chapter 15

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 15

• Overview: Locating Genes on Chromosomes

• Genes

– Are located on chromosomes

Figure 15.1


• The chromosome theory of inheritance states that

– Mendelian genes have specific loci on chromosomes

– Chromosomes undergo segregation and independent assortment


Linked Genes

• Concept 15.2: Linked genes tend to be inherited together because they are located near each other on the same chromosome

• Each chromosome

– Has hundreds or thousands of genes


• The farther apart genes are on a chromosome

– The more likely they are to be separated during crossing over


• Concept 15.3: Sex-linked genes exhibit unique patterns of inheritance


The Chromosomal Basis of Sex

• An organism’s sex

– Is an inherited phenotypic character determined by the presence or absence of certain chromosomes


• In humans and other mammals

– There are two varieties of sex chromosomes, X and Y

Figure 15.9a

(a) The X-Y system

44 +XY

44 +XX

Parents

22 +X

22 +Y

22 +X

Sperm Ova

44 +XX

44 +XY

Zygotes(offspring)


Inheritance of Sex-Linked Genes

• The sex chromosomes

– Have genes for many characters unrelated to sex

• A gene located on either sex chromosome

– Is called a sex-linked gene


• Sex-linked genes

– Follow specific patterns of inheritance

Figure 15.10a–c

XAXA XaY

Xa Y

XAXa XAY

XAYXAYa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXaYA

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaYa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

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, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm


• Some recessive alleles found on the X chromosome in humans cause certain types of disorders

– Color blindness

– Duchenne muscular dystrophy

– Hemophilia


X inactivation in Female Mammals

• In mammalian females

– One of the two X chromosomes in each cell is randomly inactivated during embryonic development


• If a female is heterozygous for a particular gene located on the X chromosome

– She will be a mosaic for that character

Two cell populationsin adult cat:

Active X

Orangefur

Inactive X

Early embryo:X chromosomes

Allele forblack fur

Cell divisionand X

chromosomeinactivation

Active X

Blackfur

Inactive X

Figure 15.11


• Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders

• Large-scale chromosomal alterations

– Often lead to spontaneous abortions or cause a variety of developmental disorders


Abnormal Chromosome Number

• When nondisjunction occurs

– Pairs of homologous chromosomes do not separate normally during meiosis

– Gametes contain two copies or no copies of a particular chromosome

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)


• Aneuploidy

– Results from the fertilization of gametes in which nondisjunction occurred

– Is a condition in which offspring have an abnormal number of a particular chromosome


• If a zygote is trisomic

– It has three copies of a particular chromosome

• If a zygote is monosomic

– It has only one copy of a particular chromosome


Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes in chromosome structure

– Deletion

– Duplication

– Inversion

– Translocation


• Alterations of chromosome structure

Figure 15.14a–d

A B C D E F G HDeletion

A B C E G HF

A B C D E F G HDuplication

A B C B D EC F G H

A

A

M N O P Q R

B C D E F G H

B C D E F G HInversion

Reciprocaltranslocation

A B P Q R

M N O C D E F G H

A D C B E F HG

(a) A deletion removes a chromosomal segment.

(b) A duplication repeats a segment.

(c) An inversion reverses a segment within a chromosome.

(d) A translocation moves a segment fromone chromosome to another,nonhomologous one. In a reciprocal

translocation, the most common type,nonhomologous chromosomes exchangefragments. Nonreciprocal translocationsalso occur, in which a chromosome transfers a fragment without receiving afragment in return.


Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number and structure

– Are associated with a number of serious human disorders


Down Syndrome

• Down syndrome

– Is usually the result of an extra chromosome 21, trisomy 21

Figure 15.15


Aneuploidy of Sex Chromosomes

• Nondisjunction of sex chromosomes

– Produces a variety of aneuploid conditions

• Aneuploidy – any abnormal number of a specific chromosome


• Klinefelter syndrome

– Is the result of an extra chromosome in a male, producing XXY individuals

• Turner syndrome

– Is the result of monosomy X, producing an X0 karyotype


Disorders Caused by Structurally Altered Chromosomes

• Cri du chat

– Is a disorder caused by a deletion in a chromosome


• Certain cancers

– Are caused by translocations of chromosomes

Figure 15.16

Normal chromosome 9Reciprocal

translocation

Translocated chromosome 9

Philadelphiachromosome

Normal chromosome 22 Translocated chromosome 22

chapter 15

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