Ch. 13- Genes, Chromosomes, and Human Genetics

Progeria is a genetic condition that causes ____.

premature aging which typically leads to death in the early teens

Linked genes are ____.

genes that do not sort independently because they are physically near each other on the same chromosome

Exceptions to the principle of independent assortment were discovered and explained by ____ as resulting from genes being physically associated with each other on the same chromosome.

Morgan

The principles of linkage and recombination were discovered using which organism?

fruit flies

Which process exchanges alleles between homologous chromosomes?

genetic recombination

Suppose that in studies of genes on the same chromosome you find the following recombination frequencies: Figure 13.1 In this case, it would be proper to say that a, c, and b are ____.

linked genes

Suppose that in studies of genes on the same chromosome you find the following recombination frequencies: Figure 13.2 Why is the total distance between a and b less than the distance between a and c plus the distance between c and b?

double crossovers between a and b decrease recombinants

Recombination frequency is ____.

used to create linkage maps

Genetic studies of an animal show that eye color is controlled by an autosomal gene with the dominant allele (R) for red eye color and the recessive allele (r) for yellow eye color. A second autosomal gene has the dominant allele (T) for paws with thumbs and the recessive allele (t) for paws without thumbs. The genetic cross RRTT x rrtt creates offspring with genotype RrTt. One of those dihybrids is mated in a testcross (RrTt x rrtt). Based on the principle of independent assortment, the testcross should produce offspring with the phenotype ratio ____.

1 red-eyed with thumbs : 1 yellow-eyed with thumbs: 1 red-eyed without thumbs: 1 yellow-eyed without thumbs

Genetic studies of an animal show that eye color is controlled by an autosomal gene with the dominant allele (R) for red eye color and the recessive allele (r) for yellow eye color. A second autosomal gene has the dominant allele (T) for paws with thumbs and the recessive allele (t) for paws without thumbs. The genetic cross RRTT x rrtt creates offspring with genotype RrTt. One of those dihybrids is mated in a testcross (RrTt x rrtt). If the two genes are completely linked (no recombination occurs between them), then the testcross should produce offspring with the phenotype ratio ____.

1 red-eyed with thumbs : 1 yellow-eyed without thumbs.

If two genes are linked on the same chromosome, then ____.

the less crossing-over occurs between them

A recombinant phenotype is ____.

a different combination of the traits seen in either parent

What is another name for map unit?

centimorgan

The map distances for four linked genes are as follows: A is 22 cM from B, B is 7 cM from C, C is 9 cM from D, B is 2 cM from D, A is 20 cM from D, and A is 29 cM from C. Based on these data, what is the order of these four genes on the chromosome?

ADBC

When two genes on the same chromosome are located 10 map units from each other, ____ of the offspring from a testcross for genetic linkage should have a recombinant phenotype.

about 10%

When two genes are located on different chromosomes, ____ of the offspring from a testcross for genetic linkage should have a recombinant phenotype.

about 50%

Recombination frequency for two genes is a function of the ____.

distance between the two genes on a single chromosome

In Drosophila melanogaster, the allele for red eyes is dominant over the allele for purple eyes, and the allele for a gray body is dominant over the allele for a black body. A testcross was done to check for genetic linkage between the genes for these traits. The results for the offspring are listed below: 478 flies with red eyes and a gray body 27 flies with red eyes and a black body 462 flies with purple eyes and a black body 33 flies with purple eyes and a gray body Which of the choices below best represents the map distance between the genes for these two traits?

6.0 map units

In Drosophila melanogaster, the allele for long wings is dominant over the allele for vestigial wings, and the allele for a gray body is dominant over the allele for a black body. A testcross was done to check for genetic linkage between the genes for these traits, with the following results for the offspring: 410 flies with long wings and a gray body 105 flies with long wings and a black body 390 flies with vestigial wings and a black body 95 flies with vestigial wings and a gray body Which of the choices below best represents the map distance between the genes for these two traits?

