1. The Chromosomal Basis of Inheritance
Chapter 15
BCOR 012

2. Drosophila melanogaster
Thomas Hunt Morgan

3. Morgan’s white-eyed fly
wild-type

4. Wild-type: evidencing the normal phenotype (as opposed to a
mutant phenotype). Usually controlled by a dominant allele.
Noted as, for example, w+ (this is the allele for normal eye color, red)
Mutant: the non-standard phenotype. Usually controlled by a
recessive allele.
Noted as, for example, w (this is the allele for white eye color)

5. These observations led Morgan to propose the the eye-color
gene is on the X-chromosome; that is to say, it is sex-linked.
All the white-eyed flies were male!

6. Genetic Linkage and Linkage Mapping

7. Mendel crossed two true-breeding pea lines
that differed in seed shape and color.
The F1 progeny were all yellow and round.
When they were crossed with one another,
what were the genotypes and phenotypes
among the F2 progeny?

8. Hypothesis: Independent Assortment
Phenotype:
yellow, round
green, wrinkled X
(This is a testcross.)
YyRr
yyrr
Genotype:
1 YyRr : 1 yyrr : yyRr : 1 Yyrr

9. ?

10. So how do you explain the non-
parental phenotypes?

12. The map distance between two genes is the recombination frequency,
expressed as a percent.

13. Clicker Question!
D, F, and J are three genes in Drosophila. The recombination frequencies for two of the three genes are shown below.
Genes D and F are:
A. either located on different chromosomes or located
very near each other on the same chromosome.
B. located very near to each other on the same chromosome.
C. located on different chromosomes.
D. either located on different chromosomes or located far from each other on the same chromosome
E. located far from each other on the same chromosome.

15. Hypothesis: Independent Assortment
Phenotype:
yellow, round
green, wrinkled X
(This is a testcross.)
YyRr
yyrr
Genotype:
Recombinants =
50%
N =
1 YyRr : 1 yyrr : yyRr : 1 Yyrr

18. Double crossovers lead to the underestimation of map distances.

20. The Genetic Basis of Sex Determination

22. Sex-Linked Traits

23. Pedigree Showing the Inheritance of Color-Blindness in a Family
Characteristics of X-linked genes:
Trait shows up more frequently in males than in females
Trait is transmitted from mother to son
Pedigree Showing the Inheritance of Color-Blindness in a Family

24. Queen Victoria and her family

26. X-Inactivation

27. In order to keep gene dosage balanced, one of the two X chromosomes becomes inactivated early in development. This phenomenon is called X inactivation.
Arrows indicate
inactivated X chromosomes

28. Coat color in the tortoiseshell
cat results from random
inactivation of the
X chromosomes.

30. Clicker Question
Q. In which cat, #1 or #2, did X-inactivation happen earlier?
A. Cat #1
B. Cat #2
C. They happened at the same relative time in embryonic
development.
D. It is impossible to tell from the photos.
Cat #1
Cat #2

31. Chromosomal Aberrations
Polyploidy: the condition in which the organism possesses three or more complete sets of chromosomes.
Aneuploidy: the condition in which certain chromosomes are present in excess or deficit (e.g., trisomy 21).

32. Aneuploidy arises as a result of nondisjunction at meiosis I or II.

33. The Down’s syndrome phenotype is the result of nondisjunction of
chromosome 21.

34. In humans, a number of aneuploid
conditions result from from non-disjunction of the X chromosomes.

36. A human cancer, chronic myelogenous leukemia, results from a translocation between chromosomes 9 and 22.