chapter 15 the chromosomal basis of inheritance. fig. 15-1 the location of a particular gene can be...

Chapter 15Chapter 15The Chromosomal Basis of The Chromosomal Basis of

InheritanceInheritance

Fig. 15-1

• The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Mendelian inheritance has its physical Mendelian inheritance has its physical basis in the behavior of chromosomesbasis in the behavior of chromosomes

The The chromosome theory of inheritance chromosome theory of inheritance states:states: Mendelian genes have specific loci (positions) on Mendelian genes have specific loci (positions) on

chromosomeschromosomes

Chromosomes undergo segregation and independent Chromosomes undergo segregation and independent assortmentassortment

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-2b

0.5 mm

Meiosis

Metaphase I

Anaphase I

Metaphase II

Gametes

LAW OF SEGREGATIONThe two alleles for each gene separate during gamete formation.

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

14

yr 1 4Yr1

4 YR

3 3

F1 Generation

14

yR

R

R

R

R

RR

R

R R R R

R

Y

Y

Y Y

Y

YY

Y

YY

YY

yr r

rr

r r

rr

r r r r

y

y

y

y

y

y y

yyyy

All F1 plants produceyellow-round seeds (YyRr)

1

2 2

1

Correlating Behavior of a Gene’s Alleles Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pairwith Behavior of a Chromosome Pair

male flies with white eyes (mutant) were male flies with white eyes (mutant) were crossed with females with red eyes (wild crossed with females with red eyes (wild type)type) The FThe F11 generation all had red eyes generation all had red eyes

The FThe F22 generation showed the 3:1 red:white eye generation showed the 3:1 red:white eye

ratio, but only males had white eyesratio, but only males had white eyes

It was determined that the white-eyed It was determined that the white-eyed mutant allele must be located on the X mutant allele must be located on the X chromosomechromosome


The Chromosomal Basis of SexThe Chromosomal Basis of Sex there are two varieties of sex there are two varieties of sex

chromosomes: a larger X chromosome and chromosomes: a larger X chromosome and a smaller Y chromosomea smaller Y chromosome

The The SRY SRY gene on the Ygene on the Ychromosome codes for chromosome codes for the development of testesthe development of testes

Females = XXFemales = XXMales = XYMales = XY

Ovum = an X chromosomeOvum = an X chromosomeSperm = either an X or a Y Sperm = either an X or a Y chromosomechromosome


Inheritance of Sex-Linked GenesInheritance of Sex-Linked Genes

The sex chromosomes have genes for The sex chromosomes have genes for many characters unrelated to sexmany characters unrelated to sex

A gene located on either sex A gene located on either sex chromosome is called a chromosome is called a sex-linked genesex-linked gene

In humans, sex-linked usually refers to a In humans, sex-linked usually refers to a gene on the larger X chromosomegene on the larger X chromosome


Sex-linked genes follow specific patterns Sex-linked genes follow specific patterns of inheritanceof inheritance

For a recessive sex-linked trait to be For a recessive sex-linked trait to be expressedexpressed Female needs two copies of the allele (XrXr) Female needs two copies of the allele (XrXr) A male needs only one copy of the allele (XrY)A male needs only one copy of the allele (XrY)

Sex-linked recessive disorders (more Sex-linked recessive disorders (more common in males): common in males): Color blindness, HemophiliaColor blindness, Hemophilia


Fig. 15-7

(a) (b) (c)

XNXN XnY XNXn XNY XNXn XnY

YXnSpermYXNSpermYXnSperm

XNXnEggs XN

XN XNXn

XNY

XNY

Eggs XN

Xn

XNXN

XnXN

XNY

XnY

Eggs XN

Xn

XNXn

XnXn

XNY

XnY

Each chromosome has hundreds or Each chromosome has hundreds or thousands of genesthousands of genes

Genes located on the same chromosome Genes located on the same chromosome that tend to be inherited together are that tend to be inherited together are called called linked geneslinked genes

Linked genes tend to be inherited together Linked genes tend to be inherited together because they are located near each other because they are located near each other

on the same chromosomeon the same chromosome


Recombination of Unlinked Genes Recombination of Unlinked Genes Independent Assortment of Independent Assortment of