25.0 map units

As an undergraduate research assistant, you are given a genetic mapping project. You are told that genes a and b are 7.4 map units from each other and that genes b and c are 5.7 map units from each other. You perform a testcross to determine the map distance between a and c. Which of the following results would indicate that c lies between a and b?

11.1 map units

Genetic map units represent ____.

relative positions of genes with respect to one another

Which process results in the production of offspring who are recombinant for two genes on the same chromosome?

crossing over between homologous chromosomes

Sex-linked genes are ____.

located on sex chromosomes

Which statement correctly describes the inheritance of sex chromosomes?

A woman will pass on an X chromosome to all of her children.

Sex-linked genes are ____.

found on the X and Y chromosomes

In humans, sex determination generally depends upon the ____.

presence of the Y chromosome

The X chromosome ____.

carries many genes that are not involved in sex determination

An autosome is a ____.

chromosome other than a sex chromosome

In birds and butterflies, males contain ____ sex chromosomes and females contain ____ sex chromosomes.

ZZ; ZW

In humans, males are the ____ sex, and females are the ____ sex.

heterogametic, homogametic

During normal human embryonic development, if ____, then the SRY gene switches development toward ____.

a Y chromosome is present; maleness

Which individual may be a carrier for a recessive X-linked trait in humans?

a heterozygous female

In Drosophila melanogaster, the gene for eye color is located on the X chromosome. The allele for red eye color (Xw+) is dominant over the allele for white eye color (Xw). You examine a vial of 100 flies that are all offspring from a single genetic cross. You see only red-eyed females present, but you see both red-eyed and white-eyed males present. The genotypes of the parents were ____.

Xw+Xw; Xw+Y

In Drosophila melanogaster, the gene for eye color is located on the X chromosome. The allele for red eye color (Xw+) is dominant over the allele for white eye color (Xw). You examine a vial of 100 flies that are all offspring from a single genetic cross. You find both red-eyed females and white-eyed females as well as both red-eyed males and white-eyed males. The genotypes of the parents were ____.

Xw+Xw; XwY

In Drosophila melanogaster, the gene for eye color is located on the X chromosome. The allele for red eye color (Xw+) is dominant over the allele for white eye color (Xw). You examine a vial of 100 flies that are all offspring from a single genetic cross. You see only red-eyed females and white-eyed males present. The genotypes of the parents were ____.

XwXw; Xw+Y

In Drosophila melanogaster, the gene for eye color is located on the X chromosome. The allele for red eye color (Xw+) is dominant over the allele for white eye color (Xw). You examine a vial of 100 flies that are all offspring from a single genetic cross. You find only red-eyed females and red-eyed males present. Flies from the vial were allowed to interbreed, and in the next generation you find only red-eyed females, but you find both red-eyed and white-eyed males. The genotypes of the original parents were ____.

Xw+Xw+; XwY

A woman with normal blood clotting mates with a man who has hemophilia. Their first child is a boy who has hemophilia. Tests show that the father and son both have the same form of hemophilia, that it is X-linked, and that the boy has normal genetic inheritance. You can predict that if the couple has more children together, it is likely that ____.

half of the boys and half of the girls will have hemophilia

A woman with normal blood clotting mates with a man who also has normal blood clotting. Their first child is a boy who has hemophilia. Tests show that the child's hemophilia is X-linked and that he has normal genetic inheritance. You can predict that if the couple has more children together, then it is likely that ____.

half of the boys and none of the girls will have hemophilia

In placental mammals, such as humans, the dosage compensation mechanism to equalize expression of sex-linked genes in males and females involves ____.

inactivating one of the two X chromosomes in most female somatic cells

If you see a male calico cat, you can be fairly certain that his diploid cells have a sex chromosome combination of ____.

