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IB Biology Topic 4 Genetics Part II Mendel
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IB Biology Topic 4 Genetics Part II Mendel

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    IB Biology Topic 4 Genetics Part II Mendel IB Biology Topic 4 Genetics Part II Mendel Presentation Transcript

    • Topic 9: Mendelian Genetics
    • Gregor Mendel
      • Austrian monk who published results of garden pea plants inheritance in 1865
      • Used artificial pollination in a series of experiments by using a small brush to place the pollen on the reproductive parts of the flowers
    • Key terminology
      • Genotype – symbolic representation of pair of alleles possessed by an organism, typically represented by two letters
        • Ex: Bb, GG, tt
      • Phenotype – characteristics or traits of an organism
        • Ex: five fingers on each hand, color blindness, type O blood
      • Dominant allele – an allele that has the same effect on the phenotype whether it is paired with the same allele or a different one; always expressed in phenotype
        • Ex: Aa give dominant trait A b/c the a allele is masked; the a allele is not transcribed and translated during protein synthesis
      • Recessive allele – an allele that has an effect on the phenotype only when present in the homozygous state
        • Ex: aa gives rise to the recessive trait b/c no dominant allele is there to mask it
      • Codominant allele – pairs of alleles that both affect the phenotype when present in a heterozygote
        • Ex: parent with curly hair and parent with straight hair can have children with different degrees of curliness as both alleles influence hair condition when both are present in the genotype
      • Locus – particular position on homologous chromosomes of a gene
      • Homozygous – having two identical alleles of a gene
        • Ex: AA is a genotype which is homozygous dominant whereas aa is the genotype which is homozygous recessive
      • Heterozygous – having two different alleles of a gene
        • Ex: Aa is a heterozygous genotype
      • Carrier – an individual who has a recessive allele of a gene that does not have an effect on their phenotype
      • Test cross – testing a suspected heterozygote plant or animal by crossing it with a known homozygous recessive (aa). Since a recessive allele can be masked, it is often impossible to tell if an organism is AA or Aa until they produce offspring which have the recessive trait.
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    • Mendel’s 1st Law Law of Segregation
      • Four parts
      • Alternative versions of genes account for variations in inherited characteristics.
      • For each characteristic, an organism inherits two alleles, one from each parent.
      • If the two alleles differ, then one, the allele that encodes the dominant trait, is fully expressed in the organism's appearance; the other, the allele encoding the recessive trait, has no noticeable effect on the organism's appearance.
      • The two alleles for each characteristic segregate during gamete production
    • Principle of Segregation: Each Parent or Gamete Contributes One Allele to Offspring Gregory Mendel's research into the breeding of pea plants was the first illustration of his genetic laws.
      • Punnet Square :
      • Used to determine the outcome of a cross between two individuals. In the example we have two parents that are heterozygous dominant for a trait
      • P p
      • P PP Pp
      • p Pp pp
      Offspring: Genotype : 1/4 PP, 1/2 Pp, and 1/4 pp Phenotype : 3/4 Purple and 1/4 white
    • Problems with predictions
      • Sex determination
      • Sex linked genes
      • Incomplete dominate
      • Codominance
      • I. Gender determination
        • Sex Chromosomes determine the gender of an individual
        • Females have two X chromosomes.
        • The X-chromosome is fairly large ands carries many genes.
        • Males have one X chromosome and one Y chromosome.
        • The Y chromosome is shorter than the X chromosome and carried fewer genes.
        • Females inherit one X from their mother and one X from their father.
        • Male inherit their X chromosome from their mother and the Y from their father.
    • Inheritance of gender:
      • The ovum (egg) always carries the X chromosome.
      • The sperm either carries the X chromosome or the Y chromosome.
      • It is therefore the male gamete that determines the gender.
      • The probability of the egg carrying the X chromosome is 1.0
      • The probability of the male carrying the X chromosome is 0.5
      • Therefore the chance of and XX fertilization is 1.0 x 0.5 = 0.5
    • II. Sex Linked Genes
    • Homologous and Non-Homologous regions of the Sex Chromosomes
      • For the male sex chromosomes their are
      • non-homologous region males in which there is only one allele per gene and that is inherited from the female on the X-chromosome
      • In the homologous region the male inherited two copies of an allele per gene.
