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					Notes – Modern genetics / Mendel’s laws                                                       Biology


` Objectives - SWBAT
 Use the terms of genetics to describe Mendel’s experiments and results
 Relate the laws of segregation and independent assortment to the behavior of chromosomes
   during meiosis
 Use a Punnett square to predict the results of a monohybrid cross


Modern genetics & Mendel’s laws
Ch 8.3
1 day


    Law of segregation
    Law of independent assortment
    How laws are implemented in meiosis (Meiosis Anaphase I – independent assortment)


Mendel’s Laws
    Basic rules of inheritance now referred to as Mendel’s Laws – to honor Mendel
    Mendel proposed the statements to explain the results of his experiments
    Mendel’s ides about factors were very similar


    At the time of Mendel, genes and chromosomes had not been discovered. Mendel did not know
    about meiosis.


Review:    Remember …
    Diploid organisms have pair chromosomes – one pair from egg, one pair from sperm
    Chromosomes of a pair are made up of matching sequences of genes
    Diploid organisms have two copies of a gene for any given trait – because diploid organisms have
    two chromosomes


Allele
The two copies of each gene are called alleles (uh-LEELZ).
    Allele – different versions of a gene for the same trait
                 T = tall    t = short / dwarf       TT = pure-bred tall   Tt = hybrid tall
Diploid organisms have alleles for each trait




                                                  -1-                                   MD 051202
Notes – Modern genetics / Mendel’s laws                                                     Biology


Rule of Segregation
From a mono-hybrid cross, a gamete receives only one allele, determined at random, from the pair of
alleles possessed by an organism, fertilization reestablishes the double number


    Mendel hypothesized that factors separate when gametes form.
    Half of an organism’s gametes have one allele from each pair, half of the gametes have the other
    allele
    Segregation of gene pairs is explained by the behavior of chromosomes during meiosis II
    anaphase
    When homologous pairs separate, the genes encoded on the chromosomes separate also


Terms:
Genotype – the genes an organism possesses
               actual genetic make-up of an organism
Phenotype – the observable attribute of an organism
               form of the trait that is observed
    genotype of the parental tall plant = TT
    genotype of the F1 tall plant = Tt
    phenotype of the genotypes TT or Tt = tall


Question: or examples:       What is the …
                                   … genotype of a true-bred parental short plant … tt
                                   … genotype of the F1 hybrids plant … Tt
                                   … phenotype of the genotypes TT … tall
                                   … phenotype of the genotype tt … short


Two classes of genotypes:
homozygous – when both alleles in the genotype are the same
heterozygous – when the two alleles in the genotype are different


What is the …
                  … genotype of a homozygous tall plant? … TT
                  … genotype of a heterozygous tall plant? … Tt
                  … genotype of a homozygous short plant ? … tt
                  … can a tallhybrid plant be homozygous? … no, their genotype is Tt




                                                    -2-                                  MD 051202
Notes – Modern genetics / Mendel’s laws                                                        Biology


Genotypic ratio
Phenotypic ratio

Punnett Square

   Punnett Square - grid for organizing genetic information
   Created by British geneticist Reginald Punnett
   Makes it easier to predict the results of a genetic cross
   Shows probabilities, not actual results!!
   How to use one ….
            1. Determine the alleles in the gametes of the parents
            2. Place the alleles of the gametes one parent along the top of the grid and those of the
                   other parent along the left side
            3. Combine the alleles inside the boxes of the Punnett square
            4. Determine genotypes and phenotypes of the offspring inside the boxes
                            Example – monohybrid cross (one-factor cross) for plant height
                                  F2 generation:                   T                 t
                                             T                    TT                 Tt
                                             t                    Tt                 tt
                   3 tall , 1 short     phenotypic 3:1
                   1 TT, 2 Tt, 1 tt     genotypic 1:2:1


Probabilistic representation
                   Pollen             Ovules                Genotypes                Phenotypes
                   ½T                 ½T         =     ¼ TT             1 TT         3 tall
                                      ½t         =     ¼ Tt             2Tt
                   ½t                 ½ T`       =     ¼ Tt
                                      ½t         =     ¼ tt             1 tt         1 short
Show/ give problems for mono-hybrid cross – Punnett squares and probabilistic


Testing Rule of Segregation

   Test cross – cross organism with a recessive homozygote
   Allows you to determine the unknown genotype of the dominant phenotype
        PP x pp  all Pp
        Pp x pp  Pp : pp
                      1 : 1
   look at results and can determine what dominant phenotype was
                                        if all Pp, then it was PP / homozygous donimant
                                        if 1:1 Pp to pp, then it was Pp hybrid / heterozygote



                                                      -3-                                 MD 051202
Notes – Modern genetics / Mendel’s laws                                                        Biology


Law of Dominance
When two copies of a pair of genes are different in a hybrid, one allele can control the trait and the
other can be hidden.
    Allele that controls the trait is the dominant allele
    Dominant allele “expresses itself”
    Dominance is the relationship of alleles and the traits they control


    Allele that is hidden is the recessive allele
    An allele can be expressed only when the organism has no copy of the corresponding dominant
    allele for the gene in question – example: TT or only one T




Law of Independent Assortment
Alleles for one gene segregate independently of alleles for another gene
    - if the genes are on different chromosomes
    - the genes are very far apart on the same chromosome (as if to appear they are on different
chromosomes)


    Gene pairs segregate into gametes randomly and independently of each other.


Mendel studied the inheritance of two traits at the same time:
    Crossed plants that had yellow round, seeds (dominant) with green, wrinkled (both recessive)
    seeds
    F1 – all yellow and round (dominant)
    F2 – all possible combinations of seed and color occurred in the same frequency
    F2 – association of traits in parent did not seem to matter
    Concluded that traits (factors) are inherited independently


    This results from the behavior of chromosomes during meiosis and from crossing over
    Separation of chromosomes pairs occurs randomly and produces many different combinations of
    chromosomes in the gametes.
    Variability almost limitless – humans – 23 pairs of chromosomes
                                                n     23
                                               2 =2        = 8,388,608 w/o considering recombination
    Because genes are on the chromosomes, the random separation of the chromosomes pairs also
    randomly separates the gene pairs.




                                                     -4-                                   MD 051202

				
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