chapter 5

EXAM 4 extranuclear inheritance

(also known as cytoplasmic inheritance) the inheritance of genetic material that is not found within the nucleus.

3. A maternal effect gene in Drosophila, called torso, is found as a functional allele (torso+) and a nonfunctional, recessive allele (torso−) that prevents the correct development of anterior- and posterior-most structures. A wild-type male (torso+torso+) is crossed to a female of unknown genotype. This mating produces 100% larva that are missing their anterior- and posterior-most structures and therefore die during early development. What is the genotype and phenotype of the female fly in this cross? What are the genotypes and phenotypes of the female fly's parents?

**need to look up answer on page 123 for genetic tip #3 Because the cross produced 100% abnormal offspring that were missing their anterior- and posterior-most structures, the mother of the abnormal offspring must have been homozygous, torso− torso−. Even so, she must be phenotypically normal in order to reproduce. As shown below, the mother of the abnormal offspring had a mother that was heterozygous for the torso alleles and a father that was either heterozygous or homozygous for the torso− allele. The mother of the abnormal offspring is phenotypically normal because her mother was heterozygous and provided the gene products of the torso+ allele from the nurse cells. However, this homozygous female will produce only abnormal offspring because she cannot provide them with the functional torso+ gene product.

EXAM 4 A female mouse that is Igf2 Igf2− is crossed to a male that is also Igf2 Igf2−. The expected outcome for the phenotypes of the offspring for this cross is all normal size. all dwarf. 1 normal : 1 dwarf. 3 normal : 1 dwarf.

1 normal : 1 dwarf.

What is the molecular explanation for maternal effect? The father's gene is silenced at fertilization. During oogenesis, nurse cells transfer gene products to the oocyte. The gene products from nurse cells are needed during the very early stages of development. Both b and c are correct.

Both b and c are correct.

C1. Define the term epigenetic inheritance, and describe two examples.

C1. Epigenetic inheritance is a pattern in which a modification occurs to a gene or chromosome that alters gene expression, but the expression is not permanently changed over the course of many generations. Examples include imprinting, XCI, and other forms of dosage compensation.

C13. Describe when X-chromosome inactivation occurs and how this leads to phenotypic results at the organism level. In your answer, you should explain why XCI causes results such as variegated coat patterns in mammals. Why do two different calico cats have their patches of orange and black fur in different places? Explain whether or not a variegated coat pattern due to XCI could occur in marsupials.

C13. X-chromosome inactivation in heterozygous females produces a mosaic pattern of gene expression. During early embryonic development, some cells have the maternal X chromosome inactivated and other cells have the paternal X chromosome inactivated; these embryonic cells will divide and produce billions of cells. In the case of a female that is heterozygous for a gene that affects pigmentation of the fur, this produces a variegated pattern of coat color. Because it is a random process in any given animal, two female cats will vary as to where the orange and black patches occur. A variegated coat pattern could not occur in female marsupials due to XCI because the paternal X chromosome is inactivated in the somatic cells of females.

C14. Describe the molecular process of X-chromosome inactivation. This description should include the three phases of inactivation and the role of the Xic. Explain what happens to the X chromosomes during embryogenesis, in adult somatic cells, and during oogenesis.

C14. X-chromosome inactivation begins with the counting of Xics. If there are two X chromosomes, in the process of initiation, one is targeted for inactivation. During embryogenesis, this inactivation begins at the Xic locus and spreads to both ends of the X chromosome until it becomes a highly condensed Barr body. The Tsix gene plays a role in the choice of the X chromosome that remains active. The Xist gene, which is located in the Xic region, remains transcriptionally active on the inactivated X chromosome. It plays an important role in XCI by coating the inactive X chromosome. After XCI is established, it is maintained in the same X chromosome in somatic cells during subsequent cell divisions. In germ cells, however, the X chromosomes are not inactivated, so an egg can transmit either copy of an active (noncondensed) X chromosome.

