Genetics questions Chapt 3
What is the principle of independent assortment? How is it related to the principle of segregation?
According to the principle of independent assortment, genes for different characteristics that are at different loci segregate independently of one another. The principle of segregation indicates that the two alleles at a locus separate; the principle of independent assortment indicates that the separation of alleles at one locus is independent of the separation of alleles at other loci.
What is the difference between genotype and phenotype?
Genotype refers to the genes or the set of alleles found within an individual. Phenotype refers to the manifestation of a particular character or trait.
Suppose that you are raising Mongolian gerbils. You notice that some of your gerbils have white spots, whereas others have solid coats. What type of crosses could you carry out to determine whether white spots are due to a recessive or a dominant allele?
If white spots are recessive, then any gerbil with white spots must be homozygous for white spots (ww), and a cross between two white-spotted gerbils (ww × ww) should produce offspring with only white spots. If white spots are dominant to solid, then a cross between a gerbil with white spots and a gerbil with a solid coat should produce either progeny all having solid coats (WW × ww Ww) or progeny where one-half have solid coats and the other half have white spots (Ww × ww -> ½ Ww ½ ww).
In which phases of mitosis and meiosis are the principles of segregation and independent assortment at work?
In anaphase I of meiosis, each pair of homologous chromosomes segregate independently of all other pairs of homologous chromosomes. The assortment is dependent on how the homologs line up during metaphase I. This assortment of homologs explains how genes located on different pairs of chromosomes will separate independently of one another. Anaphase II results in the separation of sister chromatids and subsequent production of gametes carrying single alleles for each gene locus as predicted by Mendel's principle of segregation. Mendel's principles of independent assortment and segregation do not apply to mitosis, which produces cells genetically identical to each other and to the parent cell.
Why was Mendel's approach to the study of heredity so successful?
Mendel was successful for several reasons. He chose to work with a plant, Pisum sativum, that was easy to cultivate, grew relatively rapidly, and produced many offspring whose phenotype was easy to determine, which allowed Mendel to detect mathematical ratios of progeny phenotypes. The seven characteristics he chose to study were also important because they exhibited only a few distinct phenotypes and did not show a range of variation. Finally, by looking at each trait separately and counting the numbers of the different phenotypes, Mendel adopted a reductionist experimental approach and applied the scientific method. From his observations, he proposed hypotheses that he was then able to test empirically.
How are Mendel's principles different from the concept of blending inheritance discussed in Chapter 1?
Mendel's principles assert that the genetic factors or alleles are discrete units that remain separate in an individual organism with a trait encoded by the dominant allele being the only one observed if two different alleles are present. According to Mendel's principles, if an individual contains two different alleles, then the individual's gametes could contain either of these two alleles (but not both). Blending inheritance proposes that offspring are the result of blended genetic material from the parent and the genetic factors are not discrete units. Once blended, the combined genetic material could not be separated from each other in future generations.
The inheritance of red hair was discussed in the opening story of this chapter. At times in the past, red hair in humans was thought to be a recessive trait and at other times, it was thought to be a dominant trait. What characteristics is red hair expected to exhibit as a recessive trait? What characteristics would it exhibit if it were a dominant trait?
Recessive Trait - Red hair would often appear in the children of parents who lacked red, when both parents would were heterozygous. In a mating between two red-haired parents, all of the offspring would be expected to have red hair. Dominant Trait - Red hair would only appear in children if at least one of the parents had red hair because the child would have to inherit the red hair allele from a parent, who would therefore also have red hair. A mating between two red-haired parents might produce some children with non-red hair. A cross between two non-red parents would produce only non-red offspring.
What are the addition and multiplication rules of probability and when should they be used?
The addition and multiplication rules are two rules of probability used by geneticists to predict the ratios of offspring in genetic crosses. The multiplication rule allows for predicting the probability of two or more independent events occurring together. According to the multiplication rule, the probability of two independent events occurring together is the product of their probabilities of occurring independently. The addition rule allows for predicting the likelihood of a single event that can happen in two or more ways. It states that the probability of a single mutually exclusive event can be determined by adding the probabilities of the two or more different ways in which this single event could take place. The multiplication rule allows us to predict how alleles from each parent can combine to produce offspring, while the addition rule is useful in predicting phenotypic ratios once the probability of each type of progeny can be determined.
What is the concept of dominance? How does dominance differ from incomplete dominance?
The concept of dominance states that when two different alleles are present in a genotype, only the dominant allele is expressed in the phenotype. Incomplete dominance occurs when different alleles are expressed in a heterozygous individual, and the resulting phenotype is intermediate to the phenotypes of the two homozygotes.
How is the goodness-of-fit chi-square test used to analyze genetic crosses? What does the probability associated with a chi-square value indicate about the results of a cross?
The goodness-of-fit chi-square test is a statistical method used to evaluate the role of chance in causing deviations between the observed and the expected numbers of offspring produced in a genetic cross. The probability value obtained from the chi-square table refers to the probability that random chance produced the deviations of the observed numbers from the expected numbers.
What is the principle of segregation? Why is it important?
The principle of segregation, or Mendel's first law, states that an organism possesses two alleles for any one particular trait and that these alleles separate during 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.
What characteristics of an organism would make it well suited for studies of the principles of inheritance? Can you name several organisms that have these characteristics?
