Biology I Chapter 14

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Biology I Chapter 14
2011-10-06 18:38:02
Biology Campbell Mendel Gene Idea

Chapter 14 of Campbell's Biology Textbook 8th - Mendel and the Gene Idea
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  1. What is a character in the context of genetics?
    A heritable feature that varies among individuals, such as flow color.

    Note: The variant for a character, such as purple or white color for flowers is termed the trait but some geneticists use the term synonymously.
  2. What is a train in the context of genetics?
    Each variant for a character - such as purple or white color for flowers.

    Note: The variant for a character, such as purple or white color for flowers is termed the trait but some geneticists use the term synonymously.
  3. What advantages were there to the use of pea plants?
    There are many available variables such as flower color that may be chosen that varied between two distinct alternatives.

    Pea plants have short generation time and a large number of offspring from each mating.

    Mendel could strictly control mating between plants.
  4. What does it mean to be "true-breeding"?
    Referring to plants that produce offspring of the same variety when they self-polinate.
  5. What is the mating, or crossing of two true-breeding varieties?
  6. What are the true-breeding parents known as? What are they hybrid offspring? What are they offspring of these offspring known as when they are allowed to self-polinate?
    The P generation (parental generation)

    F1 generation (first filial generation, filial - son)

    F2 generation
  7. What were Mendel's two fundamental principles of heredity?
    The law of segregation and the law of independent assortment.

    The law of segregation - The two allels for a heritable character segregate (separate during gamete formation and end up in different gametes.
  8. What was the ratio that Mendel noticed during his experiment of the purple colored flowers and white colored flowers?

    The purple flowers appeared in the ratio of 3 to 1 white flowers. The purple flowers had a dominate allele whereas white was recessive.
  9. What are the four related concepts that make up Mendel's 3:1 inheritance pattern that he observed about the F2 offspring in his pea experiments?
    1) Alternative versions of genes account for variations in inherited characters.

    The gene for flow color existed in two versions (purple and white). These alternative versions of a gene are called alleles.

    2) For each character, an organism inherits two alleles, one from each parent.

    Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent. Thus, a genetic locus is actually represented twice in a diploid cell, once on each homolog of a specific pair of chromosomes. The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel's P generation. Or the allels may differ, as in the F1 hybrids.

    3) If the two alleles at a locur differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance.

    Mendel's F1 plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is recessive.

    4) The law of segregation: The two alleles for a hertiable character segregate (separate) during gamete formation and end up in different gametes.

    An egg or a sperm gets only one of the two allels that are present in the somatic cells of the organism making the gamete. If an organism has identical alleles for a particular character (true-breeding) then that allele is present in all gametes. If different alleles are present, as in F1 hybrids, 50% of the gametes receive the dominant allele and 50% receive the recessive allele.
  10. What is a somatic cell? What is a diploid cell?
    A somatic cell is any cell in a multicellular organism except a sperm or egg cell.

    A diploid cell is a cell ontaining two sets of chromosomes (2n), one set inherited from each parent.
  11. What is the punnett square used for?
    A diagrammatic device for preddicting the allele composition of offspring from a cross between individuals of known genetic makeup.

    A captial letter is used to symbolize a dominant allele and a lowercase letter for a recessive allele.
  12. In the hybridization of a purple true-breeding flower and a white true-breeding flower, what is the resultant color of the F1 generation? What of the F2 generation (F2 comes from the self pollenization of F1)?
    F1 - Union of parental gametes produces F1 hybrids having a Pp combination. Since purple is dominat, all of these flowers are purple.

    When the hybrid plants produce gametes, the two alleles segregate - half of the gametes receive the P allele and half receive the p allele.

