Homework for Wednesday, Jan 30 P. 285: problems 4 and 7 In Student Study Guide: –Interactive...

Homework for Wednesday, Jan 30Homework for Wednesday, Jan 30• P. 285: problems 4 and 7• In Student Study Guide:

– Interactive questions: 15.2, 15.3, 15.4– Test your Knowledge:2, 6, 7, 20

CLASS WARM-UP ACTIVITYCLASS WARM-UP ACTIVITYYou get 5 minutes to prepare for the class You get 5 minutes to prepare for the class answers to one of the following.Teams of 2-answers to one of the following.Teams of 2-

3:3: • 1.What is the Chromosome theory of inheritance?

• 2. Distinguish between “wild-type”; “mutant” phenotypes, and give an example

• 3. Where are the red/white eye alleles located in fruit flies? What did Morgan discover?…….

• 4. explain Fig. 15.3

• 5. Distinguish between sex-linked genes and linked genes.

• 6. What is Genetic recombination?

• 7. Refer to Fig. 15.4-give the phenotype of the wild-type female, and the double mutant male (draw on board)

CHAPTER 15 THE CHROMOSOMAL

BASIS OF INHERITANCE

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section A: Relating Mendelism to Chromosomes

1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles

2. Morgan traced a gene to a specific chromosome3. Linked genes tend to be inherited together because they are

located on the same chromosome4. Independent assortment of chromosomes and crossing over

produce genetic recombinants5. Geneticists use recombination data to map a chromosome’s

genetic loci

The Chromosomal Theory The Chromosomal Theory of Inheritanceof Inheritance

• Genes have specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment

•Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome

• early 20th century.• Drosophila melanogaster, a fruit fly species that

eats fungi on fruit.– prolific breeders – generation time of two weeks.– Fruit flies have three pairs of autosomes and a pair of

sex chromosomes (XX in females, XY in males).

2. Morgan traced a gene 2. Morgan traced a gene to a specific chromosometo a specific chromosome

– Morgan discovered a single male fly with white eyes instead of the usual red.

• wild type: the normal or most frequently observed phenotype

• mutant phenotypes: Alternatives


Fig. 15.2

• White Eyed Male x Red-eyed female all red eyed offspring

• F1 offspring X F1 classic 3:1 phenotypic ratio in the F2 offspring.

• Surprisingly, the white-eyed trait appeared only in males.– All the females and half the males had red eyes.

• Morgan concluded that a fly’s eye color was linked to its sex.


• Morgan deduced that – eye color is linked to

sex AND – the gene for eye color

is only located on the X chromosome.

– Sex-Linked Genes


Fig. 15.3

• linked genes: Genes located on the same chromosome.

• They tend to be inherited together because the chromosome is passed along as a unit.

Linked GenesLinked Genes


Figure 15.4 Evidence for linked genes in Figure 15.4 Evidence for linked genes in DrosophilaDrosophila

Figure 15.5a Recombination due to crossing overFigure 15.5a Recombination due to crossing over

Genetic RecombinationGenetic Recombination• Production of offspring with new combos of

traits different from those combos found in the parents

• Parental types• Recombinants

• Recombinant frequency(# recombinants/total offspring)100

• 1 map unit = 1% recombinant frequency

• Can result from:• 1. Independent assortment of

chromosomes (the recombination of unlinked genes).

2. crossing over ( the recombination of linked genes)

Genetic recombination:


Figure 15.5b Recombination due to crossing overFigure 15.5b Recombination due to crossing over

Recombinant Recombinant FrequencyFrequencyProblemProblem• A wild type fruit fly (heterozygous

for gray body color and red eyes)• was mated with a black fruit fly

with purple eyes. • What is the recombinant frequency

for these genes?• Wild type – 721• Black purple – 751• Gray purple – 49• Black red - 45

Recombinant Recombinant FrequencyFrequency

Problem - AnswerProblem - Answer•Total offspring - 1566•Parental types – 1472•Recombinants – 94

• Frequency = 94/1566 *100 = 6.0%

Recap-Morgan’s test-cross Recap-Morgan’s test-cross (2 traits)(2 traits)

• Most of the F2 offspring looked like the parents( because the genes were linked-same chromosome)

• Explaining why the were greater number of recombinant phenotypes (resulted from some other force of nature-----crossing over)


– Under independent assortment (genes not linked) the testcross should produce a 1:1:1:1 phenotypic ratio.

– If completely linked, we should expect to see a 1:1:0:0 ratio with only parental phenotypes among offspring.


5. Mapping chromosomes5. Mapping chromosomes

• Alfred Sturtevant

• Linkage map: a genetic map (GENES & THEIR RELATIVE LOCTIONS) based upon recombination frequencies.


