Genetics Ch.3

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Charlie581
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Genetics Ch.3
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2015-09-12 18:51:35
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Genetics Ch.3
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  1. What is the difference between genotype and phenotype?
    The genotype is the set of genes in our DNA which is responsible for a particular trait. The phenotype is the physical expression, or characteristics, of that trait.
  2. What is the principle of segregation? Why is it important?
    The principle of segregation states that an organism possesses two alleles for any particular characteristic. These alleles separate in the formation of gametes. In other words, one allele goes into
each gamete. The principle of segregation is important because it explains how the genotypic ratios in the haploid gametes are produced.
  3. What is the concept of dominance?
    The concept of dominance states that, when two different alleles are present in a genotype, only the trait encoded by one of them (the dominant one) is observed in the phenotype.
  4. What are the addition and multiplication rules of probability and when should they be used?
    The Multiplication Rule states that the probability of two or more independent events occurring together is calculated by multiplying their independent probabilities, and should be used in such a case where one is trying to find the probability of a progeny receiving a TT allele (one T from one parent being Tt, and one T from the other parent being Tt; thus the probability would be ½ x ½ = ¼).

    The Addition Rule states that the probability of any one of two or more mutually exclusive events is calculated by adding their probabilities, and should be used when finding overall phenotypic ratios (IE: for a TtxTt cross, the probability of expressing the T phenotype would be ¼ + ¼ + ¼ = ¾ ).
  5. What is the principle of independent assortment? How is it related to the principle of segregation?
    The principle of independent assortment states that alleles at different loci segregate independently of one another. The principle of independent assortment is an extension of the principle of segregation: the Principle of Segregation states that the two alleles at a locus separate; according to the Principle of Independent Assortment, when these two alleles separate, their separation is independent of the separation of alleles at other loci.
  6. In which phases of mitosis and meiosis are the principles of segregation and independent assortment at work?
    The Principal of Segregation takes place bother before Meiosis, and during Anaphase I. The Principal of Independent Assortment takes place during Anaphase I.
  7. In cucumbers, orange fruit color (R) is dominant over cream fruit color (r). A cucumber plant homozygous for orange fruits is crossed with a plant homozygous for cream fruits. The F1 are intercrossed to produce the F2.

    A) Give the genotypes and phenotypes of the parents, the F1, and the F2.

    B) Give the genotypes and phenotypes of the offspring of a backcross between the F1 and the orange parent. 


    C) Give the genotypes and phenotypes of a backcross between the F1 and the cream parent.
    a. The parents are RR (orange fruit) and rr (cream fruit). All the F1 are Rr (orange). The F2 are 1 RR (orange), 2 Rr (orange), and 1 rr (cream) giving orange-to-cream phenotypic ratio of 3:1

    b. Half of the progeny are homozygous for orange fruit (RR) and half are heterozygous for orange fruit (Rr).

    c. Half of the progeny are heterozygous for orange fruit (Rr) and half are homozygous for cream fruit (rr).
  8. Figure 1.1 (pg. 2) shows three girls, one of whom has albinism. Could the three girls shown in the photograph be sisters? Why or why not?
  9. White (w) coat color in guinea pigs is recessive to black (W). In 1909, W. E. Castle and J. C. Phillips transplanted an ovary from a black guinea pig into a white female whose ovaries had been removed. They then mated this white female with a white male. All the offspring from the mating were black in color…

    A) Explain results of this cross.


    B) Give the genotype of the offspring of this cross.

    C) What, if anything, does this experiment indicate about the validity of the pangenesis and the germ-plasm theories discussed in Chapter 1?
    A) Although the white female guinea pig gave birth to the offspring, her eggs were produced by the ovary from the black female guinea pig. The transplanted ovary produced only eggs containing the allele for black coat color. Like most mammals, guinea pig females produce primary oocytes early in development, and thus the transplanted ovary already contained primary oocytes produced by the black female guinea pig. 

    B) The white male guinea pig contributed a w allele, and the white female guinea pig contributed the W allele from the transplanted ovary. The offspring are thus Ww.