C15. On rare occasions, a human male is born who is somewhat feminized compared with other males. Microscopic examination of the cells of one such individual revealed that he has a single Barr body in each cell. What is the chromosomal composition of this individual?

C15. The male is XXY. The person is male due to the presence of the Y chromosome. Because two X chromosomes are counted, one of the X chromosomes is inactivated to produce a Barr body.

C16. How many Barr bodies would you expect to find in humans with the following abnormal compositions of sex chromosomes? XXY XYY XXX X0 (a person with just a single X chromosome)

C16.A. One B. Zero C. Two D. Zero

C18. A black female cat (XBXB) and an orange male cat (X0Y) were mated to each other and produced a male cat that was calico. Which sex chromosomes did this male offspring inherit from its mother and father? Remember that the presence of the Y chromosome determines maleness in mammals.

C18. The offspring inherited XB from its mother and XO and Y from its father. It is an XXY animal, which is male (but somewhat feminized).

C19. What is the spreading phase of X-chromosome inactivation? Why do you think it is called a spreading phase?

C19. The spreading stage is when the X chromosome is inactivated (i.e., condensed) as a wave that spreads outward from the X-inactivation center (Xic). The condensation spreads from Xic to the rest of the X chromosome.

C2. Describe the inheritance pattern of maternal effect genes. Explain how the maternal effect occurs at the cellular level. What are the expected functional roles of the proteins that are encoded by maternal effect genes?

C2. A maternal effect gene is one for which the genotype of the mother determines the phenotype of the offspring. At the cellular level, this happens because maternal effect genes are expressed in diploid nurse cells and then the gene products are transported into the egg. These gene products are proteins that play key roles in the early steps of embryonic development.

EXAM 4 C20. When does the erasure and reestablishment phase of genomic imprinting occur? Explain why it is necessary to erase an imprint and then reestablish it in order to always maintain imprinting from the same sex of parent.

C20. The erasure and reestablishment phase occurs during gametogenesis. It is necessary to erase the imprint because each sex will transmit either inactive or active alleles of a gene. In somatic cells, the two alleles for a gene are imprinted according to the sex of the parent from which the allele was inherited.

EXAM 4 C24. How is the process of X-chromosome inactivation similar to genomic imprinting? How is it different?

C24. In some species, such as marsupials, XCI depends on the sex. This is similar to imprinting. Also, once XCI occurs during embryonic development, it is remembered throughout the rest of the life of the organism, which is also similar to imprinting. XCI in mammals is different from genomic imprinting in that it is not sex-dependent. The X chromosome that is inactivated can be inherited from the mother or the father. No marking process on the X chromosome occurs during gametogenesis. In contrast, genomic imprinting always involves a marking process during gametogenesis.

C26. What is a reciprocal cross? Suppose that a gene is found as a wild-type (functional) allele and a recessive mutant (nonfunctional) allele. What would be the expected outcomes of reciprocal crosses if a true-breeding normal individual was crossed to a true-breeding individual carrying the mutant allele? What would be the results if the gene is maternally inherited?

C26. The term reciprocal cross refers to two crosses that involve the same genotypes of the two parents, but the sexes of the parents are opposite in the two crosses. For example, the reciprocal cross of female BB × male bb is female bb × male BB. Autosomal inheritance gives the same result because the autosomes are transmitted from parent to offspring in the same way for both sexes. However, for extranuclear inheritance, the mitochondria and plastids are not transmitted via the gametes in the same way for both sexes. For maternal inheritance, the reciprocal crosses would show that the gene is always inherited from the mother.

C3. A maternal effect gene exists in a dominant N (functional) allele and a recessive n (nonfunctional) allele. What would be the ratios of genotypes and phenotypes for the offspring of the following crosses? nn female × NN male NN female × nn male Nn female × Nn male

C3. Genotypes: All Nn Phenotypes: All abnormal Genotypes: All Nn Phenotypes: All normal Genotypes: 1 NN : 2 Nn : 1 nn Phenotypes: All normal (because the mother is heterozygous and N is dominant)

C4. A Drosophila embryo dies during early embryogenesis due to a recessive maternal effect allele called bicoid−. The wild-type allele is designated bicoid+. What are the genotypes and phenotypes of the embryo's mother and maternal grandparents?

