CHAPTER 15 THE CHROMOSOMAL BASIS OF

INHERITANCE

• It was not until 1900 that biology finally caught up with Gregor Mendel.

•Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years before.

History

• Mendel’s hereditary factors are the __________ located on chromosomes.

• Around 1900, cytologists and geneticists and the behavior of Mendel’s factors.

• Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a ____________ theory of inheritance began to take form.

Fig. 15.2

2. Morgan traced a gene to a specific chromosome

• Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the early 20th century.

• Morgan’s model-Drosophila melanogaster, a fruit fly - have three pairs of _______________and a pair of ______ chromosomes (XX in females, XY in males).

Normal is called___________Red eyes White eyes

Non-normal is called__________

Fig. 15.3

• Morgan’s experiments-

Red eye X White eye

F1

3:1 Red:white

But- all __________were white, all females red

All Red eye

F2

Morgan concluded that a fly’s eye color was linked to the ___________________.

= _______

= ________

Know these symbols

Conclude- classic _______________ genetics

Fig. 15.4

Chromosome- 1.5 x 108 base pairs containing about ________ genes

• Genes located on the same chromosome,__________________, tend to be inherited together because the chromosome is passed along as a unit.

3. Linked genes tend to be inherited together because they are located on the same chromosome

0.4% of a chromosome, containing 10 genes

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•Results of crosses with linked genes deviate from those expected according to ____________________________.

• Morgan observed this linkage in ___________ and ____________.

• Gray with wings is wild type

Linked genes

Gray/wings X Black/wingless

Gray/wings

F1

P

X Black/wingless

_______ Equal numbers of each

_______ 5:5:1:1Thus, body color and wing size genes must be_________

Fig. 15.5

• A _____frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

• In contrast, _________genes, genes located on the same_______________, tend to move together through meiosis and ______________.

Morgan’s testcross for body color and wing shape did not _____________to either independent assortment or complete linkage.

• Independent assortment- expect 1:1:1:1 ratio • Completely linked- expect 1:1:0:0 • But observe _______of the flies were recombinants (5:5:1:1) , suggesting _______________linkage.

4. Independent assortment of chromosomes and crossing over produce genetic recombinants

• The actual mechanism, crossing over during_____________, results in the production of more types of gametes than one would

predict by _____________ rules alone.

Fig. 15.6

These unexpected numbers (5:5:1:1) is the basis for constructing a ________________________.

• ________________an ordered list of the genetic loci along a chromosome.

5. Geneticists can use recombination data to map a chromosome’s genetic loci

Sturtevant hypothesis- the ________________of recombinant offspring reflects the distances between genes on a chromosome.

Sturtevant used _____________________________from fruit fly crosses to map the relative position of genes along chromosomes, a ___________________________.

These genes are more likely to travel together

These genes are less likely to travel together

• Recombination frequencies (Rf) determine the relative __________ of genes

• Example: 3 fruit fly genes: body color (b), wing size (vg), and eye color (cn).

• Rf between cn and b is______

• Rf between cn and vg is_______.

• Rf between b and vg is _________• The only possible arrangement of these three genes places the ___________

gene between the other two. Fig. 15.7

• Sturtevant expressed the distance between genes, the recombination frequency, as______________.

• One map unit (sometimes called a_________________) is equivalent to a _____ recombination frequency.

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

• Rf reaches is its maximum value of ______

• genes act as if found on separate chromosomes and are inherited independently.

• For example, seed color and flower color are far enough apart that linkage is not observed.

A A’

b B

A a

B b

Meiosis

• Genes located far apart on a chromosome are mapped by adding the ___________________________ between the distant genes and ____________________genes.

Fig. 15.8

How do we map a gene if linkage is 50% ??

Answer- find another gene that does _______________

48 MU

Greybody

Blackbody

Long antennae

Short antennae

Normal wings

Short wings

Red eyes

Browneyes

67 MU

104 MU

0 MU

37 mu

22 ____or 22% ______ or 22 ___________

• Map units indicate _____________distance and order, not precise locations of genes.

• The frequency of crossing over is not actually uniform over the length of a chromosome.

• A _____________________indicates the positions of genes with respect to chromosomal features.

Gene A is located on this band

Gene B is located here

A stained chromosome15.3

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The distance between A and B can be_________

Both ________maps and physical maps are used to locate disease genes

6. The chromosomal basis of sex

Female XX

Male XYXY or XX

Male Female

Mammals use the X-Y system to determine sex

x

• Each _____ has one X chromosome.• 1/2 the _________have an X chromosome

1/2 sperm have a Y chromosome Thus, each conception has

about a __________ chance of producing a particular sex.

Other creatures may use other “systems”

• Answer- the ____ gene (sex determining region of the Y chromosome)

• determines sex (male).

• located on the ____________________

• regulates many other “male” genes.

• If lack the SRY gene, __________are produced.

Female is the____________________

What determines sex in mammals??

• The X chromosomes is loaded with________.

• One defective gene spells trouble for male

• Why? Because male has only one copy of the gene

7. Sex-linked genes have unique patterns of inheritance

• In contrast, for a recessive sex-linked trait, a female will likely have one ______________ gene and one normal one

• Heterozygous females will be_______________.

