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Chapter 5, The inheritance of single-gene differences

Specific Reading Assignment. pp 118-137, skip INHERITANCE OF ORGANELLE GENES. Crucial Figures are 3b, 4, 5, 8, 9, 12, 14, 15, 16a, 19 (You do not need to memorize pedigree figures 18-27, but you should be able to propose the type of inheritance based on a pedigree). Important Figures are 3a, 6, 7. Skip Figure: 20, 21, 22.

INHERITANCE PATTERNS (pp. 118-128)

Objectives. To learn: 1) How the behavior of homologous chromosomes in meiosis and fertilization, and the dominance/recessivity of an allele explain the inheritance of simple traits.

2) The mechanism of sex determination in mammals and some insects. 3) The behavior of sex chromosomes in meiosis. [The best way to learn the topics corresponding to this week and the following weeks, Classical Genetics, is by doing many problems. For this reason we have assigned extra end-of-chapter problems, and fewer Orientation Questions.]

1a) Diagram the transmission of a pair of chromosomes with alleles A/a through meiosis in a fungus (Fig. 11b) using the line representation of chromosomes (See question 5d in Chapter 4).

1b) In the experiments discussed in Foundations of Genetics 5-1, Mendel made two crosses: the first was of pure-breeding yellow to pure-breeding green seeds, the second was to cross the progeny to itself. Which of the two crosses supports each of the 5 parts of his hypothesis (p.125)?

2a) Using the grids (or Punnett squares) of page 123, diagram how a testcross could be used to determine whether an individual is homozygous A/A or heterozygous A/a Note that it is very important in these grids to specify the frequency with which each kind of gamete is produced. The product of those frequencies allows you to predict the frequency of each genotype in the progeny.

2b) Diagram a cross between two heterozygous individuals heterozygous for a single trait.

2c) A black mouse is crossed to a brown mouse (top of p. 123, column 2) and the progeny consists of 3 black and one brown mice. What are the genotypes of the two parents?

3) Autosomal and sex-linked genes.

3a) Draw a diagram of meiosis as in question 5d) of Chapter 4. Represent two pairs of chromosomes: X and Y and a pair of autosomes. Associate gene A to the X and B/b to the autosomes. What are all the possible gametes produced? Which will give rise to males and which to females?

3b) Who receive an X from the father, sons or daughters? Who receives an X from the mother?

3c) Which chromosome does a man always shares with his paternal grandfather? Great-grandfather?

END OF CHAPTER PROBLEMS 13, 17, 18. 

Answers to INHERITANCE PATTERNS OF INDIVIDUAL GENES

1a) Fig 11b. Note that the only difference between this figure and the one for Orientation Question 5d) in week 4, is that in the latter the plane of the equatorial plate of the cell rotates 90^o

between metaphase I and II, while in Fig. 5-3b, it does not (thus, all four products are in a line).

1b) Part 1 comes from the first and second crosses: although Agreen-ness@ is not evident in the progeny of the first cross, the determinants are not lost or mixed with Ayellow-ness@, because they re-appear after the second cross (discrete factors as opposed to fulids that mixed). Part 2 comes from the first, but especially the second crosses: The first progeny must have at least two factors for color, they have yellow, which they show, and they must have green, which they pass to their progeny. Parts 3 and 4 come from the second cross, in particular from the numerical ratios among the classes. Part 5 comes primarily from the first cross.

2a) See below

2b) See below

2c) Since Abrown@ is recessive, the brown mouse must be homozygous b/b; since they produce one brown mouse in the progeny, the black mouse must be heterozygous B/b. Note that the apparent 3:1 ratio is not relevant; the expected ratio would be 1:1, but with so few individuals it is futile to try to measure ratios.

2a)

2b)

3a) The following diagrams represent meiosis in a male mammal. Note that in Prophase I the X and Y pair with each other, which insures that they are carried to opposite poles; the pairing, however, takes place over a very short segment because the two chromosomes have totally different DNA sequences. There is no corresponding allele for the A gene on the Y chromosome. In purple lettering the X and Y chromosomes are identified as well as the autosomes (A); note that in the case of humans, for example, there would be 22 pairs of autosomes. The X-bearing sperm would give rise to female progeny and the Y-bearing sperm would give rise to male progeny.

3b) Daughters but not sons receive an X from the father. Both sons and daughters receive an X from the mother.