C4. The genotype of the mother must be bic- bic-. That is why the mother produces abnormal offspring. Because the mother is alive and able to produce offspring, her mother (the maternal grandmother) must have been bic+ bic- and passed the bic- allele to the daughter (the mother in this problem). The maternal grandfather also must have passed the bic- allele to his daughter. The maternal grandfather could be either bic+ bic- or bic- bic-.

C6. Suppose a maternal effect gene exists as a functional dominant allele and a nonfunctional recessive allele. A mother who is phenotypically abnormal produces all normal offspring. Explain the genotype of the mother.

C6. The mother must be heterozygous. She is phenotypically abnormal because her mother must have been homozygous for the nonfunctional recessive allele. However, because she produces all normal offspring, she must have inherited the functional dominant allele from her father. She produces all normal offspring because this is a maternal effect gene, and the gene product of the functional dominant allele is transferred to the egg.

C7. Suppose that a gene affects the anterior morphology in house flies and is inherited as a maternal effect gene. The gene exists in a functional allele, H, and a recessive nonfunctional allele, h, which causes a small head. A female fly with a normal head is mated to a true-breeding male with a small head. All of the offspring have small heads. What are the genotypes of the mother and offspring? Explain your answer.

C7. The mother is hh. We know this because it is a maternal effect gene and all of its offspring have small heads. The offspring are all hh because their mother is hh and their father is hh.

C9. As described in Chapter 22, researchers have been able to clone mammals by fusing a cell having a diploid nucleus (i.e., a somatic cell) with an egg that has had its nucleus removed. With regard to maternal effect genes, would the phenotype of such a cloned animal be determined by the animal that donated the egg or by the animal that donated the somatic cell? Explain. Would the cloned animal inherit extranuclear traits from the animal that donated the egg or from the animal that donated the somatic cell? Explain. In what ways would you expect this cloned animal to be similar to or different from the animal that donated the somatic cell? Is it fair to call such an animal a clone of the animal that donated the diploid nucleus?

C9. The phenotype would be determined by the animal that donated the egg, because the gene products of maternal effect genes are transferred to the egg by the nurse cells. Most likely a little of both. The egg would contain some mitochondria and so would the somatic cell. Because the egg is so much larger, however, it would probably donate many more mitochondria. The "cloned" animal would have the vast majority of its genetic traits from the animal that donated the somatic cell. However, the animal that donated the egg would govern traits determined by maternal effect genes, and mitochondrial genes would probably come from both donors. The offspring is not quite a clone, because it is not genetically identical to the animal that donated the somatic cell. Perhaps we should call it a "quasi-clone."

Phenotype turner syndrome (female)

Chromosome Composition X0 Number of X Chromosomes 1 Number of Barr Bodies 0

Phenotype normal female

Chromosome Composition XX Number of X Chromosomes 2 Number of Barr Bodies 1

Phenotype triple X syndrome (female)

Chromosome Composition XXX Number of X Chromosomes 3 Number of Barr Bodies 2

Phenotype klinefelter syndrome (male)

Chromosome Composition XXY Number of X Chromosomes 2 Number of Barr Bodies 1

Phenotype normal male

Chromosome Composition XY Number of X Chromosomes 1 Number of Barr Bodies 0

A female snail that coils to the left has offspring that coil to the right. What are the genotypes of this mother and of the maternal grandmother of the offspring, respectively? dd, DD Dd, Dd dd, Dd Dd, dd

Dd, dd

___ ___ Results in Similar Levels of Gene Expression Between the Sexes

Dosage Compensation

EXAM 4 A female born with Angelman syndrome carries a deletion in the AS gene (i.e., the UBE3A gene). Which parent transmitted the deletion to her? Her father Her mother Either her mother or father

Her mother

The Genotype of the Mother Determines the Phenotype of the Offspring for

Maternal Effect Genes

EXAM 4 An individual named Pat with Prader-Willi syndrome produced an offspring named Lee with Angelman syndrome. The other parent does not have either syndrome. How might this occur? What are the sexes of Pat and Lee?