    • Homologous and Non-Homologous regions of the Sex Chromosomes
      • On the female sex chromosomes all regions of the X chromosome are homologous.
      • There are two alleles per gene as with all other genes on all other chromosomes
      This difference in x and y chromosomes plays a large role in determining rates of genetic inherited defects
    • Sex Linkage Alleles on the non-homologous region of the X chromosome are more common in females than in males
      • A gene with two alleles where one is dominant and one is recessive.
      • Female has three possible genotypes and one is the homozygous recessive.
      • In a population the chance of being homozygous recessive is 33.3 %.
      • Males have two possible genotypes.
      • There is a 50% chance of the homozygous recessive condition in the population.
      • In sex linked conditions the recessive condition is more common in males than females.
    • Sex Linkage Alleles on the non-homologous region of the X chromosome are more common in females than in males
    • Sex Linkage Examples
      • Hemophilia is an example of a sex linkage condition.
      • The hemophilia allele is recessive to the normal allele.
      • The gene is located on the non-homologous region of the X chromosome.
      • The disease is associated with an inability to produce a clotting factor in blood.
      • Internal bleeding takes longer to stop.
    • Sex Linkage Examples
      • The homozygous genotype(*) in females has a high mortality.
      • The genotype X n Y in males has a high mortality.
    • Sex Linkage Examples
      • Red Green Color Blindness is an example of a sex linked condition.
      • Red Green Color blindness is a recessive condition.
      • The color blind allele is recessive to the normal allele.
      • Female homozygous recessives X b X b are color blind.
      • Males with the genotype X b Y are color blind.
      • Notice that in a population the probability of having a Red Green color blind genotype in males is higher.
    • Sex Linkage Examples
    • Females (homogametic sex) and X-linked alleles
      • Human females are homogametic that is their 23 rd pair of
      • chromosomes are identical and called the XX chromosomes.
      • Human females can be heterozygous for sex linked alleles.
      • e.g.
            • Hemophilia X H X h
              • or
              • Color Blindness X B X b
      • Human females can be homozygous for sex linked alleles
      • e.g.
      • Hemophilia X h X h
      • or
      • Color Blindness X b X b
    • Female carriers of sex linked alleles
      • Female heterozygote's for sex linked alleles e.g. Hemophilia X H X h or Color Blindness X B X b are carriers of the allele.
      • They are unaffected by the condition.
      • They do pass on the allele which may result in a homozygous female or a male with the sex linked recessive allele.
    • Example of a non sex linked cross : Cystic fibrosis (CF)
      • Background Calculation
      • Homozygous •Both are carriers
      • recessive disease •Use C for gene found on chromosome and F/f for the
      • 7 gene
      Answer Represent C F not carrying CF C f as carrying CF Your couple is heterozygous for CF
    • Example of sex linked traits :
      • Color Blindness
      • A sex linked gene
      • Carrier female x Normal Male
      • Hemophilia (recessive carried on X)
              • Female carrier x Normal male
      Answer Answer X B X b X B y X H X h X H y
    • III. Co dominant Blood cells Blood smear (normal) http://137.222.110.150/calnet/cellbio/cellbio.htm Sickle cell anemia
    • Codominant alleles have three phenotypes
      • Sickle cell anaemia is a genetic disease the haemoglobin of the red blood cells
      • Haemoglobin is normally a ball-shaped molecule
      • The sickle cell allele makes it form a long strands
      • The red blood cell carrying these molecules distorts into characteristic long shape
    • The genetics of sickle cell anaemia
      • The shape of the haemoglobin molecule is controlled by two alleles
      • Normal Haemoglobin allele
      • Sickle Cell Haemoglobin allele
      • There are three phenotypes
      • Normal
        • Normal individuals have two normal haemoglobin alleles
      • Sickle cell anaemia , a severe form where all the red blood cells are affected.
        • Sickle cell anaemia patients have two sickle cell alleles in their genotype
      • Sickle cell trait , a mild condition where 50% of the red blood cells are affected.
