Chap 15 AP Bio

Chromosomes and Inheritance

Locating Genes on Chromosomes

Genes Are located on chromosomes Can be visualized using certain

techniques

Karyotype

Scientist Discoveries

Morgan worked with fruit flies Because they breed at a high rate A new generation can be bred every

two weeks They have only four pairs of

chromosomes

Sex Linked Trait (White Eyes)

Morgan first observed and noted Wild type, or normal, phenotypes that

were common in the fly populations Traits alternative to the wild type

Are called mutant phenotypes

Figure 15.3

Fly Experiment

In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes

The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes

Males more often have sex linked traits

Sex Linked White Eyes

Figure 15.4

The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes,and half had red eyes.

Morgan then bred an F1 red-eyed female to an F1 red-eyed male toproduce the F2 generation.

RESULTS

PGeneration

F1

Generation

X

F2

Generation

Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes.EXPERIMENT

Morgan determined that

Genes that are close together on the same chromosome are linked and do not assort independently

Unlinked genes are either on separate chromosomes of are far apart on the same chromosome and assort independently

Parentsin testcross

b+ vg+

b vg

b+ vg+

b vg

b vg

b vg

b vg

b vg

Mostoffspring

X

or

A linkage map

Is the actual genetic map of a chromosome based on recombination frequencies

Recombinationfrequencies

9% 9.5%

17%

b cn vgChromosome

The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossoverwould “cancel out” the first and thus reduce the observed b–vg recombination frequency.

In this example, the observed recombination frequencies between three Drosophila gene pairs (b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between the other two genes:

RESULTS

A linkage map shows the relative locations of genes along a chromosome.APPLICATION

TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between twogenetic loci is proportional to the distance separating the loci. The recombination frequencies used to constructa linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depictedin Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unitequivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data.

Figure 15.7

Crossing over

The farther apart genes are on a chromosome The more likely they are to be separated

during crossing over The frequency of cross over between two

linked genes is proportional to the distance between them.

Linkage Maps are a genetic map based on recombination frequencies.

Linked genes are on the same chromosome. New combinations of linked genes are due to

crossing over.

Sex Determination

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 +XY

Sperm Ova

44 +XX

44 +XY

Zygotes(offspring)

Sex Chromosomes

The sex chromosomes Have genes for many characters

unrelated to sex A gene located on either sex

chromosome Is called a sex-linked gene

SRY gene is present on the Y chromosome and triggers male development.

Different systems of sex determinationAre found in other organisms

Ex:

Figure 15.9b–d

22 +XX

22 +X

76 +ZZ

76 +ZW

16(Haploid)

16(Diploid)

(b) The X–0 system

(c) The Z–W system

(d) The haplo-diploid system

Sex Linked disorders

Some recessive alleles found on the X chromosome in humans cause certain types of disorders Color blindness Duchenne muscular dystrophy Hemophilia

Red-Green colorblindness is a sex-linked recessive trait.

X inactivation

In mammalian females One of the two X chromosomes in each

cell is randomly inactivated during embryonic development and remains as a dark compact body in the nucleus of female somatic cells. This is called a Barr Body

Female X inactivation

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

Barr Bodies in Female somatic cells

Female Genotype?

Chromosomal Alterations

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

Nondisjunction

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

Results of Nondisjunction

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

Polyploidy

Is a condition in which there are more than two complete sets of chromosomes in an organism

Alterations of Chromosome Structure

Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion Duplication Inversion Translocation

Chromosome mutations

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.

Down syndrome

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

Figure 15.15

Aneuploidy of Sex Chromosomes

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

Certain cancersAre caused by translocations of chromosomes

Cri du chat Is a disorder caused by a deletion in a chromosome

Figure 15.16

Normal chromosome 9Reciprocal

translocation

Translocated chromosome 9

Philadelphiachromosome

Normal chromosome 22 Translocated chromosome 22

Genetic Disorders

Phenylketonuria PKU- autosomal recessive, can be controlled by regulating diet

Huntington’s- dominant single allele, does not appear until 35-45yrs of age. If one of your parents is diagnosed what is the probability you will have it?

Tay-Sachs disease (autosomal recessive)- can’t break down lipids and this affects brain development

Sickle Cell Anemia (autosomal recessive)- caused by a substitution of the wrong amino acid

Cystic Fibrosis- defect in the membrane proteins that normally function in Chloride ion transport

A pedigree

Is a family tree that describes the interrelationships of parents and children across generations

Inheritance patterns of particular traitsCan be traced and described using pedigrees

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

Pedigrees

Pedigree a diagram of a family tree showing the occurrence of heritable characters in parents offspring over multiple generations. Can also be used to make predictions

about future offspring

Karyotype- used to identify disorders

Genetic testing

Pedigrees, Karotypes, amniocentesis, and chronic villi sampling are all ways to test for genetic disorders.