Pat is the mother, who carried a deletion that encompassed both the PWS and AS genes. If a female transmits the deletion to either a son or daughter, that offspring will have AS. Therefore, Lee could be either a male or female.

1. One strain of periwinkle plants has green leaves and another strain has white leaves. Both strains are true-breeding. You do not know if the phenotypic difference is due to alleles of a nuclear gene or an organellar gene. The two strains were analyzed using reciprocal crosses, and the following results were obtained: A plant with green leaves is pollinated by a plant with white leaves: all offspring have green leaves. A plant with white leaves is pollinated by a plant with green leaves: all offspring have white leaves. Is this pattern of inheritance consistent with simple Mendelian inheritance, where green is dominant to white, and/or is it consistent with maternal inheritance?

The data are consistent with maternal inheritance, because the phenotype of the offspring correlates with inheriting the gene from the plant contributing the egg cells.

2. A human male named Phillip has an X chromosome that is missing its Xic. Is this caused by a new mutation (one that occurred during gametogenesis), or could this mutation have occurred in an earlier generation and be found in the somatic cells of one of his parents? Explain your answer. How would this mutation affect his ability to produce viable offspring?

The missing Xic must be due to a new mutation that occurred during oogenesis in Phillip's mother. Phillip will pass his X chromosome to his daughters and his Y chromosome to his sons. He cannot produce living daughters, because a missing Xic is lethal in females. However, he can produce living sons.

EXAM 4 Some human diseases are caused by mutations in mitochondrial genes. Which of the following statements is false? Human mitochondrial diseases follow a maternal inheritance pattern. Mutations associated with mitochondrial diseases often affect cells with a high demand for ATP. The symptoms associated with mitochondrial diseases tend to improve with age. Heteroplasmy plays a key role in the severity of mitochondrial disease symptoms.

The symptoms associated with mitochondrial diseases tend to improve with age.

X-Chromosome Inactivation in Mammals Depends on the X-Inactivation Center and Occurs in

Three Phases

heteroplasmy

a condition in which a cell has variation in a particular type of organelle. For example, a plant cell could contain some chloroplasts that make chlorophyll and other chloroplasts that do not.

nucleoid

a darkly staining region that contains the genetic material of mitochondria, chloroplasts, or bacteria.

Lyon hypothesis

a hypothesis to explain the pattern of X-chromosome inactivation seen in mammals. Initially, both X chromosomes are active. However, at an early stage of embryonic development, one of the two X chromosomes is randomly inactivated in each somatic cell. (calico cats)

EXAM 4 genomic imprinting

a pattern of inheritance that involves a change in a single gene or chromosome during gamete formation. Depending on whether the modification occurs during spermatogenesis or oogenesis, imprinting governs whether an offspring will express a gene that has been inherited from its mother or father. OR refers to an analogous situation in which a segment of DNA is marked, and that mark is retained and recognized throughout the life of the organism inheriting the marked DNA. Genomic imprinting happens prior to fertilization; it involves a change in a single gene or chromosome during gamete formation. Depending on whether the modification occurs during spermatogenesis or oogenesis, imprinting governs whether an offspring expresses a gene that has been inherited from its mother or father.

X-chromosome inactivation (XCI)

a process in which mammals equalize the expression of X-linked genes by randomly turning off one X chromosome in the somatic cells of females.

EXAM 4 DNA methylation

a regulatory mechanism in which an enzyme covalently attaches a methyl group (—CH3) to a base in DNA. In eukaryotes, the base is cytosine. In prokaryotes, both adenine and cytosine can be methylated.

