Mendelian Genetics

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Mendelian Genetics

• Read through core knowledge.
• What vocab do you need to learn?

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Terms

• Gene
• Allele
• Trait
• Dominant
• Recessive
• Homozygous
• Heterozygous
• Genotype
• Phenotype
• P and F1 and F2

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Definitions

• Unit of hereditary
• One of 2 or more forms of a gene at a given locus
• Genetically inherited characteristic of organism,
  varies amongst individuals
• Allele that is expressed in heterozygotes
• Allele that is only expressed in homozygotes
• Carries two copies of the allele
• Carries different allelic forms of a given gene
• Organisms hereditary make-up
• Physical characteristics of an organism
• Patrial generation, first and second filial
  generation

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Mendel why so famous?

• Worked with peas
• Used pure-breeding varieties
• Came up with idea of gene 20 years before
  chromosomes were discovered

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Law of segregation

• Of the two genes controlling each characteristic,
  only one is present in each gamete.
• During meiosis the two genes are separated.

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Independent assortment

• The segregation of one pair of alleles does not
  affect the segregation of another pair.
• There is a random arrangement of parental
  chromosomes at metaphase of meiosis.

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Monohybrid cross

• Carry out a cross for a dominant and a recessive
  homozygote where P is for pink flower and p is
  for white
• Cross the F1 and give the ratios of the F2.

P P
p
p
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Test Cross

• A cross carried out to determine an organisms
  genotype, by mating it with a homozygous
  recessive organism.
• Show how a test cross works

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Dihybrid Cross

• Carry out a cross for a dominant and a recessive
  homozygote where Y is for yellow and y for green,
  and R for round and r for wrinkled.
• Cross the F1 and give the ratios of the F2.

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Dominance

• Incomplete dominance
• Co-dominance
• Lethal Alleles

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Incomplete dominance

• Action of one allele does not completely mask the
  action of the other.
• Neither allele has dominant control over the
  trait.
• Heterozygous offspring is intermediate in
  phenotype

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Eg snapdragons

• P1 RR (Red) x rr (white)
• F1 Rr (pink)
• F2 ? (You determine the ratios)
• 1red2pink1white

R r
R
r
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Co-dominance

• Both alleles in heterozygous organism contribute
  to the phenotype.
• Both alleles are independently and equally
  expressed.

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Eg Human Blood Group AB

• P1 AA (type A) x BB (type B)
• F1 AB (type AB)

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Eg Coat colour in horses and cattle

• P1 CRCR(red) x CrCr(white)
• F1 CRCr(roan)
• F2 ? You determine the ratios
• 1 Red 2 Roan 1 white
• Roan is a blend of both white hairs and red hairs

CR Cr
CR
Cr
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Lethal Alleles

• Mutations of a gene that produce a non-functional
  gene product and affect the organisms survival.
• If dominant, may kill in single dose
• If recessive, kills when homozygote

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Eg Manx cat

• MM (normal tail)
• MML (manx - no tail)
• MLML (lethal deformity of spine in embryo)
• Carry out a cross for two heterozygotes. What is
  the phenotypic ratio?

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Eg Yellow mice

• YY (lethal yellow terminates at blastocyst
  stage)
• Yy (yellow)
• yy (not yellow)
• Again, what is the phenotypic ratio for a cross
  of heterozygotes?

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Eg Huntingtons disease

• A dominant allele is lethal.
• Nerve cell death in brain causing jerky
  involuntary movements and dementia.
• Why does it persist in the human population?
• Shows in adults 30-40 years

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Multiple alleles

• More than one allele possible at a gene locus

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Blood groups

• There are three different alleles
• A, B and O
• The alleles code for making the enzyme that hold
  the sugars together that make the different
  antigens on the RBC.

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• O is nonfunctioning (recessive)
• A is A antigen (dominant)
• B is B antigen (dominant)
• A and B antigens can act with other antibodies so
  must be matched for transfusion.

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Frequency in NZ
Phenotype Genotype Frequency in NZ
O OO 49
A AA, AO 40
B BB, BO 9
AB AB 2
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Determine the blood types
Cross Parental genotype Ratio F1 genotype Ratio F1 Phenotype
1 ABxAB 1AA2AB1BB 1A2AB1B
2 OOxOO
3 ABxAO
4 AAxBO
5 AOxOO
6 BOxOO
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Dilemma

• If a mother is type A and has a baby type B, can
  the father be type O?
• Explain your answer.
• You can now do the
• self check for this section.