XXY

The change in the chromosomes depicted between the top and the bottom in the accompanying figure represents a(n) ____.

reciprocal translocation

The change in the chromosome depicted between the top and the bottom in the accompanying figure represents a(n) ____.

inversion

The change in the chromosome depicted between the top and the bottom in the accompanying figure represents a(n) ____.

duplication

The change in the chromosome depicted between the top and the bottom in the accompanying figure represents a(n) ____.

deletion

In humans, a deletion from chromosome 5 typically leads to severe mental retardation and a malformed larynx; this disorder is known as ____.

cri-du-chat

If a hypothetical human female of genotype XX had no Barr body in any of her cells, ---____.

the genes on both X chromosomes would be expressed

The presence of genes that encode many types of hemoglobin in mammals, but not in other vertebrates that evolved earlier, is evidence of ____ of genetic material.

the duplication

Individuals who have therapy-related myeloid neoplasms (t-MN) often have deletions of chromosome 5. These deleted genes are ____.

expressed at lower levels in these individuals due to haploinsufficiency

Chemical compounds released during tobacco use can cause ____.

cancer

Nondisjunction refers to the ____.

failure of homologous pairs or sister chromatids to separate during meiosis

Individuals with extra or missing copies of one or more chromosomes are called ____.

aneuploids

Individuals with three or more copies of each of their chromosomes are called ____.

polyploids

Which is most likely to happen to a human polyploid?

natural abortion

In cells of human individuals with three or more X chromosomes, ____.

one X chromosome remains active and the others are inactivated

In humans, an extra copy of chromosome 21 typically leads to moderate to severe mental retardation and sterility, as well as to a greater likelihood of heart defects and other problems. This disorder is known as ____.

Down syndrome

Which condition would most likely lead to an apparently normal human female?

Triple-X syndrome

Which genetic condition is revealed in the karyotype display shown in the accompanying figure?

Down syndrome

About half of all flowering plant species, including many important crop plants, are ____.

polyploids

Which statement correctly describes autosomal recessive inheritance?

Individuals who are homozygous for the recessive allele display the trait.

About 10-15% of African Americans in the US are carriers for ____, an autosomal recessive genetic disorder in which a defective version of the hemoglobin protein is produced.

sickle-cell anemia

Carriers of the sickle-cell allele have a genetic advantage because they have ____.

increased resistance to malaria

About 4% of individuals of Northern European descent are carriers for ____, an autosomal recessive genetic disorder in which a defective membrane transport protein leads to deficient chloride levels in extracellular fluids.

cystic fibrosis

An individual who is a carrier of a genetically inherited disease ____.

does not have the disease but may have offspring with the disease

Most hospitals in the United States routinely test all newborns for ____, an autosomal recessive disorder in which a metabolic abnormality causes a buildup of compounds that damage brain tissue and can lead to mental retardation unless a restricted diet is followed.

phenylketonuria

This autosomal dominant genetic trait is associated with a gene on human chromosome 4 and causes a type of dwarfing due to defective cartilage formation.

achondroplasia

Which genetic condition has an X-linked recessive inheritance pattern?

Duchenne muscular dystrophy

You are a genetic counselor, and a couple comes to you with concerns that if they have a child together, he or she could have cystic fibrosis. Genetic tests reveal that the woman is a carrier for cystic fibrosis but the man is not. How should you advise them?

While none of their children will have cystic fibrosis, each child will have a 50% chance of being a carrier.

You are genetic counselor, and a couple comes to you with concerns that if they have a child together, he or she could have sickle-cell anemia. Genetic tests reveal that both the man and woman are carriers for sickle-cell anemia. How should you advise them?

All of their children will have a 25% chance of having sickle-cell anemia.

Prenatal diagnosis techniques such as amniocentesis and chorionic villus sampling remove cells produced by the embryo. These cells are then used ____.

to test for the presence of mutant alleles or chromosomal alterations

A mutation in the human gene for a fibroblast growth factor receptor (FGFR) appears to be responsible for ____.

achondroplasia

Cytoplasmic inheritance refers to genes found in ____.

mitochondria and chloroplasts

Why are mitochondrial genes inherited in a non-Mendelian pattern?

