1. Exceptions to Mendelian Genetics & Pedigrees

2. Mendel’s work led to the Chromosome Theory
Genes are located on chromosomes
Separation of genes during meiosis account for the inheritance patterns discovered by Mendel

3. However…
Inheritance of traits by a single gene may deviate from simple Mendelian patterns

4. The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan
Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

5. Several characteristics make fruit flies a convenient organism for genetic studies:
They breed at a high rate
A generation can be bred every two weeks
They have only four pairs of chromosomes
They are easy to keep alive
They don’t need a lot of space

6. In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
The F1 generation all had red eyes
The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes
Morgan determined that the white-eyed mutant allele must be located on the X chromosome

7. Fig. 15-3

8. EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + + + w w w w + w
Generation F1
All offspring had red eyes
Generation
RESULTS F2
Generation
CONCLUSION P + X w X w
Generation X  Y + w w
Sperm
Eggs F1 + + + w w
Generation w w + w
Sperm
Eggs + + w w F2 + w
Generation + w w w w + w

9. Morgan discovered sex-linked genes
Genes found on the sex chromosomes
Usually it’s found on the X chromosome

10. Difference in Human X and Y Chromosomes

11. Females are XX, and males are XY
Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome

12. For example, a gene for making a blood clotting protein is found on the X chromosome
Having a mutation of this gene causes hemophilia
Males that inherit a mutation of the gene found on their X chromosome, lack another gene on their Y chromosome and have hemophilia

13. Swelling of the joints in hemophiliac patient

14. Duchenne (pseudohypertropic) muscular dystrophy is also sex-linked

15. Classic signs of Duchenne MD

16. 8% of males have some type of color blindness
0.4% of females are color blind
The most common type is distinguishing red from green

19. Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
For example: Aa genotype gives the dominant phenotype and the “a” gene is completely hidden

20. There are exceptions!
In incomplete dominance, the phenotype of heterozygotes is somewhere between the phenotypes of the two parental varieties
NEITHER the dominant or recessive trait shows up – there’s a third phenotype

21. Such as inheritance of petal color in Japanese 4 o’clocks Fig. 14-10-3
P Generation
Red
White
CRCR
CWCW
Such as
inheritance
of petal
color in
Japanese
4 o’clocks
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

22. The sickle cell gene is also incompletely dominant.
A person heterozygous has sickle cell trait

23. In codominance, if two different alleles are present, they will both show their trait
For example, ABO human blood types

24. Sometimes there are more than 2 alleles for a trait and are called multiple alleles
Example: the A, B, and O genes in human blood types

25. Polygenic traits are controlled by more than one pair of genes and accounts for a wide variety of a trait
Skin color in humans is an example of polygenic inheritance

26. Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genes along a particular chromosome
He did this through studying how frequently two genes located on the same chromosome separate during crossing over

27. Sturtevant discovered that some genes are found on the same chromosome very close together and, if present, are often inherited together
For example, the genes for hair color and presence of freckles are both found on the same chromosome close to each other and are often inherited together

28. This deviates from Mendel’s principle of independent assortment

29. Even though there are exceptions to Mendel’s principles, basic Mendelian genetics endures as the foundation of genetics

30. A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

31. Key Male Affected male Mating Offspring, in birth order
Fig a
Key
Male
Affected
male
Mating
Offspring, in
birth order
(first-born on left)
Female
Affected
female

32. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

33. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?