Bio3400 Chapter 3 Mendelian Genetics
  1. Gregor Mendel chose the garden pea as his model system because it grows easily to maturity in one season, and can be            artificially.

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  2. Mendel examined seven visible characteristics (traits), each with two contrasting             , using true-breeding parental strains.

      A summary of the seven monohybrid crosses (matings) of the garden pea. Pollen from P[1] plants exhibiting one trait was used to fertilize the ova of plants exhibiting the other trait. In the F[1] generation, one of the two traits (dominant) was exhibited by all plants. The contrasting trait (recessive) then appeared in about 1/4 of the F[2] plants.
     
     
     
     
  3. These crosses are called             since they involve a single pair of contrasting traits. These crosses are also             since the sex of the parents did not affect the outcomes.
     
     
     
     
  4. The original parents are the     generation, and their offspring are the     generation. Offspring arising from          (self-fertilizing) the F1 generation are the     generation.

      A monohybrid cross between tall and dwarf pea plants. A heritable characteristic, such as the height of the plant, is called a phenotype. The symbols D and d designate the tall and dwarf unit factors (genes), respectively, in the genotypes of mature plants (sporophytes) and gametes. Sporophytes are shown in rectangles, and gametes are shown in circles. In the F[1] generation, all of the plants show just one of the two contrasting traits. continue


      generation, 3/4 of the plants exhibit the same (dominant) trait as the F[1] generation, and 1/4 exhibit the contrasting trait that disappeared in the F[1] generation. To explain these results, Mendel proposed the existence of particulate unit factors (genes) for each characteristic (phenotype). The two contrasting traits of the same gene are called alleles. An individual possessing two copies of the same allele is homozygous. An individual possessing different copies of an allele is heterozygous. The F[2] genotype ratio is 1:2:1; while the phenotype ratio is 3:1.
     
     
     
     
  5. To explain the results of his monohybrid crosses, Mendel proposed three postulates that form the basis for Mendelian or               genetics.
       
       
       
       
    1. Traits are inherited in       factors (genes) which exist in pairs.
       
       
       
       
    2. One form of each unit factor is           for the trait and the other is            .
       
       
       
       
    3. The paired unit factors            (separate) independently during gamete formation.
     
     
     
     
  6. The           is the genetic makeup of an individual. The            is the physical expression of the genotype.
     
     
     
     
  7. Alternative forms of a       are called          . If the two alleles for a trait in an individual are the same, the individual is             , otherwise the individual is               .

      An individual may possess a maximum of two alleles for each gene, one allele is inherited from each parent. If the two allele differ, the organism is heterozygous, and the dominant allele determines the its phenotype. Some traits, such as flower color in the pea, may have more than two alleles in the population.
     
     
     
     
  8. A          square is a tool to analyze and predict the ratios of            and             of a monohybrid cross.

      A Punnett square can be used to predict the genotype and phenotype ratios of a monohybrid cross The key step is to line up the gametes along the rows and columns of the square.


      A monohybrid cross between tall and dwarf pea plants. A heritable characteristic, such as the height of the plant, is called a phenotype. The symbols D and d designate the tall and dwarf unit factors (genes), respectively, in the genotypes of mature plants (sporophytes) and gametes. Sporophytes are shown in rectangles, and gametes are shown in circles. In the F[1] generation, all of the plants show just one of the two contrasting traits. continue


      generation, 3/4 of the plants exhibit the same (dominant) trait as the F[1] generation, and 1/4 exhibit the contrasting trait that disappeared in the F[1] generation. To explain these results, Mendel proposed the existence of particulate unit factors (genes) for each characteristic (phenotype). The two contrasting traits of the same gene are called alleles. An individual possessing two copies of the same allele is homozygous. An individual possessing different copies of an allele is heterozygous. The F[2] genotype ratio is 1:2:1; while the phenotype ratio is 3:1.
     
     
     
     
  9. A            with a homozygous            individual can be used to determine whether an organism displaying the           phenotype is homozygous or heterozygous for that trait.

      Testcross of a dominant (tall) phenotype. (a) If tall parent is homozygous. (b) If tall parent is heterozygous. The genotype of a tall parent can be determined by examining the offspring when each is crossed to the homozygous recessive dwarf plant.
     
