1. Non-Mendelian Genetics Honors Biology Ms. Day
Lecture #26
Non-Mendelian Genetics
Honors Biology
Ms. Day

2. RECALL…
Complete dominance
Heterozygote (Hh) and dominant homozygote (HH) look identical
Demonstrates or follows “Mendelian Genetics” inheritance pattern

3. “Non-Mendelian Genetics”
Incomplete (intermediate) Dominance
1 allele is not completely dominant over the other, so heterozygote (Hh) has intermediate (or mixed) phenotype between 2 alleles
(like snapdragon flowers)

4. Figure 14.10  1⁄2 P Generation White CWCW Red CRCR Gametes Pink CRCW
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW
Figure 14.10

5. “Non-Mendelian Genetics”
Codominance
2 dominant alleles affect phenotype in separate, distinguishable ways
BOTH phenotypes are present in heterzygote
Ex’s of codominance
Some flowers and Roan animals (cattle & horses)

6. Multiple Alleles A type of codominance
When there is 3+ alleles (genes) for a single trait
Ex: Human Blood type, human hair color

7. The ABO blood group in humans Is determined by multiple alleles
Table 14.2
The ABO blood group in humans Is determined by multiple alleles

8. Multiple Alleles (Codominance)
Blood Type
Genotypes A
IAIA, or
IAi B
IBIB, or
IBi AB
IAIB O ii

10. Polygenic Inheritance
Many genes (2+) determine one (1) phenotype
Many human traits
Vary in the population along a continuum
Few genes actually follow a simple Mendelian inheritance pattern
Examples:
Height, eye color, intelligence, body build and skin color

11. Polygenic Inheritance
AaBbCc
aabbcc
Aabbcc
AaBbcc
AABbCc
AABBCc
AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fraction of progeny

12. Nature and Nurture: The Environmental Impact on Phenotype
Departs from simple Mendelian genetics  phenotype depends on environment as well as on genotype
Called multifactorial inheritance
Ex: human fingerprints
hydrangea flowers
Add P to soil; need
HIGHERpH
Al in soil; need LOW pH

13. Lecture #27 Honors Biology Ms. Day
The Chromosomal Basis of Inheritance
Introduction to Sex Linkage
Honors Biology
Ms. Day

14. Gene Linkage Linked genes
Usually inherited together because located near each other on the SAME chromosome
Genes closer together on the same chromosome are more often inherited together
Each chromosome
Has 100’s or 1000’s of genes
Sex-linked genes exhibit unique patterns of inheritance; genes on the X or Y chromosome

15. Morgan’s Experimental Evidence
Thomas Hunt Morgan
Provided convincing evidence that chromosomes are the location of Mendel’s heritable alleles

16. Sex linkage explained Thomas Hunt Morgan (Columbia University 1910)
Thomas Hunt Morgan (Columbia University 1910)
Fruit Flies (Drosophila) melanogaster)
© 2007 Paul Billiet ODWS

17. Morgan’s Choice of Experimental Organism
Morgan worked with fruit flies
Lots of offspring
A new generation can be bred every two weeks
They have only 5 pairs of chromosomes

18. Morgan and Fruit Flies w+ w Morgan first observed and noted
Wild type (most common) phenotypes that were common in the fly populations
Traits alternative to the wild type are called mutant phenotypes
WILDTYPE w+ w
MUTANT

19. The case of the white-eyed mutant
Character Traits
Eye color Red eye (wild type)
White eye (mutant)
P Phenotypes Wild type (red-eyed) female x White-eyed male
F1 Phenotypes All red-eyed
Red eye is dominant to white eye

20. So far so good Hypothesis
A cross between the F1 flies should give us: 3 red eye : 1 white eye F2
Phenotypes
Red eye
White eye
Numbers
3470
82%
782
18%
So far so good

21. An interesting observation
The F2 generation showed the 3:1 red: white eye ratio, but only males had white eyes F2
Phenotypes
Red-eyed males
Red-eyed females
White-eyed males
White-eyed females
Numbers
1011
2459
782
24%
58%
18% 0%

22. A reciprocal cross
Morgan tried the cross the other way round white-eyed female x red-eyed male
Result All red-eyed females and all white-eyed males
This confirmed what Morgan suspected
The gene for eye color is linked to the X chromosome

23. Morgan’s Discovery: Sex Linked Traits
Eye color is linked on X Chromosome
Females carry 2 copies of gene; males have only 1 copy
If mutant allele is recessive, white eyed female has the trait on both X’s
White eyed male can not hide the trait since he has only one X.

24. The Chromosomal Basis of Sex
An organism’s sex
Is an inherited phenotype determined by the presence or absence of certain chromosomes
XX = girl
XY = boy

25. Inheritance of Sex-Linked Genes
The sex chromosomes
Have genes for many characters unrelated to sex (especially the X chromosome)
A gene located on either sex chromosome
Is called a sex-linked gene
(Usually on X chromosome)

26. What genes are on the X chromosome?
carries a couple thousand genes but few, if any, of these have anything to do directly with sex determination
Larger and more active than Y chromosome

27. What genes are on the Y chromosome?
Gene called SRY triggers testis development, which determines male sex characteristics
This gene is turned “on” ~6 weeks into the development of a male embryo
Y-Chromosome-linked diseases are rare

28. Sex-linked genes follow specific patterns of inheritance
Fathers  pass sex-linked alleles to ALL their daughters but NONE to their sons
XY (Father)  XX (daughter)
XY (Father)  XY (son)
Mothers  can pass sex-linked alleles to BOTH sons and daughters
XX (Mother)  XX (daughter)
XX (Mother)  XY (son)

29. Sex Linkage
If sex-linked recessive on Xn
Females have to be Xn Xn to show sex-linked trait
Xn X Females do NOT show sex-linked trait
Males have to be Xn Y to show sex-linked trait
**Most sex-linked disorders affect males; sometimes females

30. Sex-Linked Disorders
Some recessive alleles found on the X chromosome in humans cause certain types of disorders
Color blindness
Duchenne muscular dystrophy
Hemophilia
Male pattern baldness

31. X-Linked Trait = Male Pattern Baldness

32. Another X-Linked Trait = Hemophilia
About 85% of hemophiliacs suffer from classic hemophilia
1 male in
cannot produce factor VIII
The rest show Christmas disease where they can’t make factor IX
The genes for both forms of hemophilia are sex linked
Hemophiliacs have trouble clotting their blood

33. Another X-Linked Trait = Red-Green Colorblindness
Normal vision
Color blind simulation

34. Another X-Linked Trait = Duchenne Muscular Dystrophy