The mitochondrial genes are not separated by meiosis.

For most multicellular eukaryotes, including humans, mitochondria are inherited ____.

only from the mother.

When one parental allele is expressed and the other is silenced, the phenomenon is called ____.

genomic imprinting

One mechanism of genomic imprinting silences an allele by DNA ____.

methylation

Haploinsufficiency ____.

occurs when a diploid organism has only one functional copy of a gene

Examine the pedigree in the figure above. Individuals affected by the genetic condition in question are indicated by filled squares or circles. Which inheritance pattern is most likely correct for this condition in this family?

X-linked recessive

Examine the pedigree in the accompanying figure. Individuals affected by the genetic condition in question are indicated by filled squares or circles. Which inheritance pattern is most likely correct for this condition in this family?

autosomal dominant

Examine the pedigree in the figure above. Individuals affected by the genetic condition in question are indicated by filled squares or circles. Which inheritance pattern is most likely correct for this condition in this family?

X-linked dominant

Examine the pedigree in the figure above. Individuals affected by the genetic condition in question are indicated by filled squares or circles. Which inheritance pattern is most likely correct for this condition in this family?

autosomal recessive

You are a genetic counselor, and a couple comes to you with concerns that if they have a child together, he or she could have hemophilia. The man has an X-linked recessive form of hemophilia, but the woman does not. Genetic tests reveal the woman is not a carrier for hemophilia. How should you advise them?

None of their children will have hemophilia, but all of their daughters will be carriers for hemophilia.

You are a genetic counselor, and a couple comes to you with concerns that if they have a child together, he or she could have hemophilia. The woman has an X-linked recessive form of hemophilia, but the man does not. How should you advise them?

All of their sons will have hemophilia, and all of their daughters will be carriers.

You are a genetic counselor, and a couple comes to you with concerns that if they have a child together, he or she could have hemophilia. Neither of them has hemophilia, but the woman's biological father did have an X-linked recessive form of hemophilia. How should you advise them?

Each of their sons will have a 50% chance of having hemophilia, and each of their daughters will have a 50% chance of being carriers.

Suppose that you have discovered a new mutant in Drosophila melanogaster. The trait is known to be sex linked. Which crosses should you perform to determine if the mutant trait is recessive or dominant?

You should do the following crosses: Cross a male with the mutant phenotype and a female without the mutant phenotype. If none of the offspring have the mutant phenotype, the mutant phenotype is likely X-linked recessive. Alternately, if half of the male offspring and half of the female offspring have the mutant phenotype, the mutant phenotype is likely X-linked recessive. If none of the male offspring have the mutant phenotype and all of the female offspring have the mutant phenotype, the mutant trait is likely X-linked dominant. Cross a male without the mutant phenotype and a female with the mutant phenotype. If all of the male offspring and none of the female offspring have the mutant phenotype, the mutant phenotype is likely X-linked recessive. If all of the male offspring and at least 50% of the female offspring have the mutant phenotype, the mutant phenotype may be X-linked dominant.

Two genes are on the same chromosome, yet crossing over occurs between them every time meiosis occurs. Are these genes linked or unlinked and why?

Unlinked. The definition of linkage is when two genes do not assort independently. If crossing over occurs between two genes every time meiosis occurs, these genes will always assort independently.

Explain the difference in meiotic products that arise when nondisjunction occurs during the first meiotic division versus those that arise when nondisjunction occurs during the second meiotic division.

If nondisjunction occurs during the first division, all meiotic products are affected and are aneuploid. If nondisjunction occurs during the second division, half of the meiotic products are normal and the other half are aneuploid.

It has been known for some time that fetal cells can be found in a pregnant woman’s bloodstream, sometimes years after a pregnancy. A new technique has been developed that allows scientists to isolate these fetal cells from a pregnant woman’s blood. Why would this be a useful new tool for parents interested in genetic testing of their fetus?

