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Chromosomes, Mapping, and the Meiosis-Inheritance Connection

Chapter 13

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Chromosome Theory

Chromosomal theory of inheritance- developed in 1902 by Walter Sutton- proposed that genes are present on

chromosomes- based on observations that homologous

chromosomes pair with each other during meiosis

- supporting evidence was provided by work with fruit flies

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Chromosome Theory

T.H. Morgan isolated a mutant white-eyed Drosophila

red-eyed female X white-eyed male gave a F1 generation of all red eyes

Morgan concluded that red eyes are dominantParental generation

male

Parentalgenerationfemale

F1 progeny all had red eyes

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Chromosome Theory

Morgan crossed F1 females X F1 males

F2 generation contained red and white- eyed flies but all white-eyed flies were male

testcross of a F1 female with a white-eyed male showed the viability of white-eyed females

Morgan concluded that the eye color gene is linked to the X chromosome

Testcross

F1 generationfemale

F2 female progeny had red eyes, only males had white eyes

The testcross revealed that white-eyed femalesare viable. Therefore eye color is linked to the

X chromosome and absent from the Y chromosome

F1 generationmale

Parental generationmale

F1 generationfemale

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Sex Chromosomes

Sex determination in Drosophila is based on the number of X chromosomes

2 X chromosomes = female

1 X and 1 Y chromosome = male

Sex determination in humans is based on the presence of a Y chromosome

2 X chromosomes = female

having a Y chromosome (XY) = male

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Sex Chromosomes

In many organisms, the Y chromosome is greatly reduced or inactive.

genes on the X chromosome are present in only 1 copy in males

sex-linked traits: controlled by genes present on the X chromosome

Sex-linked traits show inheritance patterns different than those of genes on autosomes.

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Sex Chromosomes

Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1.

In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.

Females heterozygous for genes on the X chromosome are genetic mosaics.

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Chromosome Theory Exceptions

Mitochondria and chloroplasts contain genes.

Traits controlled by these genes do not follow the chromosomal theory of inheritance

Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent

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Chromosome Theory Exceptions

Maternal inheritance: uniparental (one-parent) inheritance from the mother

The mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization (Leber’s hereditary optic neuropathy)

In plants, the chloroplasts are often inherited from the mother, although this is species dependent

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Genetic Mapping

Early geneticists realized that they could obtain information about the distance between genes on a chromosome.

- this is genetic mapping

This type of mapping is based on genetic recombination (crossing over) between genes.

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Genetic Mapping

To determine the distance between genes:

- dihybrid organisms are testcrossed

- offspring resembling the dihybrid parent result from homologues that were not involved in the crossover

- offspring resulting from a crossover are called recombinant progeny

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Genetic Mapping

The distance between genes is proportional to the frequency of recombination events.

recombination recombinant progeny frequency total progeny

1% recombination = 1 map unit (m.u.)1 map unit = 1 centimorgan (cM)

=

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Parental male gametes

415 parentalwild type(gray bodynormal wing)

F1 generationfemalepossiblegametes

b+ vg+ b+b vg+vg b+b vg+vg

92 recombinant(gray bodyvestigial wing)

b+b vgvg b+b vgvg

88 recombinant(black bodynormal wing)

bb vg+vg bb vg+vg

405 parentalmutant type(black bodyvestigial wing)

180 ÷ 1000 = 0.18 total recomnbinant offspring18% recombinant frequency

18 cM between the two loci

bb vgvg bb vgvg

b+ vg

b vg+

b vg

b vg b vg

b+b vg+vgF1 generation

Parentalgeneration

Cross-fertilization

Testcross

b+b+ vg+vg+bb vgvg

recessive allele (vestigial wings)vg

recessive allele (black body)bdominant allele (gray body)b+

dominant allele (normal wings)vg+

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Genetic Mapping

Multiple crossovers between 2 genes can reduce the perceived genetic distance

Progeny resulting from an even number of crossovers look like parental offspring

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Genetic Mapping

Determining the order of genes can be done with a three-point testcross

The frequency of double crossovers is the product of the probabilities of each individual crossover

Therefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined

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Genetic Mapping

Mapping genes in humans involves determining the recombination frequency between a gene and an anonymous marker

Anonymous markers such as single nucleotide polymorphisms (SNPs) can be detected by molecular techniques.

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Human Genetic Disorders

Some human genetic disorders are caused by altered proteins.

The altered protein is encoded by a mutated DNA sequence

The altered protein does not function correctly, causing a change to the phenotype

The protein can be altered at only a single amino acid (e.g. sickle cell anemia)

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Human Genetic Disorders

Some genetic disorders are caused by a change in the number of chromosomes.

nondisjunction during meiosis can create gametes having one too many or one too few chromosomes

fertilization of these gametes creates trisomic or monosomic individuals

Down syndrome is trisomy of chromosome 21

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Human Genetic Disorders

Nondisjunction of sex chromosomes can result in:

XXX triple-X females may be taller but normal

XXY males (Klinefelter syndrome) male with female body characteristics and may have mental development issues

XO females (Turner syndrome) short stature sterile female, webbed neck, low-normal mental abilities, the sex organs do not mature during puberty

OY nonviable zygotesXYY males (Jacob syndrome) normal

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Human Genetic Disorders

Genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring

A specific partial deletion of chromosome 15 results in:

Prader-Willi syndrome if the chromosome is from the father (obesity, short stature, respiratory distress, mild retardation….)

Angelman syndrome if it’s from the mother (severe mental retardation, aggressive, hyperactivity, inappropriate laughter…)

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Human Genetic Disorders

Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring.

Some genetic disorders can be diagnosed during pregnancy.

amniocentesis collects fetal cells from the amniotic fluid for examination

chorionic villi sampling collects cells from the placenta for examination

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