Apbiochapter15

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The Chromosomal Basis of Inheritance (Chapter Fifteen)

I. RELATING MENDELISM TO CHROMOSOMESA. Mendelian Inheritance Has Its Physical Basis in the Behavior of

Chromosomes During Sexual Life Cycles

1. Connections between Mendels discoveries and chromosomal behaviorbegan to be made in around 1900. In 1902, Walter Sutton, TheodorBoveri, and others independently formed a chromosome theory of

inheritance,

a. which states that Mendelian geneshave specific loci on chromosomes,

and it is the chromosomes that

undergo segregation and

independent assortment.

B. Morgan Traced a Gene to a SpecificChromosome

1. Thomas Morgan was the first toassociate a specific gene with a specificchromosome.

2. Morgans Choice of ExperimentalOrganism

a. Morgan chose a species of fruit fly,Drosophila melanogaster, which

feeds on the fungi growing on fruit. A

single mating of fruit flies produces

hundreds of offspring that can mature in two weeks.

i. Additionally, fruit flies only have four pairs of chromosomes, easilydistinguishable with even a light microscope.

ii. Female fruit flies have a homologous pair of X chromosomes, whilemales have one X and one Y chromosome.

iii. He established that the normal phenotype for a character is calledthe wild type, such as red eyes in Drosophila.

iv. Alternatives to the wild type are called mutant phenotypes,because they are due to alleles assumed to have originated as

changes or mutations in the wild-type allele.

3. Discovery of Sex Linkagea. After Morgan discovered a male fly with white eyes, he mated it with a

red-eyed female. The resulting offspring had red eyes, which

suggested that red eyes were dominant. When he bred the firstgeneration to each other, he found the usual 3:1 phenotypic ratio.

i. However, the white-eye trait only showed up in males. Morgandeduced that the gene affecting his white-eyed mutant was located

exclusively on the X chromosome, where there was no

corresponding eye-color locus on the Y chromosome. Since the

mutant allele was recessive, females could only have white eyes if

she received the allele on two chromosomes.


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ii. Since a male only received one copy of thegene, there would be no wild- type allele to

offset the recessive allele.

b. Sex-linked genes are those located on a sexchromosome.

C. Linked Genes Tend to Be Inherited Together BecauseThey are Located On the Same Chromosome1. Genes located on the same chromosome tend to be

inherited together in genetic crosses because the

chromosome is passed on as a unit these are

said to be linked genes.

a. In Morgans crossing of body color and wingsize, he found that the two traits were usually

inherited together in specific combinations

because the genes are on the same chromosomes.

D. Independent Assortment of Chromosomes and Crossing Over ProduceGenetic Recombinants1. Production of offspring with new combinations of traits inherited from

two parents is genetic recombination.

2. The Recombination of Unlinked Genes: Independent Assortment ofChromosomes

a. When Mendel studied dihybrid plants, he learned that some offspringhave combinations of traits that do not match either parent in the

parental generation.

i. For example, consider crossing a pea plant with yellow-roundseeds (heterozygous for both, YyRr) and a pea plant with green-

wrinkled seeds (homozygous for both, yyrr).

Half the offspring should inherit a phenotype matching eitherone of the parents they are called parental types. But other

phenotypes display new combinations of seed shape and color:

they are called recombinants.

Since 50% of all offspring are recombinants, geneticists saythere is a 50% frequency of recombination.

b. A 50% frequency of recombination is observed for any two geneslocated on different chromosomes.

3. The Recombination of Linked Genes: Crossing Overa. Linked genes do not assort independently because they tend to move

together through meiosis and fertilization.

i. However, recombination between linked genes can occur. Morganproposed that there is some mechanism that exchanges segments

between homologous chromosomes that occasionally break

linkage between two genes.

ii. Subsequent experiments showed that such an exchange (crossingover) accounts for recombination of linked genes.


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E. Geneticists Can Use Recombination Data To Map a Chromosomes GeneticLoci

1. One of Morgans students, Alfred H. Sturtevant, discovered a method forconstructing a genetic map, an ordered list of the genetic loci a long a

particular chromosome.

a. He hypothesized that the recombination frequencies calculated fromexperiments reflect the distances between genes on a chromosome.b. Assuming that the chance of crossing over is approximately equal at

all points on a chromosome, he

predicted that the farther apart two

genes are, the higher the probability

that a crossover will occur between

them and therefore the higher the

recombination frequency.

c. A genetic map based on recombination frequencies is specificallycalled a linkage map.

i. He expressed the distance between genes in map units, definingone map unit as equivalent to a 1% recombination frequency.2. It is also possible for more than one crossover to occur and thus cancel

out a first crossover, reducing the observed number of recombinant

offspring.

