Chapter 15: The Chromosomal Basis of Inheritance

If a female is heterozygous for a particular gene located on the X chromosome, how many of her cells will express one allele? The other? How can mosaicism be observed in humans?

- about 1/2 - 1/2 - recessive X linked mutation that prevents development of sweat glands; woman who is heterozygous has some patches of normal skin and some patches of skin lacking sweat glands

What percent frequency for recombination is observed in test crosses for two genes that are unlinked? What is the physical basis of recombination between unlinked genes?

-50% -random orientation of homologous chromosomes at metaphase I of meiosis, which leads to independent assortment of the two unlinked genes

_________________________________ occurs when a reciprocal translation occurs during mitosis of cells that are precursors of white blood cells. Exchange of a large portion of chromosome 22 with a small fragment from a tip of chromosome 9 produces a much shortened, easily recognized chromosome 22, known as the __________________________________. Such an exchange causes caner by creating a new "fused" gene that leads to uncontrolled cell cycle progression.

-Chronic Myelogenous leukemia (CML) -Philadelphia chromosome

Why do aneuploid conditions affecting the sex chromosomes tend to upset genetic balance less than those involving autosomes?

-Y chromosome carries relatively few genes -Extra copies of the X chromosome simply become inactivated as Barr bodies

The fact that mitochondrial disorders are inherited only from the mother suggests a way to avoid passing along these disorders. Explain.

-chromosomes of egg of affected mother could be transferred to egg of healthy donor with chromosomes removed -"two mother" egg can be fertilized by sperm from prospective father and transplanted into womb of prospective mother, becoming an embryo with three parents

What alterations of chromosome structure most commonly occur during meiosis? Which tend to be harmful?

-deletions and duplications -duplications and translocations

What did Sturtevant hypothesize that the recombination frequency depend on? What did he assume in this hypothesis?

-distance between genes on a chromosome -random event, with chances of crossing over are approximately equal on all points of a chromosome

How are mitochondrial genes passed on in most animals and plants? (Consider which parent they come from)

-mitochondria passed onto a zygote come from cytoplasm of the egg and the few mitochondria contributed by sperm appear to be destroyed in egg by autophagy

Not all of a eukaryotic cell's genes are located on nuclear chromosomes or even in the nucleus; some genes are located in _______________________. What are these genes called?

-organelles in the cytoplasm -extranuclear or cytoplasmic genes

Among which organisms is polyploidy common? What function does it serve?

-plant kingdom -plant evolution

What disorders can ensue if defects occur in proteins produced by mitochondrial DNA?

-reduce the amount of ATP cells can make and cause a number of human disorders, most commonly in the nervous and muscular systems

What do large scale chromosomal alterations in humans or mammals often result in? Can plants or animals tolerate genetic defects better?

-spontaneous abortion of a fetus and individuals with defects exhibit developmental disorders -plants

What is the distinction between linked genes and sex-linked genes?

-two or more genes on the same chromosome that tend to be inherited together -single gene on a sex chromosome

How are genes located far apart on a chromosome mapped?

Adding recombination frequencies from crosses involving closer pairs of genes lying between the two distant genes.

The inactive X in each cell of a female condenses into a compact object called a _______________, which lies along the inside of the nuclear envelope. Most genes of X chromosome that forms this are not expressed.

Barr body

What statistical test can be used to determine if two genes assort independently or are linked from a F1 dihybrid cross?

Chi-Square Test

How is sex defined in the chromosomal basis of inheritance?

Classification into a group with a shared set of anatomical and physiological traits; largely determined by chromosomes

______________________ results from a specific deletion in chromosome 5. A child born with this deletion is intellectually disabled, has a small head with unusual facial features and has a cry that sounds like the mewing of a distressed cat. Such individuals normally die in infancy or early childhood

Cri du chat (cry of the cat)

_______________________________ is an X linked disorder which affects about 1/3,500 males born in the US. The disease is characterized by progressive weakening of muscles and loss of coordination.

Duchenne muscular dystrophy

How might a tetraploid cell originate?

Failure of a 2n zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions would then produce a 4n embryo

How might a triploid cell originate?

Fertilization of an abnormal diploid egg produced by nondisjunction of all of its chromosomes

Why does the offspring only express one allele of an imprinted gene?

Genes are imprinted differently in sperm and eggs

_________________ is an X linked recessive disorder defined by the absence of one or more proteins required for blood clotting.

Hemophilia

People with ________________________ have male sex organs, but testes are abnormally small and man is sterile. These individuals have a genotype of XXY on their sex chromosomes and though the extra X is inactivated, they tend to experience breast enlargement, other female characteristics, and subnormal intelligence.

