Chromosomes mapping and the meiosis inheritance connection
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Chromosomes, Mapping, and the Meiosis-Inheritance Connection. Chapter 13. Chromosome Theory. Chromosomal theory of inheritance - developed in 1902 by Walter Sutton - proposed that genes are present on chromosomes

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Chromosomes, Mapping, and the Meiosis-Inheritance Connection

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Chromosome theory
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


Chromosome theory1
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 red eyes are dominant


Chromosome theory2
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


Sex chromosomes
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


Sex chromosomes1
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.


Sex chromosomes2
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.


Chromosome theory exceptions
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


Chromosome theory exceptions1
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

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


Genetic mapping
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.


Genetic mapping1
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


Genetic mapping2
Genetic Mapping

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

recombinationrecombinant progeny

frequency total progeny

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

1 map unit = 1 centimorgan (cM)

=


Genetic mapping3
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


Genetic mapping4
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


Genetic mapping5
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.


Human genetic disorders
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)


Human genetic disorders1
Human Genetic Disorders

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

- nondisjunctionduring 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


Human genetic disorders2
Human Genetic Disorders

Nondisjunction of sex chromosomes can result in:

XXX triple-X females

XXY males (Klinefelter syndrome)

XO females (Turner syndrome)

OY nonviable zygotes

XYY males (Jacob syndrome)


Human genetic disorders3
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

- Angelman syndrome if from the mother


Human genetic disorders4
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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