie chart
    • What is polygenic inheritance
      it involves interaction of many alleles at many loci( the physical locations of genes on chromosomes)

      = continuous variation
    • How can environment alter gene expression
      - due to EPIGENETICS

      > this is another layer of altering gene expression that give rise to many phenotypes
    • What is monogenic inheritance
      single gene is shown

      like on a punnet square

      each gamete has 1 allele in it
    • Mendel's peas experiment

      - homozygous cross first with 2 pure bred peas

      > purple or white

      - hetrozygous cross > 3:1 PHENOTYPIC RATIO
    • Codominance
      2 different alleles occur for a gene - both equally dominant

      represented like
      C^R or C^W
    • Codominance phenotypic ratio
      1:2:1
    • What is sickle cell anemia
      -CODOMINANT

      where red blood cells become thin and elongated

      Hetrozygous sickle cell = mishapen cell

      Homozygous sickle cell - sickle cell anemia
    • Dealing with multiple genes i.e in blood.
      - more than 2 alleles

      I^A & I^B are codominant

      whilst

      I^O is recessive
    • Dihybrid crosses
      Each trait is controlled by a gene with two alleles

      each gamete has 2 alleles

      e.g mendal wrinkled pea experiment
    • How does it work Mendal's wrinkled pea experiment
      Peas can be green or yellow

      •Y - allele coding for yellow seeds (dominant)
      • y - allele coding for green seeds (recessive)

      And also they can be round or wrinkled

      •R - allele coding for round seeds (dominant)
      r - allele coding for wrinkled seeds (recessive
    • What are the four gamete combinations possible? (in the wrinkled pea experiment)
      - YR
      - Yr
      - yR
      -yr
    • How does a dihybrid cross look like and what is the phenotypic ratio.
      Unlinked genes from heterozygous parents
      = phenotypic ratio is 9:3:3:1

      > 9 is most common phenotype will be dominant

      > 3 in ratio 1 dominant/1 recessive

      - 1 in the ratio will be full recessive phenotype
    • Why do phenotypic ratios not exist in real life?
      - crossing over

      - autosomal linkage

      - sex linkage

      - epistasis
    • What is sex linkage
      - human sex is determined by the XX and XY chromosome but

      there are other genes on the sex chromosomes which are called SEX LINKED GENES

      for example :
      -red green colour blindness
      - haemophilla
    • Red green colour blindness
      - gene loci for three pigments are found on the X chromosomes

      - if one of these pigments are faulty or absent then cant see colour

      - males are more likely with XY to be colour blind than females with XX
    • X linked genes punnet square
    • what is haemophilla
      sex linked condition

      gene coding for clotting factors are found on the X chromosome
    • Recombinant
      new combination of alleles from two sources
    • What is autosomal linkage
      when one or more genes are located on the same AUTOSOME (non-sex chromosome)
    • Factors that affect autosomal linkage
      - will almost always be inherited together [segregate into gametes toegther]

      UNLESS crossing over happens {forms recombinants} between them

      - the further apart on the chromosome = the more chance of being linked
    • What is epistasis
      where one gene masks the expression of another gene
    • Why does this occur [Epistasis]
      - a gene which codes for a protein is needed for the expression of another gene e.g a promoter

      - a gene which codes for a protein, acts as a transcription repressor

      - a gene which codes for something which inhibits the protein
    • E.g Epistasis in hair
      - Gene for baldness > B or b

      - Genes for widows peak > W or w

      WW = widow
      Ww= widow
      ww = no widow

      BUT:

      WWBb = bald
      WwBb = bald
      wwBB = bald
      WwBB = bald

      ALLELES for baldness masks widows peak
    • Recessive epistasis
      when 2 copies of the recessive allele are needed to have the EPISTATIC effect

      phenotypic ratio: 9:4:3
    • Dominant epistasis
      when a dominant allele at one gene locus masks the expression of allele at another gene locus

      phenotypic ratio: 12:3:1
    • What is evolution
      a change in inherited characteristics within a group of organisms, due to changing allele frequency within the population
    • Gene pool
      sum total of all the genes in population at any given time
    • Allele frequency
      the relative frequency of a particular allele in a population
    • Hardy-Weinberg equilibrium and example
      Q) If 98 of 200 people were homozygous reccesive, calculate the number of hetrozygous and homozygous dominant individual

      p² + 2pq + = 1
      p + q = 1

      p > reccesive
      q > dominant

      - 98/200 = q²
      > √98/200 = q²
      > 0.7 = q

      so if p + q = 1

      -> 1 - 0.7 = 0.3

      so p² + 2pq + q²

      0.3² + ( 2 x 0.3 x 0.7) + 0.7²

      @ Homozygous dominant (AA) = p² = 0.3² = 0.09

      > 0.09 x 200 = 18

      @ Hetrozygous (Aa) = q² = 0.42

      > 0.42 x 200 = 84
    • If 34 of 50 people showed the dominant phenotype - calculate frequency of people who are hetrozygous
      = 25

      } dominant phenotype includes hetrozygous and dominant genotype. so you have to 50-34= 16

      and then go on from there
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