    The resulting F2 offspring from self-pollenization produce a 3:1 ratio of purple to white flowers. 1/4 of the flowers will be PP, 2/4 will be Pp and 1/4 will be pp. Thus, 3/4 will be purple (PP and Pp) while 1/4 will be white (pp).
  13. What is the character known as for an organism that has a pair of indentical alleles for a character?
    Homozygous for the gene controlling that character.

    ie PP and pp for the dominant allele and recessive allele in the F2 generation of Mendel's experiment. These are true-breeding.
  14. What is the character known as for an organism that has a pair of different alleles for a character?
    Heterozygous for that gene controlling character.

    ie. Pp for the F2 generation of Mendel's experiment. This is in contrast to PP and pp which are homozygous.

    • These are not true-breeding because they produce gametes with different alleles.
  15. Why must be distinguish between an organism's appearance or observable traits and its genetic make-up? What are the two categories that we distinguish them into?
    The different effects of dominant and recessive alleles do not always reveal its genetic composition.

    We distinguish between an organism's appearance or observable traits are known as the phenotype and its genetic makeup is known as the genotype.

    In the case of flower color in pea plants, PP and Pp plants have the same phenotype (purplse) but different genotypes.

    Note that phenotype refers to physiological traits as well as traits that relate directly to appearance. For example, there is pea variety that lacks the normal ability to self-pollinate. This physiological variation (non-self-pollination) is a phenotypic trait.
  16. What is the "testcross" and what can it reveal?
    Breeding an organism of unknown genotype with a recessive homozygote is called a test cross because it can reveal the genotype of that organism.

    The corssing is done with a white-flowered plant (pp) which will only make gametes with the recessive allele and will display any dominate traits present in the unknown plant.
  17. What is a monohybrid?
    An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid genotype Aa.
  18. What is a dihybrid? What is the F1 generation of a dihybrid cross?
    An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb.

    The resulting F1 generation of two homozygous parents is thus YyRr.
  19. In a dihybrid cross, a cross between F1 dihybrids, what is the ratio that you would obtain from the hypthesis of dependent assortment? Hypothesis of independent assortment? Which is correct?
    In the hypothesis of dependent assortment, the ratio would be 3:1, similar to that to a monhybrid cross since it assumes that that the hybrids must transmit their alleles in the same combinations in which the alleles were inherited from the P generation, producing only two classes of games in the F1 hybrids: YR and yr.

    In the hypothesis of independent assortment, the phenotypic ratio is approximately 9:3:3:1 with four classes of gametes in equal quantities: YR, Yr, yR, and yr. The resulting Punnett square would be a 4 x 4 square with 16 equally probably ways in which the alleles can combine in the F2 generation.

    Based from the results, which lined up with the hypothesis of independent assortment, the law of independent assortment was established which states that each pair of alleles segregates independently of each other pair of alleles during gamete formation.
  20. (T/F) Knowning that the probabilities of the offspring genotypes are 1/4 for YY, 1/2 for Yy, and yy for a monhybrid cross, this may be applied to a dihybrid cross with simple multiplication such as the following:
    For the probability of an F2 plant having the YYRR genotype is 1/4 x 1/4 = 1/16.
    For the probability of an F2 plant having the YyRR genotype is 1/2 (Yy) x 1/4 (RR) = 1/8.
  21. In a cross between PpYyRr x Ppyyrr, what fraction of offspring from this cross would be predicted to exhibit the recessive phenotypes for at least two of the three characters?

    Hint: The genotypes that fullfill this condition: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr.

    In a cross involving heterozygous and homozygous allel pairs (Yy x yy), the probability of heterozygous offspring is 1/2 and the probability of homozygous offspring is 1/2.
    • ppyyRr (1/4 x 1/2 x 1/2) = 1/16
    • ppYyrr (1/4 x 1/2 x 1/2) = 1/16
    • Ppyyrr (1/2 x 1/2 x 1/2) = 2/16
    • PPyyrr (1/4 x 1/2 x 1/2) = 1/16
    • ppyyrr (1/4 x 1/2 x 1/2) = 1/16