Klinefelter sndrome

The mitochondrial disease __________________________________ can produce sudden blindness in people as young as their 20s or 30s. The four mutations found to cause this disorder affect oxidative phosphorylation during cellular respiration, a crucial function for the cell.

Leber's hereditary optic neuropathy

________________ also known as ________________________ occurs about once in every 2,500 female births and is the only known viable monosomy in humans. They are phenotypically female but sex organs do not mature. When provided with HRT, they do develop secondary sex characteristics and they tend to be of normal intelligence.

Monosomy X, Turner's Syndrome

Describe the haplo-diploid system of sex determinism

No sex chromosomes in most species of bees and ants. Females develop from fertilized eggs and diploid. Males develop from unfertilized eggs and are haploid; they have no fathers

Do genes on organelle DNA display Mendelian inheritance?

No, their activities are not directed by same rules that direct the distribution of nuclear chromosomes during meiosis

Does methylation always silence a gene?

No, though heavily methylated genes tend to be inactive leading to silencing of an allele. In some alleles, indirect mechanisms involving chromatin structure and protein-DNA interactions can activate alleles

Why do male and female cells have the same effective dose (one active copy) of most X linked genes?

One of the X chromosomes in each cell in females becomes inactivated in early stages of embryonic development.

A gene on the Y chromosome- called __________ - is required for the development of testes. Without this, gonads develop into ovaries, even in an XY embryo

SRY (sex-determining region of Y)

Describe the Z-W system of sex determinism

Sex chromosomes present in egg, not sperm, determine sex of offspring. Sex chromosomes designated as W and Z. Females are ZW and males are ZZ

Describe the X-Y system of sex determinism

Sex of offspring depends on whether the sperm cell has an X or Y chromosome

___________________ at either end of the Y chromosome are the only regions that are homologous with regions on the X. These regions allow the X and Y chromosomes in males to pair and behave like homologs during meiosis in the testes.

Short segments

The genes on the Y chromosome are expressed for....?

Testis; many required for normal testicular functioning and production of normal sperm

Describe the X-0 system of sex determinism

There is one possible sex chromosome: the X chromosome, females have XX, males have one X, expressed as X0

The development of female gonads requires a gene called ______________, which encodes a protein that promotes ovary development.

WNT4

The human X chromosome contains approximately 1,100 genes, which are called ________________________, while genes on Y chromosome are called _______________________. There are very few of these, so very few disorders are transferred via the Y chromosome.

X linked genes, Y linked genes

A particular region of each X chromosome contains several genes involved in the inactivation process. The two regions, one on each X chromosome, associate briefly with each other in each cell at an early stage of embryonic development. Then one of the genes, called _____________________, does what?

XIST (X-inactive specific transcript) becomes active ONLY on chromosome that becomes Barr body. Multiple copies of RNA product apparently attach to the X chromosome on which they are made, eventually almost covering it, promoting X inactivation.

About 1/1000 males is born with a genotype of ________. They undergo normal sexual development and do not exhibit any well-defined syndrome but tend to be taller than average.

XYY

What provides the raw material on which natural selection works?

abundance of genetic variation

The frequency of Down syndrome increases with...?

age of the mother

After an X chromosome is inactivated in a particular cell, what occurs to mitotic dependents of the cell?

all descendants have the same inactive X

If either of the aberrant gametes unites with a normal one at fertilization, the zygote will have an abnormal number of a particular chromosome, known as __________________.

aneuploidy

Inactivation of the X chromosome involves modification of the DNA and histones, including....?

attachment of methyl groups (-CH3) to DNA nucleotides

When are Barr body chromosomes reactivated?

cells that give rise to eggs, resulting in every female gamete having an active X after meiosis

Symptoms of down syndrome include...?

characteristic facial features, short stature, correctable heart defects, developmental delays, increased chance of developing leukemia and Alzheimer's diseases -less risk of developing high BP, atherosclerosis, stroke, solid tumors, etc.

According to the __________________________________________, Mendelian genes have specific loci along chromosomes and it is the chromosomes that undergo segregation and independent assortment

chromosome theory of inheritance

What process accounts for the recombination of linked genes?

crossing over

_______________________ of chromosomes locate genes with respect to chromosomal features, such as stained bands, that can be seen in the microscope.

cytogenic maps

A ______________ occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes.

deletion

A _____________________ occurs when an organism possesses and extra segment to a sister or nonsister chromatid

duplication

Errors in meiosis or damaging agents such as radiation can cause breakage of a chromosome, which can lead to four different types of changes in chromosome structure

duplication, deletion, inversion, translocation

Researchers have traced Duchenne muscular dystrophy to the absence of a key muscle protein called _________________ and have mapped the gene for this protein to a specific locus on the _________ chromosome.