    C) The transplant experiment supports the germ-plasm theory. According to the Germ-Plasm Theory, only the genetic information in the germ-line tissue in the reproductive organs is passed to the offspring. The production of black guinea pig offspring suggests that the allele for black coat color was passed to the offspring from the transplanted ovary in agreement with the germ-plasm theory. According to the pangenesis theory, the genetic information passed to the offspring originates at various parts of the body and travels to the reproductive organs for transfer to the gametes. If pangenesis were correct, then the guinea pig offspring should have been white. The white-coat alleles would have traveled to 
the transplanted ovary and then into the white female’s gametes. The absence of any white offspring indicates that the pangenesis hypothesis is invalid.
  10. In cats, blood-type A results from an allele (IA) that is dominant over an allele (iB) that produces blood-type B. There is no O blood type. The blood types of male and female cats that were mated and the blood types of their kittens follow. Give the most likely genotypes for the parents of each litter.
  11. Figure 3.7 shows the results of a cross between a tall pea plant and a short pea plant. 


    A) What phenotypes and proportions will be produced if a tall F1 progeny is backcrossed to the short parent?

    B) What phenotypes and proportions will be produced if a tall F1 progeny is backcrossed to the tall parent?
  12. Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he obtains 1/2 white kittens and 1/2 black kittens. When the black kittens are interbred, all the kittens that they produce are black. On the basis of these results, would you conclude that white or black coat color in cats is a recessive trait? Explain your reasoning.
  13. Alkaptonuria is a metabolic disorder in which affected persons produce black urine. Alkaptonuria results from an allele (a) that is recessive to the allele for normal metabolism (A). Sally has normal metabolism, but her brother has alkaptonuria. Sally’s father has alkaptonuria, and her mother has normal metabolism.

    A) Give the genotypes of Sally, her mother, her father, and her brother.

    B) If Sally’s parents have another child, what is the probability that this child will have alkaptonuria?

    C) If Sally marries a man with alkaptonuria, what is the probability that their first child will have alkaptonuria?
  14. Hairlessness in American rat terriers is recessive to the presence of hair. Suppose that you have a rat terrier with hair. How can you determine whether this dog is homozygous or heterozygous for the hairy trait?
  15. What is the probability of rolling one six-sided die and obtaining the following numbers?

    2
    1 or 2
    An even number
    Any number but a 6
  16. What is the probability of rolling two six-sided dice and obtaining the following numbers?

    2 and 3
    6 and 6
    At least one 6
    Two of the same number (two 1s, or two 2s, or two 3s, etc.)
    An even number on both dice
    An even number on at least one die
  17. In guinea pigs, the allele for black fur (B) is dominant over the allele for brown (b) fur. A black guinea pig is crossed with a brown guinea pig, producing five F1 black guinea pigs and six F1 brown guinea pigs.

    A) How many copies of the black allele (B) will be present in each cell of an F1 black guinea pig at the following stages: G1, G2, metaphase of mitosis, metaphase I of meiosis, metaphase II of meiosis, and after the second cytokinesis following meiosis? Assume that no crossing over takes place.

    B) How many copies of the brown allele(b) will be present in each cell of an F1 brown guinea pig at the same stages as those listed in part a? Assume that no crossing over takes place.
  18. In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two characteristics assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots. The F1 are intercrossed to produce the F2.

    A) What are the phenotypic ratios in the F2?

    B) If an F1 plant is backcrossed with the bitter, yellow-spotted parent, what phenotypes and proportions are expected in the offspring?

    C) If an F1 plant is backcrossed with the sweet, nonspotted parent, what phenotypes and proportions are expected in the offspring?
  19. In cats, curled ears result from an allele (Cu) that is dominant over an allele (cu) for normal ears. Black color results from an independently assorting allele (G) that is dominant over an allele for gray (g). A gray cat homozygous for curled ears is mated with a homozygous black cat with normal ears. All the F1 cats are black and have curled ears.

    A) If two of the F1 cats mate, what phenotypes and proportions are expected in the F2?

    B) An F1 cat mates with a stray cat that is gray and possesses normal ears. What phenotypes and proportions of progeny are expected from this cross?

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