1. Chromosomes, Mapping, and the Meiosis-Inheritance Connection
Chapter 13

2. 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

3. 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 dominant

4. 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

7. Sex Chromosomes
Sex determination in Drosophila is based on the number of X chromosomes
2 X chromosomes = female
1 X = male (typo mistake in original)
Sex determination in humans is based on the presence of a Y chromosome
having a Y chromosome (XY) = male

8. 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.

10. 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.

12. 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

13. 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

14. 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.

17. 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

18. 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) =

21. 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

23. 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

25. 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.

27. 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)

30. 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

32. 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)

34. 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 it’s from the mother

35. 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