        • Sickle cell trait individuals are heterozygotes, having one of each allele
    • Symbols for codominant alleles
      • Both alleles are expressed in the heterozygote both take a CAPITAL CASE letter
      • An index letter identifies the allele
      • Therefore:
      • Normal haemoglobin allele is Hb N
      • Sickle cell allele is Hb S
    • Codominant genotypes Sickle cell anaemia Hb S Hb S Sickle cell trait Hb N Hb S Normal haemoglobin Hb N Hb N Phenotypes Genotypes
    • Unusual proportions
      • In codominance, heterozygotes have their own phenotype
      • This gives rise to different proportions amongst to offspring of some genetic crosses
      25% 50% 25% Proportions Sickle cell anaemia Sickle cell trait Normal Offspring Hb S Hb S Hb N Hb S Hb S Hb N Hb S Hb N Hb N Hb N Hb S Hb N Hb S Hb N Hb S Hb N Gametes Hb N Hb S Hb N Hb S Genotypes Sickle cell trait x Sickle cell trait Phenotypes
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    • Blood Groups ABO
      • An example of codominance as well as multiple alleles is blood groups with 3 alleles
    • Multiple Alleles: ABO Blood Groups Blood type O: Universal donor. Blood type AB: Universal acceptor
    •  
    • IV. Incomplete dominance
      • produces an intermediate phenotype between dominance and recessiveness
      • Segregation and independent assortment still apply
    • Pedigree Chart
      • Another way to visualize a monohybrid crosses or determining a genotype is by using a pedigree chart
    • Pedigree Chart
      • Another way to visualize a monohybrid crosses or determining a genotype is by using a pedigree chart
      • Knowing the phenotype of individuals in a family will sometimes allow genotypes to be determined.
      • In genetic counseling this enables probabilities to be determined for the inheritance of characteristics in children.
    • Pedigree Chart
      • White circle : Normal female
      • White Square: Normal male
      • Black Circle: affected female
      • Black square: affected male
      • (1) and (2)..Normal Parents
      • (3) affected female
      • (4),(5) and (6) normal
    • Pedigree Chart The chart shows the inheritance of the hemophilia gene through the Royal families of Europe
    •  
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    • The following examples of pedigree relate to two human diseases:
      • 1.Phenylkentonuria(PKU)
      • is a genetic disease in which an
      • individual is unable to
      • Produce the enzyme PKU. This
      • enzyme changes the R group of
      • the amino acid Phenylalanine to
      • Tyrosine.
      • Answer the following
      • questions on this pedigree. 
    • 1. Phenylketonuria (Pku)
      • Using the allele key provided state the genotype of parents 1 and 2?
      • Give the genotype and phenotype of individual 5 ?
      • Is it possible that the condition is sex linked ?
      • What is the genotype and phenotype of individuals 7 and 8?
      • Which two individuals have the incorrect pedigree
      Answer
    • 2. Muscular Dystrophy
      • What type of genetic disease is muscular dystrophy?
      • Give the genotype and phenotype of 1?
      • Give the genotype and phenotype of 2?
      • Give the genotype and phenotype of 8 ?
      • Give the genotype and phenotype of 5 and 6 ?
      Answer
    • Dihybrid Crosses
    • Law of independent assortment Each pair of alleles segregates into gametes independently (occurs during metaphase I) Pg 245
    • Mendel's Second Law Law of Independent Assortment
      • This law states that allele pairs separate independently during the formation of gametes.
      • Therefore, traits are transmitted to offspring independently of one another
    • Mendel's dihybrid cross with Peas on different chromosomes
      • Phenotypes: Smooth Yellow Seeds X Rough Green Seeds
      • The chromosomes are shown as homologous pairs.
      • Smooth is dominant to Rough
      • Yellow is dominant to green
      • Meiosis:
      • Reduces the chromosome number and randomly assorts the alleles for each gene
      • As the parents are homozygous they each produce only one type of gamete.
      • Fertilization:
      • Random fertilization of the gametes.
      • Offspring:
      • The offspring are heterozygous at each gene loci. Notice the chromosome number is restored to the that of the parents.
