1. Chapter 15 Chromosomal Basis of Inheritance

2. Mendel & Chromosomes
Mendel was ahead of his time. 19th C cytology suggested a mechanism for his earlier findings. What did they find?
Chromosomes and genes are both present in pairs in diploid cells.
Homologous chromosomes separate and alleles segregate during meiosis.
Fertilization restores the paired condition for both chromosomes and genes.

3. Chromosome Theory of Inheritance
Mendelian genes have specific loci on chromosomes
Chromosomes are what physically undergo segregation and independent assortment.

4. Morgan first associated a specific gene with a specific chromosome.
Morgan’s Fruit Flies
Morgan first associated a specific gene with a specific chromosome.
Why fruit flies?
Breed quickly (two week generations)
4 pairs of chromosomes (3 pair of autosomes, 1 pair of sex chromosomes)
Females = XX
Males = XY

5. Wild Type flies are the most common natural phenotype. (Red Eyes)
Morgan’s Fruit Flies
Wild Type flies are the most common natural phenotype. (Red Eyes)
After a series of crosses, Morgan produced mutants with white eyes.
After a few generations, Morgan noted that only males displayed the white eyes.
He concluded that certain genes are located on the sex chromosome and thus linked to sex.
Sex-linked genes (ie: hemophilia)

6. Sex-linked Traits

7. Males (XY) only need one copy of recessive allele to show trait.
Sex-linked Traits
Morgan concluded the gene with the white-eyed mutation is on the X chromosome. Y chromosome = no info
Males (XY) only need one copy of recessive allele to show trait.

8. Linked Genes
All genes located on the same chromosome tend to be inherited together.
Chromosome passed on as a unit.
Testcross results varied from those predicted by the law of independent assortment.
This showed that certain genes will assort together. (on same chromosome)

9. Linked Genes

10. Linked Genes
Body color and wing shape are usually inherited together (same chromosome)

11. Recombinants
Where did the other phenotypes come from? (grey-vestigial and black normal)
Genetic recombination= offspring with new combinations of traits inherited from two parents
How??
independent assortment of genes (non-homologous)
crossing over of genes (homologous)

12. Recombinants Mendel’s dihybrid crosses produced recombinant genotypes.
50% parental : 50% recombinant genotypes typical for nonhomologues
Metaphase I
YR, Yr, yR, and yr
Seed shape and color tetrads are independent from one another

13. Recombinants
Linked genes tend to move together during meiosis/fertilization
If Independent assortment of genes
Expect a 1:1:1:1 phenotype ratio
If Complete linkage of genes
1:1:0:0 ratio (all parental)
Observed 17% recombinant flies
Suggested Incomplete linkage of genes

14. Crossing Over Prophase I: homologous chromosomes can “swap” alleles
More variable gametes than simple mendelian rules would predict

15. Therefore, Crossing Over Explains:

16. Linkage Maps Ordered list of genetic loci along chromosome
Based on recombination frequencies between two genes
Higher % of recombination = farther apart
More places in between genes for crossing over to occur and separate the genes

17. Linkage Maps The recombination frequency between cn and b is 9%.
The recombination frequency between cn and vg is 9.5%.
The recombination frequency between b and vg is 17%.

18. Linkage Maps
Map units are the distances between genes on a chromosome.
1 map unit = 1% recombination
50% recombination = so far apart that crossing over is all but certain
Remember, 50% recomb. = ind. assortment (non-homologous)
Linkage maps show relative order/distance
More recent studies show exact distances and order

19. Sex Chromosomes

20. X-Y Sex Determination
X and Y behave as homologues
Each egg receives an X from XX mother
One sperm receives X and one Y
Results in 50/50 chance of male or female
SRY Gene
Present (on Y) : gonads develop into testes (male)
Not present (no Y): gonads become ovaries (female)
SRY also regulates other genes

21. Sex-Linked Genes
Sex chromosomes also contain other genes. (ie: drosophila eye color)
Females must be homozygous recessive to display trait (XX – second X can mask recessive)
Females can be carriers
Males only need to inherit a single copy to show trait
Can a male be a carrier?

22. Sex-Linked Disorders Duchenne Muscular Dystrophy 1/3500 males
Progressive muscular weakening
Die by mid-20’s
Missing X-linked gene
No production of dystrophin (muscle protein)

23. Sex-Linked Disorders Hemophilia
Absence of one or more clotting factors
affected individuals cannot stop bleeding normally
treated with protein injections

24. Barr Bodies Only one of the females X chromosomes is active
The other becomes a Barr body
When assorted into an ovum, the Barr body becomes activated again
Which X becomes Barr body is random in each cell
Approx. 50% express each allele (if hetero)

25. X-Inactivation in Females

26. Nondisjunction Errors with meiotic spindle
Meiosis I: Homologous tetrad doesn’t separate OR
Meiosis II: Sister chromatids don’t separate
Some gametes receive two of the same type of chromosome and another gamete receives no copy

27. Aneuploidy Results from fertilization involving nondisjoined gamete(s)
Trisomy three copies of a particular chromosome (2n + 1)
Monosomy only one copy of a particular chromosome (2n – 1)

28. Down Syndrome Three copies of chromosome 21
1/700 children born each year
Definite link with maternal age

29. Aneuploidy in Sex Chromosomes
XXY Male (Klinefelter’s Syndrome)
Male sex organs, sterile w/ femininity
XYY Males
Tend to be taller than normal

30. Aneuploidy in Sex Chromosomes
XXX Females
Will develop as normal females
XO Females (monosomy – Turner syndrome)
Immature females
1/2500 live female births

31. Changes in Chromosomes