ChromosomesChromosomes Mendel observed that combinations of traits in Mendel observed that combinations of traits in

some offspring differ from either parentsome offspring differ from either parent Offspring with a phenotype matching one of the Offspring with a phenotype matching one of the

parental phenotypes are called parental phenotypes are called parental typesparental types Offspring with nonparental phenotypes (new Offspring with nonparental phenotypes (new

combinations of traits) are called combinations of traits) are called recombinant recombinant typestypes, or , or recombinantsrecombinants


Fig. 15-UN2

YyRr

Gametes from green-wrinkled homozygousrecessive parent ( yyrr)

Gametes from yellow-roundheterozygous parent (YyRr)

Parental-type

offspring

Recombinantoffspring

yr

yyrr Yyrr yyRr

YR yr Yr yR

Recombination of Linked Genes: Recombination of Linked Genes: Crossing OverCrossing Over

It was proposed that some process must It was proposed that some process must sometimes break the physical connection sometimes break the physical connection between genes on the same chromosomebetween genes on the same chromosome

That mechanism was the That mechanism was the crossing over crossing over of of homologous chromosomeshomologous chromosomes


Animation: Crossing OverAnimation: Crossing Over

Fig. 15-10Testcrossparents

Replicationof chromo-somes

Gray body, normal wings(F1 dihybrid)

Black body, vestigial wings(double mutant)

Replicationof chromo-somes

b+ vg+

b+ vg+

b+ vg+

b vg

b vg

b vg

b vg

b vg

b vg

b vgb vg

b vg

b+ vg+

b+ vg

b vg+

b vg

Recombinantchromosomes

Meiosis I and II

Meiosis I

Meiosis II

b vg+b+ vgb vgb+ vg+Eggs

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

b+ vg+

b vg b vg

b vg b+ vg

b vg b vg

b vg+

Sperm

b vg

Parental-type offspring Recombinant offspring

Recombinationfrequency =

391 recombinants2,300 total offspring

100 = 17%

Alterations of chromosome number or Alterations of chromosome number or structure cause some genetic disordersstructure cause some genetic disorders

Large-scale chromosomal alterations often Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) lead to spontaneous abortions (miscarriages) or cause a variety of developmental disordersor cause a variety of developmental disorders


Abnormal Chromosome NumberAbnormal Chromosome Number

In In nondisjunctionnondisjunction, pairs of homologous , pairs of homologous chromosomes do not separate normally chromosomes do not separate normally during meiosisduring meiosis

As a result, one gamete receives two of As a result, one gamete receives two of the same type of chromosome, and the same type of chromosome, and another gamete receives no copyanother gamete receives no copy


Fig. 15-13-3

Meiosis I

Nondisjunction

(a) Nondisjunction of homologous chromosomes in meiosis I

(b) Nondisjunction of sister chromatids in meiosis II

Meiosis II

Nondisjunction

Gametes

Number of chromosomes

n + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n

Alterations of Chromosome StructureAlterations of Chromosome Structure

Breakage of a chromosome can lead to Breakage of a chromosome can lead to four types of changes in chromosome four types of changes in chromosome structure:structure: Deletion Deletion removes a chromosomal segmentremoves a chromosomal segment Duplication Duplication repeats a segmentrepeats a segment Inversion Inversion reverses a segment within a reverses a segment within a

chromosomechromosome Translocation Translocation moves a segment from one moves a segment from one

chromosome to anotherchromosome to another


Fig. 15-15

DeletionA B C D E F G H A B C E F G H(a)

(b)

(c)

(d)

Duplication

Inversion

Reciprocaltranslocation

A B C D E F G H

A B C D E F G H

A B C D E F G H

A B C B C D E F G H

A D C B E F G H

M N O C D E F G H

M N O P Q R A B P Q R

Down Down Syndrome Syndrome

(Trisomy 21)(Trisomy 21) Down Down

syndrome syndrome is is a condition a condition that results that results from three from three copies of copies of chromosome chromosome 2121


Sex ChromosomesSex Chromosomes

Nondisjunction of sex chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid produces a variety of aneuploid conditionsconditions

Klinefelter syndrome Klinefelter syndrome is the result of an is the result of an extra chromosome in a male, producing extra chromosome in a male, producing XXY individualsXXY individuals

Monosomy X, called Monosomy X, called Turner syndromeTurner syndrome, , produces X0 females, who are sterile; it is produces X0 females, who are sterile; it is the only known viable monosomy in the only known viable monosomy in humans humans


chapter 15 the chromosomal basis of inheritance. fig. 15-1 the location of a particular gene can be...

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