1. Mutations
Mutations – heritable changes in genetic information (changes to the DNA sequence)
Two types - gene and chromosomal mutations
Remember mutations are not always negative – catalysts for evolution
Mutations can be caused by chemical or physical agents (mutagens)
Chemical – pesticides, tobacco smoke, environmental pollutants
Physical – X-rays and ultraviolet light

2. 17.5 Gene Mutations Point Mutations
Substitutions: mutations that affect a single nucleotide
Insertions or Deletions: can cause frameshifts that shift the reading frame of the genetic message.
Can change the entire protein so it does not work
Disastrous effects whenever the number of nucleotides deleted or inserted is not a multiple of three (size of a codon).

3. 17.5 Gene Mutations

4. Silent mutations
Silent: a substitution in the DNA sequence that does not result in an amino acid change because many codons code for the same amino acid. For instance: GAA and GAG both code for amino acid GLU

5. 17.5 Missense mutations
Missense mutation: a substitution that changes one amino acid to another one. These mutations may have little effect on the protein: the new amino acid may have similar properties to the one it replaces or be physically located in a non-critical region of the protein. Some missense are serious like sickle cell anemia.

6. 17.5 Nonsense mutations
Nonsense mutation: substitution of one base in the DNA code results in a “stop” codon therefore shortening the protein. Translation is stopped prematurely; the resulting polypeptide will be shorter. Nearly all nonsense mutations lead to nonfunctional protein.

7. 15.4 Chromosomal Mutations
Chromosomal mutation: mutation that changes the number or structure of chromosomes.

8. 15.4 Chromosomal Mutations
Types of chromosomal mutations:
Deletion: The loss of all or part of a chromosome
Duplication: A segment is repeated
Inversion: part of the chromosome is reverse from its usual direction.
Translocation: one chromosome breaks off an attaches to another chromosome.

9. Karyotype of a normal male
Nondisjunction
Karyotype of a Patau’s male (notice chromosome #13 has three chromosomes instead of two)
Karyotype of a normal male

10. 15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Thomas Hunt Morgan: experimental embryologist from Columbia University – famous for fruit fly (Drosophila melanogaster) experiments early 20th century.
Drosophila melanogaster: only four pairs of chromosomes 3 are autosomes and 1 is the sex chromosomes (XX or XY).
Wild type: Red eyes (w+)
Mutant: White eyes (w)

11. 15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)

12. 15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Significance of these findings: a correlation between a particular trait and an individual’s sex suggested that a specific gene is carried on a specific chromosome

13. 15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Significance of these findings: a correlation between a particular trait and an individual’s sex suggested that a specific gene is carried on a specific chromosome

14. The Chromosomal Basis of Sex
In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
Only the ends of the Y chromosome have regions that are homologous with the X chromosome
The SRY gene on the Y chromosome codes for the development of testes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

15. Fig. 15-5 X Y
Figure 15.5 Human sex chromosomes

16. Some disorders caused by recessive alleles on the X chromosome in humans:
Color blindness
Duchenne muscular dystrophy
Hemophilia
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. X Inactivation in Female Mammals
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development
The inactive X condenses into a Barr body
If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

18. 15.2 - X Inactivation in Females
Sugar (girl)
Spice (girl)

19. Each chromosome has hundreds or thousands of genes
Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome
Each chromosome has hundreds or thousands of genes
Genes located on the same chromosome that tend to be inherited together are called linked genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. b+ vg+
b vg 
Parents
in testcross
b vg
b vg
b+ vg+
b vg
Most
offspring or
b vg
b vg

21. Black body, vestigial wings
Fig
Testcross
parents
Gray body, normal wings
(F1 dihybrid)
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
b vg
b vg
Replication
of chromo-
somes
Replication
of chromo-
somes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
Meiosis I
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
b+ vg+
b vg
b+ vg
b vg+
Figure Chromosomal basis for recombination of linked genes
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
944
Black-
vestigial
206
Gray-
vestigial
185
Black-
normal
b vg
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Sperm
Parental-type offspring
Recombinant offspring
Recombination
frequency
391 recombinants =
 100 = 17%
2,300 total offspring

22. However, nonparental phenotypes were also produced
Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)
He noted that these genes do not assort independently, and reasoned that they were on the same chromosome
However, nonparental phenotypes were also produced
Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

23. Recombination of Unlinked Genes: Independent Assortment of Chromosomes
Mendel observed that combinations of traits in some offspring differ from either parent
Offspring with a phenotype matching one of the parental phenotypes are called parental types
Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants
A 50% frequency of recombination is observed for any two genes on different chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

24. Recombination of Linked Genes: Crossing Over
Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes
Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome
That mechanism was the crossing over of homologous chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

25. Mapping the Distance Between Genes Using Recombination Data
Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome
Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Mapping the Distance Between Genes Using Recombination Data
A linkage map is a genetic map of a chromosome based on recombination frequencies
Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency
Map units indicate relative distance and order, not precise locations of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

27. RESULTS Recombination frequencies 9% 9.5% Chromosome 17% b cn vg
Fig
RESULTS
Recombination
frequencies 9%
9.5%
Chromosome
17%
Figure Constructing a linkage map b cn vg

28. Mapping the Distance Between Genes Using Recombination Data
Genes that are far apart on the same chromosome can have a recombination frequency near 50%
Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings