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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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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
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
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 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
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.
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.
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
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
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.
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
The distance between genes is proportional to the frequency of recombination events.
frequency total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM)
Multiple crossovers between 2 genes can reduce the perceived genetic distance
- progeny resulting from an even number of crossovers look like parental offspring
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
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.
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)
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
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)
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
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