3. Since the frequency of crossing over is not actually uniform over thelength of the chromosome, map units do not have absolute size. Linkage

map portrays a sequence of genes on a chromosome, but not the precise

location of genes.

4. Cytological maps of chromosomes locate genes with respect tochromosomal features.

II. SEX CHROMOSOMESA. The Chromosomal Basis of Sex Varies With The Organism

1. In humans and other mammals, there are two varieties of sexchromosomes, designated X and Y.

a. As previously stated, females result from two X chromosomes, while amale results from a combination of X and Y.

b. In both testes and ovaries, the two sex chromosomes separate duringmeiosis, and each gamete receives one. Each ovum contains one X

chromosome, while half the sperm contain and X and half the sperm

contain a Y chromosome.

c. The SRY gene (sex-determining region ofY) prompts development of gonads intotestes rather than ovaries. It is a chemical

trigger that codes for a protein that

regulates many other genes.

2. In the X-O system, females are XX while malesare XO. This occurs in grasshoppers, crickets,

roaches, and some other insects.


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3. In birds, some fishes, and some insects, the variable that determines sexis present in the ovum, where males are ZZ and females are ZW (so as not

to be confused with X and Y).

4. In most species of bees and ants, females develop from fertilized (diploid)ova, while males developed from unfertilized eggs (haploid).

B. Sex-Linked Genes Have Unique Patterns ofInheritance1. In humans, the term sex-linked usually refers

to genes on the X chromosome.

a. If a sex-linked trait is due to a recessiveallele, a female will express the phenotype

only if she is a homozygote.

i. Because males only have one locus, anymale receiving the recessive allele from

his mother will express the trait.

Thus, far more males than femaleswill have disorders from sex-linkedrecessives.

2. Sex-Linked Disorders in Humansa. Duchenne muscular dystrophy is characterized by a progressive

weakening of muscles and loss of coordination, and is caused by the

absence of a key muscle protein called dystrophin.

b. Hemophilia is a sex-linked recessive trait defined by the absence ofone or more of the proteins required for blood clotting. Bleeding is

prolonged because a firm clot is slow to form.

3. X Inactivation in Female Mammalsa. Although female mammals inherit two X chromosomes, one X

chromosome in each cell becomes almost complexly inactivatedduring embryonic development.

i. As a result, cells of females and males have the same effective dose(one copy) of genes with loci on the X chromosome.

ii. The inactive X in each cell of a female condenses into a compactobject, called a Barr body,

which lies along the inside of the nuclear envelope. Most of the genes of the X chromosome forming the Barr body

are not expressed, but some remain active.

b. It is completely random in each embryonic cell as to which Xchromosome will form the Barr body.

i. Females will then contain a mosaic of two types of cell: ones withan active X from the father, and ones with an active X from the

mother.

ii. After an X chromosome is inactivated in a particular cell, allmitotic descendants of that cell will have the same inactive X. This

results in the mosaicism seen in the coloration of a tortoiseshell

cat.


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c. Inactivation involves attachment of methyl groups to cytosine.i. A gene called XIST (X-inactive

specific transcript) is found on

the Barr-body chromosome.

ii. The genes product produces anRNA molecule that attach to theX chromosome and cover it. This

seems to initiate X inactivation.

III.ERRORS AND EXCEPTIONS INCHROMOSOMAL INHERITANCE

A. Alterations of Chromosome Number ofStructure Cause Some Genetic Disorders

1. Alterations of Chromosome Number; Aneuploidy and Polyploidya. There is an occasional error called a nondisjunction, where members

of a pair of homologous chromosomes do not move apart properly

during meiosis I or sister chromatids fail to separate during meiosis II.

i. One gamete will receive two of the same type of chromosomewhile the other will receive no copy.ii. If either of the irregular gametes unites with a normal one, the

offspring will have an abnormal chromosome number, known as

aneuploidy.

If there are three copies of a chromosome in the fertilized eggs,the aneuploid cell is trisomic for that chromosome, while it is

monosomic for the chromosome if there is a missing

chromosome.

b. Some organisms have more than two complete chromosome sets. Thegeneral term is polyploidy, which the specific terms triploidy and

tetraploidy for three or four chromosomal sets.i. Polyploidy is common in the plant kingdom, while they are much

less common in the animal kingdom.

Having entire extra sets seems to disrupt balance less than oneextra or missing chromosome.

