1. Chromosomal Theory of Inheritance
Chapter 15

2. Learning target 1 I can explain the chromosome theory of inheritance.
I can explain why linked genes do not assort independently.
I can explain how sex linked inheritance works in mammals and in birds.
I can distinguish between parental and recombinant phenotypes.
I can explain how crossing over can unlink genes.
I can create a linkage map.
I can describe the inheritance patterns and symptoms of color blindness, Duchenne muscular dystrophy and hemophilia.
Learning target 1

3. The Theory Genes are located on chromosomes
Chromosomes segregate and independently assort during meiosis

4. Evidence Cytologists (study cells): Geneticists (study genes):
1879 – Mitosis worked out
1890 – Meiosis worked out
Geneticists (study genes):
1860 – Mendel proposed laws of segregation & independent assortment
1900 – Mendel’s work rediscovered

5. Linkage & Chromosome maps
Linked genes are genes that are located on the same chromosome and DO NOT assort independently
Why some traits tend to be inherited together, such as red hair and freckles
We also know that chromosomes will also ‘cross-over’ during gamete formation
Based on the number of crossovers observed, we are able to determine the order of genes, and relative distance of genes from one another

6. Genetic Recombination
Due to crossing over, offspring may or may not look like their parents
Parental types do
Recombinant types don’t
Crossing over can unlink genes on the same chromosome, depending on distance from each other

7. Making a Linkage Map
In fruit flies, gray body is dominant to black body and normal wings are dominant to vestigial wings. Flies heterozygous for both gray bodies and normal wings are crossed with flies that had black bodies and vestigial wings. The following results were obtained:
The results indicate that the genes for wings and body color are on the same chromosome. The recombinant offspring are a result of crossing over. How many map units (expressed as a percent) apart are the two genes?
Phenotype
Number of flies
Gray body/normal wings
482
Black body/vestigial wings
472
Gray body/vestigial wings
103
Black body/normal wings 92

8. The formula for calculating recombination frequency is:
Which phenotypes are recombinants?
Gray body/vestigial wings (103)
Black body/normal wings (92)
Use the formula:
(103+92)/1149 = 17%
This means that the genes for body color and wing shape are located 17 map units from each other on the chromosome

9. Sex Determination Determined by presence of Y chromosome in humans
XX: female
XY: male
Other systems in birds, insects:
XO: insects
ZW: birds
Haplo-diploid: bees

10. SRY gene Sex determination region On Y chromosome
Triggers events that lead to testicular formation

11. Sex-linkage (or, X-linkage)
Some traits are only inherited because they are on the X chromosome
Typically recessive traits
Males inherit these more often
Why?

12. Sex linked disorders Colorblindness Hemophilia
Duchenne Muscular Dystrophy

13. Sample cross Colorblindness XCXc x XCY In any sex-linked cross:
XC = normal vision
Xc = colorblind
XCXc x XCY
In any sex-linked cross:
Affected males Xc get from Mom
Affected females get one Xc from Mom and one from Dad
More males affected than females

14. I can explain how X inactivation results in Barr Bodies.
I can describe the process of X inactivation in female mammals.
Explain how this phenomenon produces the tortoiseshell coloration in cats.
Learning target 2

15. X inactivation Females are XX but only need one X
one X condenses and genes become silenced
Inactivation happens during embryonic development
Inactivation is random
Females are mosaics of 2 cell types
paternal X inactive
maternal X inactive

16. I can describe how alterations of chromosomes occur and how they can cause genetic disorders
I can explain how nondisjunction can lead to aneuploidy
I can Identify human disorders caused by chromosomal alterations; trisomy 21, Klinefelter syndromes, turner syndrome.
I can identify alterations in chromosome structure as the result of: deletions, duplications, inversions, and translocations.
Learning target 3

17. Whole Chromosome Mutations
Nondisjunction
Failure of chromosomes to separate during meiosis

18. Disorders due to Nondisjunction
Down Syndrome (trisomy 21)
Klinefelter Syndrome – male who is XXY
Turner Syndrome – female who is XO

19. Chromosomal Alterations
Mutations of parts of chromosomes
Replication error or crossing over error

20. I can identify and explain inheritance patterns that are exceptions to the standard chromosome theory.
I can explain how genomic imprinting works.
Learning target 4

21. Back to Mendel for a minute…
Looking at Mendel’s peas, it didn’t matter if the traits came from the maternal or paternal parent
Each trait would have the same bearing on the offspring
Scientists, within the past decade or so, have found some genes that are inherited differently depending on which parent passed it along

22. Genomic Imprinting
Variations in phenotype depending on whether the allele is passed on from the male or female parent
This differs from sex linkage due to the fact that most imprinted genes are found on autosomes

23. Imprinting Occurs during gamete formation
These genes are expressed differently in eggs or sperm – the trait is ‘silenced’ in one gender
The developing embryo will only express the trait from one parent in all of its body cells
Effect is determined by which chromosome’s gene is expressed

24. Example – Igf2
Insulin-like growth factor 2 (Igf2) is needed in mice for normal growth and development
In crosses between wild type and homozygous recessive dwarf mice, individuals produced heterozygous offspring
Offspring differed in phenotype depending on whether the gene was maternal or paternal

25. I can explain how inheritance works with extra-nuclear genes found in organelles such as mitochondria.
Learning target 5

26. One More Genetic Exception…
Not all genes are found on nuclear chromosomes
Organelles such as mitochondria and chloroplasts have circular DNA molecules that carry genes
These are almost always passed on from the mother because the egg houses these organelles, while the sperm only carries chromosomes