Useful characteristics - Are easy to grow and maintain - Grow rapidly, producing many generations in a short period - Produce large numbers of offspring - Have distinctive phenotypes that are easy to recognize Examples of organisms that meet these criteria - Neurospora, a fungus - Saccharomyces cerevisiae, a yeast - Arabidopsis, a plant - Caenorhabditis elegans, a nematode - Drosophila melanogaster, a fruit fly
What is the chromosome theory of inheritance? Why was it important?
Walter Sutton developed the chromosome theory of inheritance. The theory states that genes are located on the chromosomes. The independent segregation of pairs of homologous chromosomes in meiosis provides the biological basis for Mendel's two rules of heredity.
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?
We will use h for the hairless allele and H for the dominant. Because H is dominant to h, a rat terrier with hair could be either homozygous (HH) or heterozygous (Hh). To determine which genotype is present in the rat terrier with hair, cross this dog with a hairless rat terrier (hh). If the terrier with hair is homozygous (HH), then no hairless offspring will be produced. However, if the terrier is heterozygous (Hh) then we would expect one-half of the offspring to be hairless.
What is the probability of rolling one six-sided die and obtaining the following numbers?
a. 2 Because 2 is only found on one side of a six-sided die, then there is a 1/6 chance of rolling a two. b. 1 or 2 The probability of rolling a 1 on a six-sided die is 1/6. Similarly, the probability of rolling a 2 on a six-sided die is 1/6. Because the question asks what is the probability of rolling a 1 or a 2, and these are mutually exclusive events, we should use the additive rule of probability to determine the probability of rolling a 1 or a 2: (p of rolling a 1) + (p of rolling a 2) = p of rolling either a 1 or a 2 1/6 + 1/6 = 2/6 = 1/3 probability of rolling either a 1 or a 2 c. An even number The probability of rolling an even number depends on the number of even numbers found on the die. A single die contains three even numbers (2, 4, 6). The probability of rolling any one of these three numbers on a six-sided die is 1/6. To determine the probability of rolling either a 2, a 4, or a 6, we apply the additive rule: 1/6 + 1/6 + 1/6 = 3/6 = ½. d. Any number but a 6 The number 6 is found only on one side of a six-sided die. The probability of rolling a 6 is therefore 1/6. The probability of rolling any number but 6 is (1 - 1/6) = 5/6.
What is the probability of rolling two six-sided dice and obtaining the following numbers?
a. 2 and 3 To calculate the probability of rolling two six-sided dice and obtaining a 2 and a 3, we will need to use the product and additive rules. There are two possible ways in which to obtain the 2 and the 3 on the dice. There is a 1/6 chance of rolling a 2 on the first die and a 1/6 chance of rolling a 3 on the second die. The probability of this taking place is therefore 1/6 × 1/6 = 1/36. There is also a 1/6 chance of rolling a 3 on the first die and a 1/6 chance of rolling a 2 of the second die. Again, the probability of this taking place is 1/6 × 1/6 = 1/36. So the probability of rolling a 2 and a 3 would be 1/36 + 1/36 = 2/36 or 1/18. b. 6 and 6 There is only one way to roll two 6's on a pair of dice: the first die must be a 6 and the second die must be a 6. The probability is 1/6 × 1/6 = 1/36. c. At least one 6 There are 3 ways in which to get at least one 6 in the roll of two dice. The first is to roll 6 on both dice, which we already determined has a probability of 1/36. The second way is to roll a 6 on the first die (1/6) and something other than a 6 on the second (5/6). When the multiplication rule is applied to this second possibility, we achieve an overall probability of 5/36. The third way would be to roll something other than a 6 in the first die (5/6) and a 6 on the second die (1/6) for an overall probability of 5/36. Using the addition rule to add the probabilities of these three different ways to achieve at least one 6, we arrive at the final answer of 11/36 chance. d. Two of the same number (two 1's, or two 2's, or two 3's, etc.) There are several ways to roll two of the same number. You could roll two 1's, two 2's, two 3's, two 4's, two 5's, or two 6's. Using the multiplication rule, the probability of rolling two 1s is 1/6 × 1/6 = 1/36. The same is true of two 2's, two 3's, two 4's, two 5's, and two 6's. Using the addition rule, the probability of rolling either two 1's, two 2's, two 3's, two 4's, two 5's, and two 6's is 1/36 + 1/36 +1/36 +1/36 +1/36 + 1/36 = 6/36 = 1/6. e. An even number on both dice Three out of the six sides of a die are even numbers, so there is a 3/6 probability of rolling an even number on each of the dice. The chance of having an even number on both dice is (3/6)(3/6) = 9/36, or ¼. f. An even number on at least one die Three out of the six sides of a die are even numbers, so the probability of rolling an even number on the one die is 3/6. The probability of not rolling an even number is 3/6. An even number on at least one die could be obtained by rolling (a) an even on the first but not on the second die (3/6 × 3/6 = 9/36), (b) an even on the second die but not on the first (3/6 × 3/6 = 9/36), or (c) an even on both dice (3/6 × 3/6 = 9/36). Using the addition rule to obtain the probability of either a or b or c, we obtain 9/36 + 9/36 + 9/36 = 27/36 = ¾.