1. Figure 15.1 The chromosomal basis of Mendel’s laws

2. Figure 15.2 Morgan’s first mutant

3. Figure 15.3 Sex-linked inheritance

4. Drosophila Testcross

5. Figure 15.4 Evidence for linked genes in Drosophila

6. Recombination Due to Crossing Over

7. Recombination Due to Crossing Over

8. If the two genes were on different chromosomes, the alleles from the F1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring.
Since most offspring had a parental phenotype it can be concluded that the genes for body color and wing size are located on the same chromosome.

9. Linked genes tend to be inherited together because they are located near each other on the same chromosome.
Recombinants have different phenotypes
than the parents.

10. Morgan, who discovered that genes
were located on chromosomes, proposed
that some process must occasionally
break the physical connection between
genes on the same chromosome.
This is now called crossing over, which
accounts for the recombination of linked
genes.

11. Sturdevant, a student of Morgan’s,
devised a way to construct genetic maps, an ordered list of the genetic loci along a
particular chromosome.
He predicted that the further apart two genes are on a chromosome, the higher
the probability that crossing over will
occur.

12. Using recombination frequencies to construct a genetic map
1 map unit is equal to 1%
recombination frequency

13. A genetic map based on recombination
frequencies is called a linkage map and
is based on the assumption that the
probability of a crossover between two
genetic loci is based on the distance
separating the loci.
The distances between genes are expressed as map units, with one map unit equivalent to a 1% recombination frequency.

14. Partial Genetic Map of a Drosophila Chromosome
The numbers represent the distance between that locus and the locus for aristae length.

15. Some chromosomal systems of sex determination

16. Although female mammals, including humans, inherit two X chromosomes, one X chromosome in each cell becomes almost completely inactivated during embryonic development. The inactive X in each cell of a female condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope.

17. If a female is heterozygous for a
sex-linked trait, about half her cells
will express one allele, while the
others will express the alternate
allele.

18. X inactivation and the Tortoiseshell Cat

19. Calico Cat

20. Meiotic Nondisjunction

21. Translocation

22. Alterations of Chromosome Structure

23. Down Syndrome

24. Klinefelter Syndrome

25. XYY karyotype