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Gene-gene interactions

• When a characteristic is influenced by more than
  one gene at two different loci or even on
  different chromosomes altogether.

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Epistasis (standing upon)

• Involves two non-allelic genes (different loci)
• Action of one gene masks or alters expression of
  other genes
• Three forms collaboration, complementary,
  supplementary

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Eg Albinism

• Occurs in rodent that are homozygous recessive
  for colour even if they have alleles for agouti
  or black fur.
• The gene for colour is epistatic
• gene 1 gene 2
• A ? B ? C
• coat colour show one colour/another colour

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Collaboration

• Ratio 9331
• (Although the ratio is typical, it is unusual
  that some of the phenotypes may not have been
  shown in the parents)
• Four different phenotypes depending on the
  presence or absence of certain genes

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Eg comb shape in chickens

• P_R_ walnut
• P_rr pea
• ppR_ rose
• pprr single
• Carry out a cross for two heterozygotes PpRr x
  PpRr

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Supplementary Genes (Epistasis)

• Ratio 934
• A dominant allele at one locus is necessary for
  the expression of alleles at another
• Typically three phenotypes
• Carry out a cross for two heterozygotes CcBb x
  CcBb

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Eg Coat colour in mice

• Gene C controls the production of melanin
• Gene B indicates whether the colour is black or
  brown
• Without the production of melanin, there will be
  no colour.
• gene C gene B
• enzyme 1 enzyme 2
• no pigment ? melanin produced ? Black
• ? Brown
• C_B_ Black
• C_bb Brown
• cc__ No colour

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Complementary Genes

• Ratio 97
• Development of a characteristic requires the
  presence of at least one dominant allele at both
  of 2 loci
• Typically there are two phenotypes
• Carry out a cross for two heterozygotes PpQq x
  PpQq

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Eg Purple pigment in sweet pea flowers

• Gene P makes white intermediate
• Gene Q converts white to purple
• gene P gene Q
• enzyme 1 enzyme 2
• Colourless ? Colourless ? Coloured precursor
  intermediate product
• (white pp_ _) (white P_ _ _) (purple P_Q_)

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Duplicate genes

• Ratio 151
• A characteristic is developed if EITHER or BOTH
  of the dominant alleles at two loci is/are
  present.
• Carry out a cross for two heterozygotes AaBb x
  AaBb

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Eg Fruit width in Shepherds Purse

• Gene A and B code for two different enzymes which
  can form wide fruit.
• gene A gene B
• enzyme A enzyme B
• Substance ? Active ? Substance
• X Product Y
• wide (A_B_,A_bb, aaB_)
• narrow (aabb)

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Practice

• Self check page 111/112
• Create a table to compare
• Exam questions
• Study book pg 31 Qb

Type of interaction F2 phenotype ratio Number of phenotypes Example organism Example trait
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• A women who owned a purebred female albino
  (lacking pigments) poodle (an autosomal recessive
  phenotype) wanted white puppies, so she took the
  dog to a breeder, who said he would mate her
  female with an albino stud male, also from a pure
  stock. When six puppies were born they were all
  black, so the women sued the breeder, claiming
  that he replaced the stud with a black dog,
  giving her six unwanted puppies. You are called
  in as an expert witness, and the defence asks you
  if it is possible to produce black offspring from
  two pure-breeding recessive albino parents.
• (a) Discuss what evidence you would give by
  explaining what gene-gene interrelationship is
  involved in each of the parents and using
  appropriate allele symbols, draw biochemical
  pathways to obtain an albino phenotype and a
  black phenotype.
• Clear well-labelled diagrams may be used to help
  you answer this question.
• (b) Explain the expected possible F2 phenotypes
  ratios if two of the black puppies were allowed
  to interbreed.

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Exams 4U 2007 Q5

• It is definitely a form of epistasis (that is
  there is obviously more than one gene involved.
• We know of 3 types of epistasis collaboration,
  supplementary, complementary
• Use the process of elimination
• There are only 2 phenotypes, therefore it is not
  collaboration
• There is no intermediary product, therefore it is
  not supplementary
• Both genes are required to create colour,
  therefore it is complementary

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• So, both parent dogs are claimed to be pure
  breeding therefore homozygous
• What are our options for this with 2 genes?
• AABB, aabb, but also AAbb and aaBB
• Note a P1 cross of AABB x aabb and a P1 cross of
  AAbb x aaBB both make the F1 generation AaBb,
  which produces the complementary ratio of 97 in
  the F2

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Pleiotropy

• A single gene may produce a product that can
  influence a number of traits in the phenotype.