1. Exploring Mendelian Genetics
Going Beyond Simple Dominance:
X-linked (sex-linked) Traits
Incomplete Dominance
Codominance
Multiple Alleles Blood Types
Polygenic Traits
Pleiotropy
Epistasis
Multifactorial

2. SeX-linked Traits Trait can be dominant or recessive
Probability of inheritance is altered because the trait is on the X chromosome
Females- XX Males- XY

3. Examples of X-linked traits
Colorblindness
Hemophilia
Duchenne
Muscular
Dystrophy
Female carriers do not express phenotype

4. Colorblindness Test

5. Incomplete Dominance
In some cases, neither allele truly dominates over the other.
No allele is really dom. or rec.
The heterozygous genotype shows a MIX of the two traits.
Example- Four O’Clocks, Snapdragons
R- gene for red flowers, W- gene for white flowers:
RR- red, WW- white, RW- pink

6. Codominance In some cases, both alleles are dominant.
No allele is really recessive.
The heterozygous genotype shows BOTH of the two traits.
Example- Chicken feathers
B- gene for black feathers, W- gene for white feathers:
BB- black, WW- white,
BW- “erminette” Black and White!
Rhododendron and Roan Cow

7. Multiple Alleles
Many genes have more than just two alleles for a trait
Remember, you can still only have 2 alleles at a time b/c of the diploid nature of our cells
It is still just ONE gene, but lots of possibilities
Example: Alleles for rabbit fur
C-full color, dominates over Cch, Ch, c
Cch – chinchilla, dominates over Ch and c
Ch- himalayan, dominates over c
c- albino, recessive to all

9. Polygenic Traits
Many traits result from the interaction of several genes.
Multiple genes, perhaps on different chromosomes even, produce one phenotype
Polygenic traits can produce a large range of phenotypes
Examples: human skin color (at least 4 genes), human eye color, human height
(eye color explained)

11. Capital letters = dark; small letters = light; more dark alleles = darker!!

12. Gradation of Human Skin Color

13. Height in Humans
Range of phenotypes resulting from polygenic trait

14. Human Blood Types Multiple alleles, Polygenic and Codominance!
Multiple alleles- A, B, O
Polygenic- one gene controls type, another gene controls rH factor (+, -)
Codominance- A and B are codominant but both dominate over O

15. Multiple Alleles
Most genes have more than two alleles in a population—called multiple alleles.
The ABO blood groups in humans are determined by three alleles, IA, IB, and i.
Both the IA and IB alleles are dominant to the i allele
The IA and IB alleles are codominant to each other.
Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

18. Pleiotropy Ability of 1 gene to affect more than one phenotype
Discovered because Mendelian #s didn’t work
Most common examples include some physical phenotype and viability “phenotype”
nice example of the coat color in mice functioning also as a lethal gene

19. Epistasis (p380)
The interaction between 2 genes to control a single phenotype; you need both genes to receive a certain outcome.
A “cascade” of proteins

20. NATURE VERSUS NURTURE
WHAT DO YOU THINK?

21. Nature vs. Nurture Environmental effects
National Children’s Health Study (JH)
Twin studies
Norm of Reaction
Range of phenotypic possibilities over which there may be variation due to environmental influence

22. Environment and Phenotype
Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.
Black fur only occurs on extremities.

23. Other Chromosomal Actions
Linkage  Genes occurring on the same chromosome can be inherited together
X-Inactivation  if men only need one X chromosome, do women need both X’s?
If not, which one “works”?
Non-disjunction  how gametes can end up with the wrong # of chromosomes in meiosis

24. Linkage
Really it is the chromosomes that are segregated independently, not necessarily individual genes.
Some genes are LINKED if they are on the same chromosome
Ex: you get all of the genes on chromosome 1 from your mom if you get her chr.1
Is that always the case though? What do you know might happen?

25. Crossing Over
Depending on how FAR APART genes are on chromosomes, they may be switched during meiosis
Prophase I
Must be homologous

26. Gene Maps Where actual genes are located on chromosomes.
Discovered by Alfred Sturtevant in Thomas Morgan’s lab at Columbia in 1931.
~ 1 million bp = 1 map unit (m.u.) or centimorgan
By looking at statistical inheritance patterns, he calculated the “recombination frequency” of alleles.
If genes are far apart, crossing over is more likely
1 cM = 1% chance a marker at one genetic locus on a chromosome will be separated from a marker at a second locus due to crossing over in a single generation

27. X-Chromosome Inactivation
Mary Lyon
Men are XY: One X chromosome is enough
Women are XX: The “extra” copy is not needed. It condenses into a “Barr body.”
Remember, you have trillions of cells. Which X chromosome that “switches off” is random in each cell

28. Mosaicism

29. Nondisjunction “Not coming apart”
Homologous chromosomes fail to separate in meiosis
Result in Trisomy or Monosomy
Examples:
Klinefelter syndrome (trisomy X)
Down syndrome (trisomy 21)
Turners syndrome (X_ monosomy)