Currently, chorionic villus sampling and amniocentesis are the methods of choice for acquiring fetal cells for genetic testing. A blood sample from the mother would be a much less invasive way to obtain fetal cells for testing.

A certain genetic disorder appears to be inherited from mother to child. What are two possible modes of inheritance for this disorder?

This could be maternal inheritance with the disorder being caused by a mutation in the mitochondrial DNA. This could also be a disorder related to epigenetics, where the paternal copy of a gene is inactivated so the only active copy is the maternal copy which, if mutated, would cause the disorder.

You are working in a research lab on the organism Drosophila melanogaster and you have been given the job of determining the relative positions of three linked genes for your undergraduate research project. A graduate student in the lab tells you that gene a is 5.0 centimorgans from gene b, and that gene c is 2.7 centimorgans from gene b. What should you do to finish mapping these three genes? What are the likely results that may occur, and how would you interpret such results?

To complete this task, you should perform crosses that will allow determination of the recombination frequency between gene a and gene c, which will give the map distance these genes. There are two likely outcomes: an a-c map distance of about 2.3 centimorgans and an a-c map distance of about 7.7 centimorgans. The results may not be exactly these values, but they should be close to one or the other. A result of close to 2.3 centimorgans would indicate that gene c is between genes a and b, while a result of close to 7.7 centimorgans would indicate that gene b is between genes a and c.

Your cousin learns that she is a carrier for phenylketonuria (PKU). Her husband's biological mother has PKU. She asks you to explain what this could mean if she and her husband have children and asks what, if anything, could be done to best protect any child that may have. What would you tell her?

First, I would tell her that PKU is easily treated by dietary adjustments, and that with proper guidance and behavior, people with PKU are able to lead full, productive, and essentially normal lives. I would tell her that PKU demonstrates autosomal recessive inheritance; therefore, her husband must be a carrier. He received a normal PKU allele from his father, and the mutant PKU allele from his mother. The normal copy from his father prevents him from being affected. I would explain to her that, as a carrier, she is in the same situation--she has one normal allele and one abnormal allele. Since the odds of passing on any allele are essentially the same as flipping a coin, any child that she has will have a 50% chance of receiving a normal allele and a 50% chance of receiving an abnormal allele. The same will apply for her husband. Since having PKU requires inheriting an abnormal allele from both parents, each of their children will have a 25% chance of having PKU. Then, I would tell her that nearly all hospitals in the United States screen newborn infants for PKU, and she should ensure any child she has is tested. If the results of the test indicate the child has PKU, she should follow the dietary instructions for the child closely to protect the child's brain development.

Turner Syndrome (XO) is the only viable monosomy found in humans. Some sex chromosome trisomies are also viable, including XXX, XXY, and XYY. There are no viable autosomal monosomies, and a small handful of viable autosomal trisomies exist, such as trisomy 21 (Down syndrome), and trisomy 13 and 18, both of which are lethal within the first year of life. Clearly, aneuploidy is not tolerated well in humans. Speculate as to why there are so few viable monosomies (only one) in humans, and why there are so few viable trisomies. Also, speculate as to why aneuploidy involving the sex chromosomes is better tolerated than those involving autosomes.

Monosomic cells are missing an entire chromosome. Because only one monosomy is viable, this suggests human cells must require two of each chromosome to function normally. Possibly, some autosomal genes must be present in two copies to achieve adequate expression levels in order to be functional for the cell. Trisomic cells have an extra copy of a chromosome. Due to the low tolerance for human trisomies, human cells seem to require only two copies of each gene, and having three copies of some genes is lethal as there may be too much gene function. The few viable autosomal trisomies are either lethal shortly after birth (trisomy 13 and 18) or have a variety of associated symptoms, including a slightly shortened lifespan (trisomy 21). During human development, all X chromosomes beyond one per cell are inactivated. Therefore, aneuploidy is better tolerated when involving sex chromosomes because there is already a natural process in place for partially “haploidizing” those chromosomes. Despite this, there are symptoms associated with XO, XXY, XYY, and XXX individuals.