     
     
     
  10. A           cross involves two pairs of contrasting traits. Mendel proposed a         postulate from his dihybrid crosses:


        F[1] and F[2] results of Mendel's dihybrid crosses between yellow, round and green, wrinkled pea seeds and between yellow, wrinkled and green, round pea seeds.


        Independent assortment of two traits results in an equal frequency of the formation of all the possible combinations of gametes, with a predicted 9:3:3:1 dihybrid ratio in the F[2] generation. 2


        A 9:3:3:1 dihybrid ratio in the F[2] generation is characteristic of the independent assortment of two traits. One method of obtaining all the possible combinations of gamete genotypes is using the FOIL procedure similar to the multiplication of two binomial expressions.


        Using FOIL to multiply two binomial expressions.
       
       
       
       
    1. Traits         independently during gamete formation.


      Assuming 2 pairs of traits are inherited independently (independent assortment), the product law predicts that the combined probability of the two phenotypes is equal to the product of their individual probabilities.
     
     
     
     
  11. Mendel's laws are based on probability, and simple mathematical        and laws of                 and           can be used to find various combinations in crosses.

      In a AaBBCcDd x AaBBCcDd cross, the heterozygous gene pairs (n) is 3 (note: B is not heterozygous). Assuming independent assortment, each parent can produce 8 (2^3) different gametes and yield 27 (3^3) different genotypes and 8 (2^3) different phenotypes in the F[2].
     
     
     
     
    • The probability of allele frequencies in the         is the          of the probabilities of allele frequencies in the          .

        Random fertilization.

        When a heterozygote (Rr) forms gametes, segregation of alleles is like the toss of a coin.

        We can determine the probability for any genotype among the offspring of 2 heterozygotes by multiplying the individual probabilities of a gamete having a particular allele (R or r).



        Assuming 2 pairs of traits are inherited independently (independent assortment), the product law predicts that the combined probability of the two phenotypes is equal to the product of their individual probabilities.
       
       
       
       
    • The frequency probabilities of multiple allele combinations is the      of the frequency probabilities of the individual alleles.
     
     
     
     
  12. A           shows a family tree with respect to a given trait, and is a valuable tool in human genetic studies.

      Standard pedigree conventions Circles represent females and squares designate males. A diamond indicates unknown sex. Individuals exhibiting the phenotype ("affected") have shaded symbols. If the parents are related (consanguineous, such as first cousins), they are connected by a double line. Offspring are called sibs (siblings) and are connected by a horizontal sibship line. A number in a symbol represents numerous sibs of the same phenotypes. A proband (p) is the first known affected individual in the pedigree, indicated by an arrow.


      Autosomal Recessive Trait. Since neither II-3 nor II-4 is affected, this must be a recessive trait that has skipped a generation; and II-3 and II-4 must both be heterozygous. Approximately 1/4 of their offspring should be affected, and II-3 and II-4 individuals can be represented by a shaded dot within their symbols.


      Autosomal Dominant Trait. All affected individuals have a parent that also expresses the trait. A dominant trait should not skip a generation.


      Pedigrees have been very useful in determining how human genetic diseases such as hemophilia are inherited.


      Hemophilia exhibits X-linked inheritance, where affected fathers pass the trait to all of their daughters, who are usually heterozygous carriers.
     
     
     
     
  13. Mendel's principles of independent              and independent             can be explained by the modern              theory of heredity and events that occur during          .

      Mendel's pairs of unit factors are really genes located on homologous pairs of chromosomes.


      Members of each pair of homologs separate in meiosis I, resulting in independent segregation of the 2 alleles, which end up in different gametes.


      In anaphase I, one half of each tetrad (one pair of sister chromatids), now called a dyad is pulled, attached to one kinetochore, toward one pole of the dividing cell (disjunction). Each dyad is potentially a mosaic of maternal and paternal genetic material due to crossing over. telephase I


      Genes on nonhomologous chromosomes align randomly during metaphase I, resulting in independent assortment of alleles on different chromosomes.


      In metaphase I, the paired tetrads line up on the metaphase plate, the sister chromatids of each chromosome held together by a single centromere, which does not divide. The terminal chiasmata reach the ends of the chromatids and separate. The alignment of maternal and paternal chromosomes with respect to the poles is random. anaphase I