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CHAPTER 15. THE CHROMOSOMAL BASIS OF INHERITANCE. I. Concept 15.1: Behavior of Chromosomes. A. Background 1. Genetics 1860’s— Mendel proposed that discrete inherited factors segregate and assort independently during gamete formation 2. Cytology

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chapter 15



i concept 15 1 behavior of chromosomes
I. Concept 15.1: Behavior of Chromosomes

A. Background

1. Genetics

1860’s—Mendelproposed that discrete inherited factors segregate and assort independently during gamete formation

2. Cytology

1875—Cytologists worked out process of mitosis

1890’s—Cytologists worked out process of meiosis

3. Genetics

1900—Correns, von Tschermak, and de Vries independently discovered Mendel’s work


4. Cytology and Genetics

1902—2 areas converge as Walter Sutton, Theodor Boveri, and others noticed parallels between the behavior of Mendel’s factors and the behavior of chromosomes:

  • Chromosomes and genes both present in pairs in diploid cells.
  • Homologous chromosomes separate and allele pairs segregate during meiosis.
  • Fertilization restores the diploid condition for both.

5. Chromosomal Theory of Inheritance is based on these observations. According to this theory:

Mendelian factors (genes) are located on chromosomes.

 Chromosomes segregate and assort independently during meiosis

b contributions to the chromosomal theory
B. Contributions to the Chromosomal Theory

1. Walter Flemming(1882)

  • First to observe chromosomes in nuclei of dividing salamander cells
  • Called process “mitosis”

2. August Weismann (1887)

  • Each gamete has half the number of chromosomes as a fertilized egg.
  • Proposed that a special division process reduced the chromosome number by one half

3. Theodore Boveri(1888)

  • Determined that chromosomes were essential for fertilization and development
  • Discovered the centriole
  • Actually observed meiosis in cells of Ascaris

4. Walter Sutton (1902)

  • Found relationship between meiosis and Mendel’s laws
  • Predicted gene linkage

5. Thomas Hunt Morgan (1910)

  • Began experiments with Drosophila melanogaster (fruit fly)
  • Discovered X-linked (sex linked) inheritance
  • Discovered sex determination (X and Y chromosomes)
  • Provided convincing evidence that Mendel’s factors are located on chromosomes
c morgan and drosophila
C. Morgan and Drosophila

1. Reasons for using Drosophila

  • Easy to raise
  • Produce a large number of offspring
  • Short life cycle (2 weeks)
  • Mutants easily recognized
  • Only 4 pairs of chromosomes (3 pr. of autosomes, 1 pr. of sex chromosomes XX female, XY male)

2. Sex-linked Traits

  • Wildtype—normal character phenotype

Ex: w+-red eyes

  • Mutant phenotype

-alternatives to wild type

-due to mutations in wild type gene

Ex: w-white eyes

  • Discovered by Morgan
  • Refers to genes on the X chromosome
  • From this experiment, Morgan deduced that:

a.Eye color is linked to sex.

b. The gene for eye color is located on the X chromosome.


Fig. 15-4b





All offspring

had red eyes






3. Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support for the chromosome theory of inheritance

ii concept 15 2 sex linked genes
II. Concept 15.2 Sex-Linked Genes
  • Heterogameticsex—produces 2 kinds of gametes which determine the sex of offspring
  • Homogameticsex—produces 1 kind of gamete with respect to sex chromosome
  • In humans and some other animals, there is a chromosomal basis of sex determination
  • Each gamete has only 1 sex chromosome.

A. The Chromosomal Basis of Sex

  • In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
  • Only the ends of the Y chromosome have regions that are homologous with the X chromosome

The Chromosomal Basis of Sex

3. The SRY gene on the Y chromosome codes for the development of testes

  • SRY- Sex determining Region of Y
    • -The presence of this gene on the Y chromosome
    • codes for the development of testes. In the absence of
    • this gene, the gonads develop into ovaries

B. Systems of Sex Determination

1. X-Y

  • Mammals, humans, Drosophila
  • ♂--heterogametic (XY)
  • ♀--homogametic (XX)
  • Y chromosome determines sex of offspring

2. X-O

  • Grasshopper, cricket, roach, and other insects
  • ♂--heterogametic (XO)
  • ♀--homogametic (XX)

3. Z-W

  • Birds, some fish, some insects—butterflies and moths
  • ♀--heterogametic (ZW)
  • ♂--homogametic (ZZ)

4. Haplo-diploidy

  • Bees and ants
  • Have no sex chromosomes
  • ♀--develops from fertilized egg (2n)
  • ♂--develops parthenogeneticallyfrom unfertilized egg (n)

C. Inheritance of Sex-Linked Genes

  • The sex chromosomes have genes for many characters unrelated to sex
  • A gene located on the X chromosome is called a sex-linked gene
  • Genes on the Y chromosome are called holandric genes and are found in males only
  • In humans, sex-linked usually refers to a gene on the larger X chromosome
  • Sex-linked genes follow specific patterns of inheritance