• The farther apart two genes are, the higher the probability that a crossover will occur between them & therefore a higher recombination frequency.


• A linkage map provides an imperfect picture of a chromosome.– Map units indicate relative distance

and order, not precise locations of genes.

• Cytological maps.– These indicated the positions of genes with

respect to chromosomal features.-like banding


three fruit fly genes, body color (b), wing size (vg), and eye color (cn).– The recombination frequency between cn and b is 9%.– The recombination frequency between cn and vg is

9.5%.– The recombination frequency between b and vg is 17%.


Fig. 15.6

• Map units: the distance between genes

• 1 map unit =1% recombination frequency

• Centimorgan• relative distance and order, not

precise locations of genes.

– .


Practice: Interactive Question Practice: Interactive Question 15.2 from student work book15.2 from student work book

• Recombination frequencies for gene pairs. Create linkage map, show map units between gene loci.

• J,k 12%• J,m 9%• K,l 6%• l,m 15%

answeranswer

6 6 9

K L J M

• Some genes on a chromosome are so far apart that a crossover between them is virtually certain.

• Frequency of recombination reaches has a maximum value of 50%

• Same as recom. Freq. as if found on separate chromosomes, and are inherited independently.


Homework for Wednesday, Jan 30Homework for Wednesday, Jan 30• P. 285: problems 4 and 7• In Student Study Guide:

– Interactive questions: 15.2, 15.3, 15.4– Test your Knowledge:2, 6, 7, 20

CHAPTER 15 THE CHROMOSOMAL BASIS OF

INHERITANCE


Section B: Sex Chromosomes1. The chromosomal basis of sex varies with the organism2. Sex-linked genes have unique patterns of inheritance

1. In the X-Y system, X and Y rarely undergo crossing over.

The chromosomal basis of sex The chromosomal basis of sex varies with the organismvaries with the organism


• 2. X-0 system – some insects:– XX=female; XO-male

• Z-W system – Birds:– females determine sex

ZZ-male; ZW-females

• the haplo-diploid system: – Bees, ants: no sex chromosomes-

Females develop from fertilized eggs (2n);

Males from unfert. eggs (haploid)


Fig. 15.8

• In humans, the anatomical signs of sex first appear when the embryo is about two months old.

• In individuals with the SRY gene (sex-determining region of the Y chromosome), the generic embryonic gonads are modified into testes.– Activity of the SRY gene triggers a cascade of

biochemical, physiological, and anatomical features because it regulates many other genes.

– In addition, other genes on the Y chromosome are necessary for the production of functional sperm.

• In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.


The Chromosomal Basis of Sex in The Chromosomal Basis of Sex in HumansHumans

• Males-heterogametic (XY)• Females homogametic ( XX)• Whether an embryo develops into

male of female depends upon the Y chromosome.

• SRY gene– Triggers cascade of events—normal

testicular development– SRY absent-gonads develop into ovaries

2. Sex-linked genes have unique 2. Sex-linked genes have unique patterns of inheritancepatterns of inheritance


Fig. 15.9

Sex-linked traitsSex-linked traits

• Males are hemizygous–More males than females have sex-linked disorders

• Sex-linked disordersSex-linked disorders• 1. Duchenne muscular dystrophy

affects one in 3,500 males born in the United States.– rarely live past their early 20s.– This disorder is due to the absence of an X-

linked gene for a key muscle protein, called dystrophin.

– progressive weakening of the muscles and a loss of coordination.


• 2.Hemophilia sex-linked, recessive absence of one or more clotting factors that normally slow and then stop bleeding.

• prolonged bleeding – Bleeding in muscles and joints can be

painful and lead to serious damage.– treated with intravenous injections of the

missing protein.


Sex Linkage ProblemSex Linkage ProblemHemophilia is caused by a sex-linked

recessive allele. A man with hemophilia marries a woman

with normal clotting factors. What is the probability that they will have a

daughter with hemophilia? What is the probability that they will have a

son with hemophilia? What is the probability that their son will

have hemophilia?

X-Inactivation in FemalesX-Inactivation in FemalesCompensating for the missing X.Lyon Hypothesis:

In female mammals, only one X is fully functional

• Barr body– Inactive X chromosome (random)– condenses– Reactivated in gonadal cells at meiosis

• Mosaic of inactive maternal and paternal X chromosomes (calico cats)

• XIST gene– Only active on the Barr body– Plays a role in inactivation

• In humans, this mosaic pattern is evident in women who are heterozygous for a X-linked mutation that prevents the development of sweat glands.– A heterozygous woman will have patches

of normal skin and skin patches lacking sweat glands.


• Similarly, the orange and black pattern on tortoiseshell cats is due to patches of cells expressing an orange allele while others have a nonorange allele.