      • New homologous pairs are produced
      • The offspring are crossed = F1 x F1 (F1 self)
    • The grid shows all offspring genotype combinations from random fertilization
      • Phenotype Key:    
      • Phenotypic ratio:
        • 9 Smooth Yellow:
        • 3 Smooth green:
        • 3 Rough Yellow:
        • 1 Rough Green
                                            
    • Dyhybrid crosses
      • Think about 2 traits, controlled by 2 genes on 2 different chromosomes
      • What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous for both genes?
      • First determine the type of gametes the plant will create
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    • Mendel's Law of Independent Assortment
      • Meiosis creates genetic variation in the gametes of
      • an individual. Combinations of genes occur in the
      • gametes that do not occur in the parental organism
      • This is do to crossing over that occurs during...
      • Prophase I:
      • New genetic variation are created on linked genes (The matching chromosomes or pairs of homologous
      • chromosomes)
      • Anaphase II:
      • Genetic variation of unlinked genes (different chromosomes) by random assortment of the homologous pairs.
    • Dihybrid Ratio :
      • This ratio is called the F2 Dihybrid Ratio and results from a cross between two heterozygous dihybrids
      • The ratio is a prediction of the offspring ratio.
      • Actual numbers may deviate from this ratio as each fertilization is a random process
      • The ratio only sets the probability of a particular offspring phenotype arising.
    • Recombination
      • Recombination occurs with the re assortment of genes
      • or characters into different combinations from those of
      • the parents.
      • Causes of Recombination
      • 1. Unlinked Genes ( Prophase I)
      • Unlinked genes are not on the same chromosomes.
      • Recombination occurs by chromosome assortment.
    • Recombination
      • 2. Linked Genes (Metaphase II)
      • Linked genes are on the same chromosome.
      • They tend to be inherited together.
      • New combinations are caused by crossover
    • Recombinants from Cross over
      • The diagram below shows the loci of genes A and B.
      • The genes are on the same chromosome (A and B are a linkage group)
    • Recombinants from Cross over continued
      • The cross over involves the exchange of lengths of DNA.
      • In doing so the chromosomes of a homologous pair exchange their alleles.
      • When the chromatids separate at anaphase II this will produce new combinations of alleles in the gametes (recombinants).
      • The recombinants in this example are aB and Ab
    • Linkage group:
      • genes on the same chromosome
      • genes inherited together
      • genes that do not show the expected Mendelian ratios as predicted by the Laws Independent Assortment.
      The genes A and B are a linkage group. If this genotype (AaBb) was crossed with itself would produce a 3:1 ratio not a 9:3:3:1
    • Dihybrid Cross Recombinant
      • Recombination occurs with the reassortment of genes or characters into different combinations from those of the parents. In the example below color blindness is carried only on the X chromosome Xb
      Allele Key: XB= Normal vision Xb= Colorblind Y = Male chromosome T= Tongue roller t = non-tongue roller Allele key: C= Color produced c = no color produced   A = Banding (agouti/back yellow tip) a =Non banding (black) Answer Answer XbXBTt X XByTt CcAa X CcAa
    • Linked Genes
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    • Autosomal Gene Linkage vs Sex Linkage
    • Epistasis
      • Interaction between two nonallelic genes in which one modifies the expression of the other
        • If the expression of second gene is depending on the first gene, the first gene is said to be epistatic to the 2nd gene.
        • Notes: A dihybrid cross involving epistasis will not yield the typical 9:3:3:1 ratio
    • Rodent fur pigment genes C = pigment deposition is epistatic to: B = melanin production Black = dominant B brown = recessive b Albino = no pigment
    • Topic 10.3 Polygenic Inheritance
    • Polygenic Inheritance (multi alleles)
      • Definition: 'A single characteristic that is controlled by two or more genes‘
      • Each allele of a polygenic character often contributes only a small amount to the over all phenotype. This makes studying the individual alleles difficult.
      • In addition environmental effects smooth out the genotypic variation to give continuous distribution curves.
    • Polygenic inheritance
      • Quantitative character which varies on a continuum within a population
        • Caused by additive effects of two or more genes expressed in a single phenotype
          • e.g. human skin coloration
            • AABBCC = very dark skin
            • aabbcc = very light skin
            • AaBbCc = intermediate-colored skin
              • AABbcc also expresses as intermediate since alleles are additive
    • “ Doses” of pigmentation Polygenic traits usually produce a “bell-shaped” distribution curve
    • Example 2: Human skin color
      • This is controlled by as many as 6 genes each with its own alleles.