6. For a recessive sex-linked trait to be expressed:

  • A female needs two copies of the allele
  • A male needs only one copy of the allele

7. Sex-linked recessive disorders are much more common in males than in females

8. If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is homozygous.

9. A heterozygous female is a carrier.

10. Males only need 1 allele of a sex-linked trait to show the trait.

11. Males are hemizygous (only one copy of a gene is present in a diploid organism)


Fig. 15-7

N= normal color vision







12. Fathers pass sex-linked alleles to only and all of their daughters

XA—Normal gene

Xa—Sex-linked gene

XAXA—Normal Female

XAXa—Carrier Female

XaXa—Affected Female

XAY—Normal Male

XaY—Affected Male

14. If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex.
  • Daughters who do not have the disorder will be carriers, whereas males without the disorder will be completely free of the recessive allele
15 examples of sex linked traits
15. Examples of Sex-linked Traits

a. Color Blindness


-can’t distinguish certain colors

-red-green most common

b. Duchenne’s Muscular Dystrophy


-atrophy of muscle

c. Hemophilia


-blood fails to clot because of lack of a clotting factor

d x inactivation in females
D. X-inactivation in Females

1. Female mammals have only one fully functional X chromosome in diploid cells.

2. Proposed by Mary F. Lyon and known as the Lyon Hypothesis

3. Each of the embryonic cells inactivates one of the two X chromosomes.

4. Inactive X contracts into densely staining object called a Barr Body.

5. Ex:

  • Mosaic coloration of calico cats
  • Normal sweat gland development in humans
e other disorders associated with sex
E. Other Disorders Associated with Sex

1. Sex-limited Traits

  • Appear exclusively in one sex
  • Ex: uterine cancer in ♀; prostate cancer in ♂

2. Sex-influenced Traits

  • Expression is influenced by presence of ♀ or ♂ sex hormones
  • Acts as a dominant in one sex and recessive in the other
  • Ex: ♂--baldness, stomach ulcers;

♀--breast cancer

both—length of index finger as compared to ring finger

-dominant in ♂ shorter index finger

-dominant in ♀ longer index finger

iii concept 15 3 linked genes
III. Concept 15.3: Linked Genes
  • Each chromosome has hundreds or thousands of genes
  • Genes located on the same chromosome that tend to be inherited together are calledlinked genes
  • Do not assort independently
  • Dihybrid crosses deviate from expected 9:3:3:1 phenotypic ratio

A. Genetic Recombination

1. Production of offspring with new combinations of traits different from those combinations found in the parents (crossing over)

2. Results from the events of meiosis and random fertilization

B. Recombination of Unlinked Genes: Independent Assortment

  • Parentaltypes—progeny that have the same phenotype as one or the other of the parents
  • Recombinanttypes—progeny whose phenotypes differ from either parent

Parental Types- offspring that inherit a phenotype that matches

one of the parental phenotypes.

Gametes from yellow-round

heterozygous parent (YyRr)





Gametes from green-

wrinkled homozygous

recessive parent (yyrr)











Recombinant Types or Recombinants- offspring that have new

combinations of phenotypes.


P YyRr x yyrr (testcross)

(yellow, round) (green, wrinkled)

F1 ¼ YyRr ¼ yyrrParental types 50%

¼ yyRr ¼ YyrrRecombinant types 50%

  • When half the progeny are recombinants, there is a 50% frequency of recombination.
  • A 50% frequency of recombination usually indicates that the two genes are on different chromosomes, because it is the expected result if the two genes assort randomly.
C. Recombination of Linked Genes: Crossing OverCrossing Over- accounts for the recombination of linked genes.

b—black body vg—vestigial wings

b+--gray body vg+--wild type wings

b+ b vg+ vg x b b vg vg (testcross)

(gray, normal wings) (black, vestigial wings)


What type of ratios would you expect to see in the testcross

offspring if the genes were located on different chromosomes?

What if they were located on the same chromosome and parental

alleles are always inherited together?


Recombination Frequency (RF) = # recombinants x 100

total # offspring

  • Proposed by Morgan
  • Process of crossing over during meiosis accounts for the recombination of linked genes (genes on same chromosome)
  • Crossing over—breakage and exchange of corresponding segments between homologous chromosomes

--results in new allelic combination

  • Probability of crossing over (recombination) between two genes is proportional to the distance separating those genes
  • The closer together two genes are, the less likely that a cross over will occur.
  • Proved by A. H. Sturtevant
d chromosome mapping
D. Chromosome Mapping

1. Recombination frequencies are used to construct chromosome maps [show locations of genes on a particular chromosome (linear)]

2. Sturtevant constructed chromosome maps for Drosophila using recombination frequencies

3. 1 map unit = 1% recombination frequency (now called centimorgans)

  • Genetic Map- an ordered list of the genetic loci along a particular chromosome.
  • Linkage Map- a genetic map based on recombination frequencies. Displays order but not precise location.
    • Distances are expressed in map units- equivalent to 1% recombination frequency. (Centimorgans)

4. How to use XO (crossover) data to construct a chromosome map:

  • Ex: RF

b vg 17%

cn b 9%

cn vg 9.5%

a. Establish the distance between the genes with the highest RF


b. Determine RF between third gene and first.


c. Consider the two possible placements of the third gene.




d. Determine the RF between the third gene and the second gene to eliminate the incorrect sequence



--correct sequence b-cn-vg


5. If linked genes are so far apart on a chromosome that the RF is 50%, they are indistinguishable from unlinked genes that assort independently.