Fig. 15.10

Figure 15.10x Calico catFigure 15.10x Calico cat

X-inactivation Produces X-inactivation Produces Tortoiseshell CatsTortoiseshell Cats

• X-linked system with two alleles – heterozygous females express both alleles in a

mosaic pattern, depending on active gene

CHROMOSOMAL BASIS OF CHROMOSOMAL BASIS OF INHERITANCE:INHERITANCE:

ERRORS AND EXCEPTIONSERRORS AND EXCEPTIONS

Nondisjunction and Abnormal Nondisjunction and Abnormal Chromosome NumbersChromosome Numbers

• aneuploidy (an abnormal chromosome number)– normal gamete fuses with a gamete

from a nondisjunction (effect usually severe)

– trisomic 2n + 1 total chromosomes– monosomic 2n - 1 chromosomes.

• Nondisjunction spindle incorrectly separates chromosome during karyokinesis

• usually lethal

PolypoidyPolypoidy• organisms with more than two

complete sets of chromosomes (effect often less severe)

• usually occurs when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes– produces a triploid (3n) zygote (2n + 1n)

Polyploid is Polyploid is Common in Plants, Common in Plants, butbut Rare in Animals Rare in Animals

• polyploidy plays an important role in the evolution of plants– economically important

– at least one species of rodent may be a pro- duct of polyploidy

• Polyploids are more nearly • normal in phenotype than • aneuploids.

Abnormal Chromosome Abnormal Chromosome Numbers and Human Numbers and Human

Disorders Disorders • most aneuploid zygotes are non-viable,

and spontaneously abort early in development– developmental problems result from an

imbalance among gene products• certain aneuploid conditions upset the

balance less, survive to term and beyond– individuals have a set of symptoms - a

syndrome - characteristic of the type of aneuploidy

Down SyndromeDown Syndrome• trisomy 21, three copies of

chromosome 21– affects one in 700 children born in U.S.

• severely alters an individual’s phenotype in specific ways.

Incidence of Trisomy Increases Incidence of Trisomy Increases with Age in Womenwith Age in Women

• usually result from nondisjunction during gamete production in one parent

• frequency of Down syndrome correlates with age of mother– may be linked to some age-dependent

abnormality in spindle checkpoint during meiosis I, leading to nondisjunction

• incidence of other trisomies also increase with maternal age, but are usually lethal if autosomal

Nondisjunction of Sex Nondisjunction of Sex ChromosomesChromosomes

• produces a variety of viable aneuploid conditions in humans

• unlike autosomes, aneuploidy in sex chromosomes upsets genetic balance less severely.– may be because Y chromosome

contains relatively few genes– extra copies of X chromosome become

inactivated as Barr bodies in somatic cells

XX or XY NondisjuntionsXX or XY Nondisjuntions • XXY males Klinefelter’s syndrome,

(1/2000)– sterile individuals with male sex organs– may be feminized. They are of normal

intelligence• XYY males often taller than average• XXX females Trisomy X (1/2000)

– produces healthy females• XO females Turner’s syndrome (1/5000)

– produces phenotypic, but immature females

Structural Changes in ChromosomesStructural Changes in ChromosomesBreakage of a chromosome can lead to four types of changes in chromosome

structure.

• 1. A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division.-Usually lethal

• 2. A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid.


Fig. 15.13a & b

• 3. An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.

• 4. In translocation, a chromosomal fragment joins a nonhomologous chromosome.– Some translocations are reciprocal, others are

not.


Fig. 15.13c & d

• Deletions and duplications are common in meiosis.– Homologous chromatids may break and

rejoin at incorrect places, such that one chromatid will lose more genes than it receives.

• .– Duplications and translocations are

typically harmful.• Reciprocal translocation or inversion

can alter phenotype because a gene’s expression is influenced by its location.


Structural Changes and Structural Changes and Human DisordersHuman Disorders

• cri du chat syndrome caused by a specific deletion in chromosome 5– mentally handicapped, small head,

unusual facial features, cry sounds like the meowing of a distressed cat.

– fatal in infancy or early childhood• chronic myelogenous leukemia

(CML) – CML translocation between tips of

chromosome 22 and chromosome 9

• effects of some mammalian genes depends on whether they were inherited from mother or father

• alleles for most genes have same expression regardless of source (maternal or paternal)

• expression of some traits in mammals, vary depending on origin of inherited alleles.– genes involved are not linked to sex and

may or may not lie on the X chromosome

2. Phenotypic imprinting2. Phenotypic imprinting

Prader-Willi and Angelman Prader-Willi and Angelman SyndromesSyndromes

• both due to a deletion of a specific segment of chromosome 15

• different phenotypic effects are due to genomic imprinting– Prader-Willi syndrome, mental

retardation, obesity, short stature, and unusually small hands and feet• abnormal chromosome from their father.