3c) The Y, in both cases.

Objectives. To learn: 1) The symbols used to represent hereditary traits and familial relationships in humans. 2) How Mendelian inheritance applies to humans.

4a) Draw an invented pedigree in which two normal parents have six children. The eldest of the children are two daughters, then came a pair of identical twins. The youngest is a boy, and the remaining child is male affected with a hereditary condition, which led the family to see a genetics counselor. Include all appropriate labels.

4b) In the example above, assuming that the disease is transmitted from the parents, rather than being a new mutation, what type of inheritance would you suggest?

4c) In the same example, how could you confirm a particular type of inheritance?

4d) In Fig. 5-12, how can you explain that individuals A/a have normal pigmentation? What does ALys@ represent in that figure?

4e) Pooling data from many pedigrees, what features characterize X-linked recessive conditions?

4f) What are the characteristics of pedigrees in which a dominant trait is transmitted?

4g) Draw a pedigree for a rare, recessive X-linked trait; the pedigree should extend several generations and the trait should display a Askipping@ of generations. Indicate the genotype of each individual.

Note: While it is common practice to represent females and males by their chromosomal constitution, XX and XY respectively, and this presents no difficulty, in representing genotypes for X-linked genes, it is not advisable to employ the nomenclature used in the book (an X with a superscript, ^AX, like ornaments hanging from a Christmas tree). It is much better to use the same symbols you were using in the meiosis and Mendelian exercises: a line representing the chromosome below the allele letter, or a slash (A/a/ in this case the second line is sometimes omitted: A/a). In the case of the Y chromosome, use the letter Y, or a short bent line:

The value of this suggested symbolism will become apparent in the next chapter, when we need to indicate the presence and order of two or more genes on the same X chromosome.

END OF CHAPTER PROBLEMS 10a, 10b, 11a, 20a, 20b,  25a, 25b.

5a) In a mating of two heterozygotes for an autosomal recessive condition: What is the probability that they will have an affected child? What is the probability of having a child homozygous for the normal allele? What is the probability that a normal child is homozygous for the normal allele?

5b) What is the probability that the son of a woman heterozygous for hemophilia, an X-linked trait, is affected? What is the probability that a woman heterozygous for hemophilia will have an affected child?

5c) What is the probability that the son of a man carrying an X-linked trait, is affected?

5d) A man who is color blind has a color-blind father, maternal uncle (brother of his mother) and sister. His sister is married to a normal man and has two sons, and the man is married to a normal woman and has a son and a daughter. Diagram the pedigree of this family. What are the genotypes and phenotypes of the four children in the last generation?

END OF CHAPTER PROBLEMS 10c, 11b, 19, 20c, 21, 23, 25c.

4a)

4b) It could be autosomal (pink genotypes) or sex linked recessive (purple genotypes):

4c) I would study other families with the same trait, confirm that it is recessive and see whether males and females are equally affected (autosomal) or whether it is usually males that are affected.

4d) Because they are heterozygotes and the one normal copy of the enzyme gene is sufficient for normal function (See haplo-sufficiency in p83). The mutation in this family is one in which the amino acid threonine was replaced with lysine in the polypeptide chain of the enzyme tyrosinase.

4e) The overall feature of pooled data is that there is an excess of males affected with the trait. (In individual extended pedigrees of X-linked recessive traits, women usually have normal phenotypes and some of them have affected sons, but not daughters. There is a certain skipping of generations when an affected male passes the mutant allele to a daughter, who being heterozygous, is normal, and then the daughters has affected sons.)

4f) In these pedigrees, all affected individuals always have an affected parent. (While in pedigrees of recessive traits most affected individuals have normal parents, because these traits are usually rare conditions and parents are usually heterozygous rather than homozygous).

4g)

5a) The probability of a/a is 0.25, the same as the probability of A/A. The probability of A/A among normal children is 0.67. (Among normal children (A/-) 2/3 will be A/a and 1/3 A/A, because among all the progeny there are twice as many A/a as A/A, 2/4 to 1/4.)

5b) The probability that the son will be affected is 0.5. The probability of an affected child is 0.25 (which is the product of two probabilities that of having a male child, 1/2, and that of transmitting the mutant chromosome to that child, 1/2).

5c) Essentially zero because men pass their Y chromosome, not the X, to their sons.

Problems 24, 26, 28.