      • As the number of genes increases the amount of phenotypic variation increases.
      • The alleles control the production of melanin which is a pigment that colors skin.
      • In this example the calculation is performed with 3 genes each with 3 alleles. The cross is between two individuals heterozygous at both alleles
      Allele Key A= add melanin a= no melanin added B= adds melanin b= no melanin added
    • Example 3: Finch Beak Depth
      • Finches are seed eating birds that use their beaks to break open seeds. The depth of beak is under polygenic control of three genes with two alleles each.
      • Produce the Punnett square for the following cross.
      • Compare the data frequency against the graphic representations to the right.
    • Example 4
      • Cross: between two bird which are heterozygous at
      • All three loci
            • Allele key :
            • A= add depth
            • a= no depth added
            • B= add depth
            • b= no depth added
            • C= add depth
            • c= no depth added
      Answer
    • Example: Wheat Color Wheat polygenics was first worked out in 1909 by Nilsson- Ehile
      • Each of the alleles (for the three genes) of the wheat has a small effect on the phenotypic variation for color. The additive effect produces the continuous variation that is found in Wheat red coloration
    • Example: Wheat Color
      • Cross :
      • Two plants heterozygous at each 3 gene loci
      • calculate the phenotypic ratio and draw a graph of the phenotypic variation
      • Allele Key :
      • 3 genes with 2 alleles each. A,B and C
      • Each 'dominant' allele produces one unit of color.
      • The homozygous dominant at all three loci produces dark red wheat.
      • The homozygous recessive at all three loci produces pale white color.
      Answer
    • Example 5 : Wild Columbine
      • Another example of how the height of a plant is controlled by a polygenic system. Here there are two genes each with two alleles. Again cross two heterozygote's at each loci.  
          • Allele key :
          • M1= Tall plant
          • M2= Small plant
          • N1=Tall plant
          • N2=Small plant
      Answer
    • Example 6: Chicken Combs
      • Controlled by two genes this produces four phenotypes. Using the allele key:
      • gene 1: P and p
      • gene 2: R and r
      • Exercise :
      • Produce a completed
      • allele key and a punnett
      • grid with genotypic and
      • phenotypic ratios that
      • explain the phenotypes
      • to the right
    • Practice
      • Dihybrid Calculations   
      •   Example 1 : The Capercaille is a ground dwelling bird of the pine forest of Scotland (simple dominance/ recessive). The length and color of the primary feather is controlled by two unlinked genes. Using the allele key to the right calculate the phenotypic ratio of an F2 beginning with a cross between a hen bird that is homozygous recessive for both genes and a cock bird that is homozygous dominant for both genes.
      Allele Key: Q- Long feather q-short feather A- dark feather color a-light feather color
    • Example2: Sweet peas (codominance/ dominance)
      • Sweet pea flowers color is controlled by a codominant pair of alleles. The flowers are white, pink and red. The height of the mature plant is controlled by a gene showing dominance. Calculate the F2 phenotypic ratio for a cross between two heterozygous plants at both gene loci.
      Allele key: A1= white, A2= red B= Tall, b= short Answer
    • Example 3: Dihybrid Test Cross
      • A suspect heterozygote Guinea pig for coat color and length is crossed with a double homozygote recessive. As with a monohybrid test cross there is a predictable dihybrid test cross ratio which is 1:1:1:1
      Allele key: L = long hair coat l = short hair coat B = Black coat b =brown coat Answer
    • Answer to capercaille * Practice
    • Sweat Pea Example Back
    • Guinea Pig Back
    • Example 4
    • Sweet Pea Example
    • chi Back
    • Example Back
    • Example: Wheat Color Back
    • Example: Wild Columbine Back
    • Cystic fibrosis Back to slide
    • Color Blindness Back to slide
    • Hemophilia
    • PKU answer Back
    • Muscular Dystrophy (MS) Back
    • Tongue Roller Back
    • Mice Back
    • Sickle Cell Cross