– Angelman syndrome, spontaneous laughter, jerky movements, and other motor and mental symptoms• abnormal chromosome from mother

Fragile X SyndromeFragile X Syndrome• leads to various degrees of mental

retardation, appears to be subject to genomic imprinting.– abnormal X chromosome in which tip

hangs on by a thin thread of DNA.– disorder affects 1/1,500 males and

1/2,500 females• inheritance of fragile X is complex • syndrome is more common when

abnormal chromosome is inherited from mother– consistent with higher frequency in males– caused by maternal imprinting

Genomic ImprintingGenomic Imprinting• a gene on one homologous

chromosome is silenced, other is expressed

• imprinting status of a given gene depends on whether gene resides in a female or a male– effects of alleles on offspring, depend on

whether they arrive in zygote via ovum or via sperm

• silencing mechanism depends on presence or absence of methylation, not well understood

• involves several hundred mammalian genes– imprinting is critical for normal

development

Genomic Genomic Imprinting is Imprinting is Reset Each Reset Each GenerationGeneration

• each new generation, all imprints are “erased” in gamete-producing cells.

• chromosomes are all “reimprinted” according to individuals sex

• Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.

– These individDevelopmental problems result from an imbalance among gene products.

– These individuals have a set of symptoms - a syndrome - characteristic of the type of aneuploidy.


• Chromosomal translocations • Some individuals with Down

syndrome have the normal number of chromosomes but have all or part of a third chromosome 21 attached to another chromosome by translocation.


• For most genes it is a reasonable assumption that a specific allele will have the same effect regardless of whether it was inherited from the mother or father.

• However, for some traits in mammals, it does depend on which parent passed along the alleles for those traits.– The genes involved are not sex linked and may or

may not lie on the X chromosome.

2. The phenotypic effects of 2. The phenotypic effects of some mammalian genes depend some mammalian genes depend on whether they were inherited on whether they were inherited from the mother or the father from the mother or the father

(imprinting)(imprinting)


• Two disorders with different phenotypic effects, Prader-Willi syndrome and Angelman syndrome, are due to the same cause, a deletion of a specific segment of chromosome 15.– Prader-Willi syndrome is characterized by

mental retardation, obesity, short stature, and unusually small hands and feet.

– These individuals inherit the abnormal chromosome from their father.

– Individuals with Angelman syndrome exhibit spontaneous laughter, jerky movements, and other motor and mental symptoms.

– This is inherited from the mother.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The difference between the disorders is due to genomic imprinting.

• In this process, a gene on one homologous chromosome is silenced, while its allele on the homologous chromosome is expressed.

• The imprinting status of a given gene depends on whether the gene resides in a female or a male.– The same alleles may have different effects on

offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.


• In the new generation, both maternal and paternal imprints are apparently “erased” in gamete-producing cells.

• Then, all chromosomes are reimprinted according to the sex of the individual in which they reside.


Fig. 15.15

• Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.– This disorder is named for an abnormal X

chromosome in which the tip hangs on by a thin thread of DNA.

– This disorder affects one in every 1,500 males and one in every 2,500 females.

• Inheritance of fragile X is complex, but the syndrome is more common when the abnormal chromosome is inherited from the mother.– This is consistent with the higher frequency in males.– Imprinting by the mother somehow causes it.


• Not all of a eukaryote cell’s genes are located in the nucleus.

• Extranuclear genes are found on small circles of DNA in mitochondria and chloroplasts.

• These organelles reproduce themselves.• Their cytoplasmic genes do not display

Mendelian inheritance.– They are not distributed to offspring during

meiosis.

3. Extranuclear genes exhibit a non-3. Extranuclear genes exhibit a non-Mendelian pattern of inheritanceMendelian pattern of inheritance


• Karl Correns first observed cytoplasmic genes in plants in 1909.

• He determined that the coloration of the offspring was determined only by the maternal parent.

• These coloration patterns are due to genes in the plastids which are inherited only via the ovum, not the pollen.


Fig. 15.16

• Because a zygote inherits all its mitochondria only from the ovum, all mitochondrial genes in mammals demonstrate maternal inheritance.

• Several rare human disorders are produced by mutations to mitochondrial DNA.– These primarily impact ATP supply by producing

defects in the electron transport chain or ATP synthase.

– Tissues that require high energy supplies (for example, the nervous system and muscles) may suffer energy deprivation from these defects.

– Other mitochondrial mutations may contribute to diabetes, heart disease, and other diseases of aging.


Homework for Wednesday, Jan 30 P. 285: problems 4 and 7 In Student Study Guide: –Interactive...

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