chap 15 ap bio
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Chap 15 AP Bio. Chromosomes and Inheritance. Figure 15.1. 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 - PowerPoint PPT PresentationTRANSCRIPT
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Chap 15 AP Bio
Chromosomes and Inheritance
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Locating Genes on Chromosomes
Genes Are located on chromosomes Can be visualized using certain
techniques
Karyotype
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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
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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
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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
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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
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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
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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
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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.
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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)
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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.
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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
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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.
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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
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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
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Barr Bodies in Female somatic cells
Female Genotype?
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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
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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)
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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
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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
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Polyploidy
Is a condition in which there are more than two complete sets of chromosomes in an organism
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Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion Duplication Inversion Translocation
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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.
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Down syndrome
Is usually the result of an extra chromosome 21, trisomy 21
Figure 15.15
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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
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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
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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
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A pedigree
Is a family tree that describes the interrelationships of parents and children across generations
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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)
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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
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Karyotype- used to identify disorders
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Genetic testing
Pedigrees, Karotypes, amniocentesis, and chronic villi sampling are all ways to test for genetic disorders.