  • Can map such genes if RF can be determined between those two genes and intermediate genes.

6. Maps from XO data give relative positions of linked genes

7. Cytological mapping pinpoints actual location of genes and real distance between them.

  • May differ from XO maps in distance but not sequence.
iv concept 15 4 alternations of chromosome number or structure
IV. Concept 15.4: Alternations of Chromosome Number or Structure
  • Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
  • Meiotic errors and mutagens can cause major chromosomal changes such as altered chromosome numbers or altered chromosomal structure.

A. Alterations of Chromosomal Number

1. Nondisjunction

      • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis
      • Results in one gamete receiving two of the same type of chromosome (n+1) and the other gamete receiving none (n-1)

2. Aneuploidy

  • Aneuploidyresults from the fertilization of gametes in which nondisjunction occurred
  • Offspring with this condition have an abnormal number of a particular chromosome
  • a chromosomal aberration in which one or more chromosomes are present in extra copies or deficient in number
  • A monosomiczygote has only one copy of a particular chromosome
  • A trisomiczygote has three copies of a particular chromosome
      • Ex: Down’s Syndrome or Trisomy 21

3. Polyploidy

  • Polyploidyis a condition in which an organism has more than two complete sets of chromosomes
    • a chromosomal alteration in which the organism possesses more than two complete chromosome sets.
      • Triploidy (3n) is three sets of chromosomes
      • Tetraploidy(4n) is four sets of chromosomes
  • Polyploidy is common in plants, but not animals
  • Polyploids are more normal in appearance than aneuploids
b alterations of chromosomal structure
B. Alterations of Chromosomal Structure

1. Breakage of a chromosome can lead to four types of changes in chromosome structure:

a. Deletion removes a chromosomal segment


c. An inversion occurs if the fragment reattaches to the original chromosome in reverse order.

Inversion reverses a segment within a chromosome


C. Chromosomal Alterations in Human Disease

1. Alterations of chromosome number and structure are associated with some serious disorders

2. Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond

3. These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy


4. Examples of Autosomal Aneuploidy:

a. Down’s Syndrome

  • Trisomy 21
  • Related to age of parent
  • Most common birth defect in U. S. (1/700 births)

b. Patau Syndrome

  • Trisomy 13

c. Edward’s Syndrome

  • Trisomy 18

5. Examples of Sex Chromosome Aneuploidy: (less severe)

a. Klinefelter’s Syndrome--Genotype usually XXY

b. Extra Y--XYY

c. Trisomy X (metafemales)--XXX

d. Turner’s Syndrome (monosomy X)--XO

6. Examples of Deletions:

a. Cri du chat Syndrome--Deletion on chromosome 5

7. Examples of Translocations:

a. Chronic Myelogenous Leukemia (CML)--Portion of chromosome 22 switched with a fragment of chromosome 9

b. Type of Down’s Syndrome--Translocation from chromosome 21 to chromosome 15

v concept 15 5 exceptions to chromosome theory
V. Concept 15.5: Exceptions to Chromosome Theory
  • There are two normal exceptions to Mendelian genetics
  • One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus
v concept 15 5 exceptions to chromosome theory1
V. Concept 15.5: Exceptions to Chromosome Theory

A. Genomic Imprinting

  • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits
  • Such variation in phenotype is called genomic imprinting
  • Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production
  • “a variation in phenotype depending on whether an allele is inherited from the male or female parent.

4. It appears that imprinting is the result of the methylation (addition of –CH3) of DNA

5. Genomic imprinting is thought to affect only a small fraction of mammalian genes

6. Most imprinted genes are critical for embryonic development


B. Inheritance of Organelle Genes

  • Extranuclear genes (or cytoplasmic genes)are genes found in organelles in the cytoplasm
  • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules
  • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg
  • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant

5. Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems

6. Cytoplasmic genes described in plants by Karl Correns (1909)

you should now be able to
You should now be able to:
  • Explain the chromosomal theory of inheritance and its discovery
  • Explain why sex-linked diseases are more common in human males than females
  • Distinguish between sex-linked genes and linked genes
  • Explain how meiosis accounts for recombinant phenotypes
  • Explain how linkage maps are constructed

Explain how nondisjunction can lead to aneuploidy

  • Define trisomy, triploidy, and polyploidy
  • Distinguish among deletions, duplications, inversions, and translocations
  • Explain genomic imprinting
  • Explain why extranuclear genes are not inherited in a Mendelian fashion