dystrophin, X

Most genomically imprinted genes are crucial for which process in mammals?

embryonic development -improper imprinting can result in abnormal development and certain cancers

Sturtevant determined that it was possible to map genes in his prediction that: The ______________ apart two genes, the ______________ probability that a crossover will occur between them and therefore the _____________ the recombination frequency.

farther, higher, higher

A _______________________ is an ordered list of the genetic loci along a particular chromosome developed by Alfred H. Sturtevant

genetic map

_________________________ is the production of offspring with combinations of traits that differ from those found in either P generation parent

genetic recombination

It is possible for genes very far apart on a chromosome to appear to be ____________________________, and thus, assort independently, though they are _______________________________.

genetically unlinked, physically unconnected

A variation in phenotype depending on whether an allele is inherited from the mother or father is called _________________________________. (most are on autosomes)

genomic imprinting

__________________________ occurs during gamete formation and results in the silencing of a particular allele of certain genes.

genomic imprinting

If an X linked trait is due to a recessive allele, a female will express the phenotype only if she is __________________ for the allele. Because males only have one locus, the terms homozygous and heterozygous lack meaning for describing their X linked genes; term ___________________ is used in such cases. Any male receiving the recessive allele from his mother will express the trait

homozygous, hemizygous

Meiosis and random fertilization generate genetic variation among offspring of sexually reproducing organisms due to....

independent assortment of chromosomes, crossing over in Meiosis I, and possibility of any sperm fertilizing any egg

A chromosomal fragment may reattach to the original chromosome but in the reverse orientation, producing an __________________.

inversion

Comparing a linkage map with a physical map or a cytogenic map or the same chromsome, we find that the _________ order of genes in identical in all maps but the _____________ is not.

linear, spacing

A genetic map based on recombination frequencies is called a _________________________.

linkage map

Genes located near one another on the same chromosome are often inherited together. These are called ________.

linked genes

Sturtevant expresses the distances between genes on a linkage map in _________________, defining one as equivalent to a 1% recombination frequency.

map units

Which organelles in the DNA contain small circular DNA molecules that carry a number of genes?

mitochondria, chloroplasts

The mitochondrial disease, _______________________ causes weakness, intolerance of exercise, and muscular deterioration

mitochondrial myopathy

Nondisjunction can also occur during _______1_______. If such an error takes place in early embryonic development, then the aneuploid condition is passed along by ______1______ to a large number of cells and is likely to have a substantial effect on the organism.

mitosis

An aneuploid zygote is said to be ________________ for a chromosome if it has just one copy of that chromosome

monosomic

Females consist of a _________ of two cell types: those with active X derived from father and those with active X derived from mother

mosaic

Traits that are alternative to the wild type are called ___________ phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in the wild type allele.

mutant

A _________________ occurs when members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. Each gamete receives two of the same type of chromosomes and another gamete receives no copy. Other chromosomes are distributed normally

nondisjunction

A linkage map does portray the __________ of genes along a chromosome, but it does not accurately portray the precise _________________ of those genes.

order, locations

Filial phenotypes that match either of the parental phenotypes are known as ________________________

parental types

In general _______________ are more normal in appearance than ___________________.

polyploids, aneuploids

A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division.

polyploidy

What do most mitochondrial genes code for?

protein complexes of ETC and ATP synthase

British geneticist Mary Lyon demonstrated that the selection of which X chromosome will form the Barr body occurs _______________ and _____________________ in each embryonic cell present at the time of X inactivation.

randomly, independently

_________________________ are phenotypes of offspring that are nonparental, they do not match either of the parental phenotypes.

recombinant types/ recombinants

An gene located on either sex chromosome is called a __________________.

sex linked gene

Imprinting can involve either ________________ an allele in one type of gamete (eggs or sperm) and __________________ it in another. In many cases, the imprint seems to consist of _________________ groups added to _____________ nucleotides in one of the alleles, which may silence the allele.

silencing, activating, methyl (-CH3) groups, cytosine

Morgan's finding of the correlation between a particular ___________ and an individual's _______ provided support for the chromosome theory of inheritance: namely that a specific gene is carried on a specific chromosome.

trait, sex

A fourth possible result of chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement known as a ____________________.

translocation

___________________ indicates three chromosomal sets and ________________ indicates four chromosomal sets.

triploidy (3n), tetraploidy (4n)

An aneuploid zygote is said to be _________________ for a chromosome if it has three copies for that chromosome

trisomic

Females with _____________________, which occurs one in approximately 1,000 births are healthy and have no unusual physical features other than being slightly taller than average. They are at risk for learning disabilities but are fertile.

trisomy X (XXX)

The phenotype for a character most commonly observed in natural populations is the ____________________.

wild type