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Chapter 24: Patterns of Chromosome Inheritance

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Viewing the Chromosomes

A karyotype is a display of chromosomes paired according to their size, location of the centromere, and staining patterns.

A karyotype reveals abnormalities in chromosome number or structure.

Humans have 23 pairs of chromosomes; 22 pairs of autosomes and one pair of sex chromosomes.

Females are XX and males are XY.

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Normal male karyotype

2 copies of 22 autosomal chromosomes

2 copies of 1 sex chromosome

Fig. 24.1

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Down syndrome karyotype

Total of 47 chromosomes

(Trisomy 21)

Fig. 24.1

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Changes in Chromosome Number

Nondisjunction occurs when: 1.) both members of a homologous pair

go into the same daughter cell or

2.) when sister chromatids fail to separate and both daughter chromosomes go into the same gamete.

The result is a trisomy or a monosomy.

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trisomy monosomy

Nondisjunction in meiosis I

Fig. 24.2

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monosomy

trisomy

Nondisjunction in meiosis II

Fig. 24.2

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The Barr body is an inactive X chromosome and is seen whenever more than one X chromosome is present (i.e., XX, XXY, XXX).

Cells of females function with a single chromosome just as those of males do.

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Down SyndromeDown syndrome is caused by trisomy 21,

a result of nondisjunction.

Nondisjunction risk increases after age 40.

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Changes in Sex Chromosome Number

The presence/absence of a Y chromosome determines maleness.

An abnormal number of sex chromosomes is the result of inheriting too many or too few X or Y chromosomes.

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Turner syndrome: females with one X chromosome; XO.

Klinefelter syndrome: males that have two or more X chromosomes and a Y chromosome.

A Poly-X female has more than two X chromosomes and extra Barr bodies.

Jacobs syndrome males are XYY.

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Changes in Chromosome Structure

A mutation is a permanent genetic change.

Radiation, organic chemicals, or even viruses may cause chromosomes to break, leading to mutations.

A change in chromosome structure is a chromosome mutation.

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Chromosomal mutations include: 1.) deletion

2.) duplication

3.) inversion

4.) translocation

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DeletionsDeletions occur when part of the chromosome is lost.

An individual with a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome may result.

Fig. 24.5

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Duplication: a chromosome segment is repeated in the same chromosome.

Duplications

Fig. 24.6

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Translocation

Translocation is exchange of chromosomal segments between two, nonhomologous chromosomes (Ex: part of chromosome 2 is swapped with part of chromosome 7).

Fig. 24.7

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InversionInversion involves a segment of a

chromosome being turned 180 degrees; the reverse sequence of alleles can alter gene activity.

AA BB CCDDEEFF

AA BB CC DD EE FF NORMAL

INVERSION

See Fig. 24.8

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Sex-Linked TraitsTraits controlled by genes on the X or Y

chromosomes are called sex-linked (i.e., on sex chromosomes).

An allele on the X chromosome that is in the region where the Y chromosome has no alleles will express even if recessive; it is termed X-linked.

A female would have to have two recessive genes to express the trait; a male would only need one.

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XX = female XY = male

X Y

X

X

XX XY

XX XY

Y Chromosome determines sex of offspring

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Male FemaleX Y X X

No alternate alleles available.

Comparison of the sex chromosomes

Alternate alleles available.

Alternate alleles available.

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X-Linked AllelesThe key for an X-linked problem shows

the allele attached to the X as in: XB = normal vision

Xb = color blindness. Females with the genotype XBXb are

carriers because they appear to be normal but each son has a 50% chance of being color blind depending on which allele the son receives.

XbXb and XbY are both colorblind.

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Cross involving an X-linked alleleA woman is not color blind but her father was color blind.

She has a child with a man that is not color blind. Could her children be color blind?

Woman XB Xb Man XB Y

XB

XB

Xb

Y

XB XB XB Y

XB Xb Xb Y

Color blind

Not Color blind

1

3

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X-Linked Disorders

In pedigree charts that show the inheritance pattern for X-linked recessive disorders, more males than females have the trait.

X-linked recessive disorders include red-green color blindness, muscular dystrophy, and hemophilia.

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X-linked recessive pedigree chart(Color Blindness)

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Color Blindness

Three types of cones are in the retina detecting red, green, or blue.

Genes for blue cones are autosomal; those for red and green cones are on the X chromosome.

Males are much more likely to have red-green color blindness than females.

About 8% of Caucasian men have red-green color blindness.

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Other X-linked Disorders

Muscular dystrophy is characterized by the wasting of muscles.

Hemophilia blood does not clot normally.

Fragile X syndrome one of the most common forms of mental retardation.

-CGG Repeats