2. Alterations of Chromosome Structurea. A deletion occurs when a chromosomal

fragment is lost during cell division,

which results in the absence of certain

genes.

b. A fragment might become attached asan extra segment to a sister chromatidand result in duplication.

c. A chromosomal fragment might alsoreattach to the original chromosome in

reverse orientation, producing an

inversion.


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d. A fourth possible result is translocation, in which the fragment wouldjoin a nonhomologous chromosome.

e. Deletions and duplications are especially likely to occur duringmeiosis.

i. Homologous chromatids sometimes break and join at incorrectplaces, with one partner giving up more genes than it receives. Alarge deletion may result in the absence of essential genes.

f. Duplications and translocations also tend to have harmful effects.g. Inversions and translocations can alter phenotype because gene

expression is influenced by location among neighboring genes

3. Human Disorders Due to Chromosomal Alterationsa. Most aneuploidy in humans would have disastrous results and are

naturally aborted before birth.

i. One aneuploid condition, Down syndrome, results from an extracopy of chromosome 21.

Down syndrome results in characteristic facial features, shortstature, heart defects, susceptibility to respiratory infection,and mental retardation.

b. Nondisjunction of sex chromosomes produces a variety of aneuploidconditions in humans.

i. Most of these tend to upset genetic balance less than aneuploidconditions involving autosomes.

c. An extra X chromosome in a male, producing an XXY, results inKlinefelter syndrome, in which males will have abnormally small

testes and be sterile.

i. The syndrome also includes other feminine body characteristics.ii. Males with an extra Y chromosome (XYY) tend to be somewhat

taller than average.iii. Females with trisomy X (XXX) are healthy and cannot be

distinguished from XX females except by karyotype.

iv. Monosomy X, called Turner syndrome, is the only known viablemonosomy in humans, and results in sterile females.

d. Alteration in chromosome structure can also cause disorders, such ascri du chat, where part of chromosome 5 is deleted.

i. Children with this specific deletion are mentally retarded, with asmall head and unusual facial features, and a cry that sounds like

the mewing of a cat.

e. Chromosomal translocation can result in certain cancers, such aschronic myelogenous leukemia, which affects cells that give rise towhite blood cells.

i. A portion of chromosome 22 has switched places with a smallfragment from a tip of chromosome 9.

B. The Phenotypic Effects of Some Mammalian Genes Depend on Whether Theywere Inherited From the Mother or Father (Imprinting)

1. Most of the time, a specific allele will have the same effect whetherinherited from the mother or father. However, this is not always the case.


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2. In the Prader-Willi syndrome, there is mental retardation, obesity, shortstature, and unusually small hands and feet.

3. People with the Angelman syndrome exhibit spontaneous laughter, jerkymovements, and other motor and mental systems.

a. This seems to be a result of deletion of a particular segment ofchromosome 15.

b. If the abnormal chromosome is from the father, they will have Prader-Willi syndrome, whereas they will have Angelmann syndrome if it is

from the mother.

4. A process called genomic imprinting can explain the Prader-Willi/Angelmann enigma and some similar phenomena.

a. In this process, a gene on one chromosome is somehow silenced,while its allele on the homologous chromosome is left free to be

expressed.

b. The same alleles may have different effects on offspring depending onwhether they were from the sperm or the egg.

5. Imprinting involves the addition of methyl groups to the cytosinenucleotides of one of the alleles.a. Heavily methylated genes are usually inactive and suggests that in

these cases, the animal uses the allele that is not imprinted. In other

cases, absence of methylation plays a role in silencing it.

6. Another genomic imprinting disorder is one called fragile X syndrome,where there is an abnormal X chromosome whose tip hangs on by a thin

thread of DNA.

C. Extranuclear Genes Exhibit a Non-Mendelian Pattern of Inheritance1. Not all of a eukaryotic cells genes are located in the nucleus.2. Extranuclear genes are found on small circles of DNA in mitochondria and

in plants plastids, which reproduce themselves and transmit genes todaughter organelles.

3. They do not follow Mendelian inheritance.4. Karl Correns found that coloration of offspring was determined only by

the maternal parent (source of seeds) rather than the paternal parent

(pollen source).

5. Maternal inheritance is also the rule for mitochondrial genes in mammals.Scientists have learned that mutations in mitochondrial DNA cause a

number of rare human disorders.

6. Defects in proteins of the ETC or ATP synthase may result in energydeprivation.

7. Mitochondrial mutations could also contribute to some cases of diabetesand heart disease, or disorders that debilitate the elderly, such as

Alzheimers disease.

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