• Example- Duchenne muscular dystrophy affects one in 3,500 males born in the United States.

• Affected individuals rarely live past their early 20s.

• Absence of an X-linked gene called _____________

• The disease is characterized by a progressive weakening of the muscles

• Another example: Hemophilia is a __________________ trait defined by the absence of one or more clotting factors.

• prolonged bleeding because a firm clot forms slowly.

• Bleeding in muscles and joints can be painful and lead to serious damage.

• Individuals can be treated with intravenous injections of the missing protein.

• In female mammals only _____ X chromosome is active.

• Therefore, males and females have the same effective _______ (one copy ) of genes on the X chromosome.

• The inactivated X chromosome is a _______________.

• This inactivates most of its genes.

• reactivated in ovarian cells that produce ova.

• After Barr body formation, all descendent cells have the same inactive X.

•If a female is _______________________for a sex-linked trait, approximately half her cells will express one allele and the other half will express the other allele.

• A _______________________mutation-

• A heterozygous woman will have patches of normal skin and skin patches lacking sweat glands.

• _________________cats -orange and black pattern

• ½ cells express an orange allele while others have a nonorange allele.

Examples of X-linked diseases

Fig. 15.11

• Answer- One X chromosome expresses the XIST gene (X-inactive specific transcript) and XIST RNA _______ the chromosome.

mouse autosome

11

12

13

21

24

12

13

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p

q

mouse autosome

11

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How is the X chromosome inactivated?

Xist RNA

Inactivated X chromosome

• Xist is necessary and ________________ for X inactivation (using 450kb YAC)

XIST RNA

•Evidence: If insert Xist transgene on autosome results in an ______________autosome

Figure not in text

• ________________occurs when problems with the meiotic spindle cause errors in daughter cells.

• This may occur if tetrad chromosomes do not separate properly during meiosis I.

• Alternatively, sister chromatids may fail to separate during meiosis II.

8. Alterations of chromosome number or structure cause some genetic disorders

Fig. 15.12

• Nondisjunction results in too many or to few chromosomes termed ________________.

• some gametes receive two of the same type of chromosome and another gamete receives no copy.

• ___________ cells - three copies of a particular chromosome type and have 2n + 1 total chromosomes.

• _____________cells - one copy of a particular chromosome type and have 2n - 1 chromosomes.

• Normally results in embryonic death, but some survive

Organisms with more than two ____________sets of chromosomes, have undergone ________________e.g. 3n or 4n (rare in mammals)

• Polyploids are more nearly normal in phenotype than____________

• A deletion - a chromosome fragment lacking a ________________ is lost during cell division.

• A _____________- a fragment becomes attached as an extra segment to a sister chromatid.

Other chromosome problems

• An inversion - a chromosomal fragment ____________ to the original chromosome but in the reverse orientation.

• In_________________, a chromosomal fragment joins a _________________ chromosome.

Fig. 15.14c & d

•Some translocations are ___________others are not.

• Homozygous embryos with a large ___________ normally die

• A deletion in the X chromosome is ____________in males

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

Results of chromosome errors

Example- Leukemia is due to a growth gene being placed next to an active region resulting in cancer

• Approx. ________ of human embryos are aneuploid and die early in development

Some are viable-• Chromosome 15 trisomy- die at birth• Chromsome 21 trisomy- Down’s syndrome

• Down syndrome- trisomy 21.

• One in 700 children born in the United States.

• result from nondisjunction during gamete production

Fig. 15.15• correlates with the age of the mother.

Karyotype

Phenotype

Extra sex chromosomes are allowed

• _______________________________an XXY male, occurs once in every 2000 live births.

• Male sex organs, but are__________.

• Feminine characteristics; normal intelligence.

• Males with an extra Y chromosome (XYY) tend to somewhat _________ than average.

• Trisomy X (XXX), which occurs once in every 2000 live births, produces _______________ females.

• Monosomy X or Turner’s syndrome (X0), which occurs once in

every 5000 births, produces phenotypic, but immature females.

• XYY and XXYY- males often found in ____________.

• A few genes (not most!!) are expressed only if derived from the mother (or from the father)

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

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• _____________________________

• A gene on ________ homologous chromosome is silenced

• The ________________ status of a given gene depends on whether the gene resides in a female or a male.

• The difference between the disorders is due to___________________________

• 1. Prader-Willi syndrome and Angelman syndrome are due a deletion of a specific segment of ______________________.

• Prader-Willi syndrome - mental retardation, obesity, short stature, and unusually small hands and feet.

• Inherit the abnormal chromosome from their father.

• Angelman syndrome -spontaneous laughter, jerky movements, and other motor and mental symptoms.

• This is inherited from the mother.

Examples of imprinted genes

2. ________________ syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting

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• ________________genes are found on small circles of DNA in mitochondria and ____________.

• These organelles________________ themselves.

• Their cytoplasmic genes do not display Mendelian inheritance.

• They are not distributed to offspring during meiosis.

10. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

• A zygote inherits all its mitochondria only from the_______.

• Mitochondrial mutations may contribute to diabetes, heart disease, and other diseases of aging.