X-inactivation center (Xic)

a site on the X chromosome that appears to play a critical role in X-chromosome inactivation.

Barr body

a structure in the interphase nuclei of somatic cells of female mammals that is a highly condensed X chromosome.

EXAM 4 A cross is made between a green four-o'clock plant and a variegated one. If the variegated plant provides the pollen, the expected outcome of the phenotypes of the offspring will be all plants with green leaves. 3 plants with green leaves to 1 plant with variegated leaves. 3 plants with green leaves to 1 plant with white leaves. some plants with green leaves, some with variegated leaves, and some with white leaves.

all plants with green leaves.

need to also know chapter 15.3, 3.6, and 16.1

also finish connect online study questions active with book for chapter 10

maternal effect

an inheritance pattern for certain nuclear genes in which the genotype of the mother directly determines the phenotypic traits of her offspring. We will see that maternal effect inheritance is explained by the accumulation of gene products that the mother provides to her developing oocytes (immature eggs).

In fruit flies, dosage compensation is achieved by X-chromosome inactivation. doubling the expression of genes on the single X chromosome in the male. decreasing the expression of genes on the two X chromosomes by 50% in the female. all of the above.

doubling the expression of genes on the single X chromosome in the male.

EXAM 4 The marking process for genomic imprinting initially occurs during gametogenesis. fertilization. embryonic development. adulthood.

gametogenesis.

EXAM 4 Extranuclear inheritance occurs due to chromosomes that may become detached from the spindle apparatus during meiosis. genetic material that is found in chloroplasts and mitochondria. mutations that disrupt the integrity of the nuclear membrane. none of the above.

genetic material that is found in chloroplasts and mitochondria.

EXAM 4 monoallelic expression

in the case of genomic imprinting, refers to the phenomenon that only one of the two alleles of a given gene is transcriptionally expressed.

EXAM 4 maternal inheritance

inheritance of DNA that occurs through the cytoplasm of the egg.

The process of XCI can be divided into three phases:

initiation, spreading, and maintenance

EXAM 4 In mice, the copy of the Igf2 gene that is inherited from the mother is never expressed in her offspring. This happens because the Igf2 gene from the mother always undergoes a mutation that inactivates its function. is deleted during oogenesis. is deleted during embryonic development. is not transcribed in the somatic cells of the offspring.

is not transcribed in the somatic cells of the offspring.

EXAM 4 which statements are true about genomic imprinting? occurs in several species including flowering plants, insects and mammals only occurs in placental and marsupial mammals only involves a single gene determines x-chromosome inactivation in marsupials may involve a single gene, a part of a chromosome, or the entire chromosome

occurs in several species including flowering plants, insects and mammals determines x-chromosome inactivation in marsupials may involve a single gene, a part of a chromosome, or the entire chromosome

According to the Lyon hypothesis, one of the X chromosomes is converted to a Barr body in somatic cells of female mammals. one of the X chromosomes is converted to a Barr body in all cells of female mammals. both of the X chromosomes are converted to Barr bodies in somatic cells of female mammals. both of the X chromosomes are converted to Barr bodies in all cells of female mammals.

one of the X chromosomes is converted to a Barr body in somatic cells of female mammals.

At the molecular and cellular level, the non-Mendelian inheritance pattern of maternal effect genes can be explained by the process of ______ in female animals (Figure 5.2a). As an animal oocyte Page 105matures, many surrounding maternal cells called nurse cells provide the oocyte with nutrients and other materials.

oogenesis

Dosage compensation

the phenomenon that in species with sex chromosomes, one of the sex chromosomes is altered so that males and females have similar levels of gene expression, even though they do not contain the same complement of sex chromosomes.

Epigenetics

the study of mechanisms that lead to changes in gene expression that can be passed from cell to cell and are reversible, but do not involve a change in the DNA sequence.

true or false Female Gametes Receive Gene Products from the Mother That Affect Early Developmental Stages of the Embryo

true