    Total = 6/16 or 3/8
  22. For any gene with a dominant allele C and recessive allele c, what proportions of the offspring from a CC x Cc cross are expected to be be homozygous dominant, homozygous recessive, and heterozygous?
    • Homozygous dominant: 2/4
    • Homozygous recessive: 0
    • Heterozygous: 2/4
  23. An organism with the genotype BbDD is mated to one with genotype BBDd. Assuming independent assortment of these two genes, write the genotypes of all possible offspring of this cross and use the rules of probability to calculate the chance of each genotype occuring.
    • BBDD 1/4
    • BbDD 1/4
    • BBDd 1/4
    • BbDd 1/4
    • In a mate with heterozygous and homozygous allele pairs are 1/2 (ie. Bb x dd = 1/2)
    • In a mate with homozygous pairs, the allele pairs are also 1/2.
  24. (T/F) Characters are always determined by one gene, for which there are only two alleles. This is why Mendel's example is used so widely.

    There are many cases in which the relationship between the genotype and phenotype is rarely so simple. Mendel's experiment is used as the fundamentals in which the characters were determined by single genes but this may not always be the case.

    The basic prinicples of segregation and independent assortment apply even to more complex patterns of inheritance.
  25. What is the concept of complete dominance? Incomplete dominance? Codominance?
    Complete dominance: The situtation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

    In such situations, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

    Incomplete dominance: The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.

    ie. pink flowers in Mendel's experiment did not occur because there was no incomplete dominance; they all exhibited complete dominance.

    Codominance: The two alleles both affect the phenotype in separate, distinguishable ways.
  26. What is Tay-Sachs disease? Why does this display an important concept?
    An inherited disorder in humans where the brain cells of a child cannot metabolize certain lipids because a crucial enzyme does not work properly. As these lipids accumulate in brain cells, the child begins to suffer seizures, blindness, and degeneration of motor and mental performance and dies within a few years.

    Only children who inherit two copies of the Ray-Sachs allele (homozygotes) have the disease. Thus, at the organismal level, the Tay-Sachs allele qualifies as recessive.

    However, the activity level of the lipid-metabolizing enzyme is heterozygotes is intermediate between that in individuals with Tay-Sachs disease. The intermediate phenotype observed at the biochemical level is characteristic of incomplete dominance of either allele.

    Also, the heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. At the molecular level, the normal allele and the Tay-Sachs allele is codominant.

    This is important as it shows that it depends on the level that the phenotype is analyzed for classification.
  27. (T/F) The dominant allele for a particular character would always be more dominant allele for a particular character in a poplulation than the recessive allele for that character.

    1/400 in the US are born with an extra finger or toe, known as polydactyly. The low frequency of polydactyly indicates that the recessive allele, which results in five digits per appendage, is far more prevalent than the dominant allele in the polulation.
  28. (T/F) There are only two alleles that exist for any characteristic.

    An example is blood type: A, B, AB or O.
  29. Genes that have multiple phenotypic effects are known as

    a) phenotype
    b) polydactyly
    c) pleiotropy
    d) pleidotrapy
  30. What is epistasis?
    A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.
  31. Mendel studied characters that could be classified on an ___________, such as purple versus white flower color.

    a) either-or basis
    b) this-and basis
    c) do-what basis
  32. What is a quantitative character? What is a polygenic inheritance?
    Quantitative character: A heritable feature that varies continuously over a range rather than in an either-or fashion

    Polygenic inheritance: An additive effect of two or more genes on a single phenotypic character.
  33. What is the "norm of reaction" for a genotype? What are characters that are multifactorial?
    The norm of areaction is the range of phenotypes produced by a single genotype, due to environmental influences.

    Multifactorial refers to a phenotypic character that is influenced by multiple genes and environmental factors.
  34. Incomplete dominance and epistasis are both terms that define genetic relationships. What is the most basic distinction between these terms?
    Incomplete dominance describes the relationship between two alleles of a single gene, whereas epistasis relates to the genetic relationship between two genes (and the respective alleles of each)
  35. If a man with type AB blod marries a woman with type O blood, what blood types would you expect in their children?
    Half of the children would be expected to have type A blood and half type B blood.

    • AB = both A and B allele
    • O = neither A or B allele.
  36. What is a pedigree?