1. 11- 3: Exploring Mendelian Genetics & 11-5: Linkage and Gene Maps

2. What are the chances?
Do people with brown hair always have brown eyes?
Do people with brown hair have a higher chance of having brown eyes?
Is a round pea seed always yellow? Can a short plant have purple flowers?

3. INDEPENDENT ASSORTMENT
The inheritance of one gene does not influence the inheritance of another.
Mendel named this:
The Law of Independent Assortment
In meiosis, the chromosomes line up randomly on the equator to be separated.
If your parents are heterozygous for any traits, this leads to lots of possibilities!

6. How did Mendel figure this out?
Through experiments…
He crossed two different plants
Each was true-breeding for 2 different traits
P gen: rryy (wrinkled & green)
x RRYY (ROUND & YELLOW)
F1 gen: RrYy
F2: a bunch of possibilities!

7. Segregation of Chromosomes Video

9. Two Trait Crosses

10. Aligning the Punnett Square
F1 gen now has: RrYy
Distribute the 1st letter of the 1st set to each of the 2 letters in the 2nd set.
Distribute the 2nd letter of the 1st set to each of the 2 letters in the 2nd set
R r Y y
4 possibilities: RY, Ry, rY, ry

12. Patterns
A Het x Het dihybrid cross yields the typical phenotypic ratio of:
9: 3: 3: 1

13. Now You Try!
In humans, the gene that causes a unibrow (u) is recessive to not connected eyebrows (U); the gene for thick lips (T) is dominant over the gene for thin lips (t). If a male that is homozygous for not connected eyebrows and heterozygous for thick lips mates with a woman who has a unibrow and is heterozygous for thick lips, what is the phenotypic ratio of the offspring?

14. How do you set it up? Genotypes
--Dad = UUTt --Mom = uuTt
Possible Different Combinations of Alleles
--Dad = UT or Ut --Mom = uT or ut
U = not connected eyebrows
u = unibrow
T = thick lips
t = thin lips
1. Make one column for each possible different combination of alleles from dad
dad UT Ut uT ut
2. Make one row for each possible different combination of alleles from mom
mom
3. Fill in the boxes

15. Results of the Cross UT Ut uT ut UuTT UuTtut
UuTT
UuTt
Only include the phenotypes present in the results.
UuTt
Uutt
Phenotypic Ratio
3 not connected eyebrows & thick lips : 1 not connected eyebrows & thin lips

16. Summing It Up: Mendel’s Principles
1. Parents pass on characteristics, sexually, through genes to their offspring
2. When there are multiple alleles (appearances) for one gene, some are dominant & some are recessive
During formation of parental gametes, alleles are segregated into separate gametes. Each parent is then able to pass ONE allele to the child. The child therefore gets ONE allele from EACH parent
The chromosomes (and therefore alleles) from each parent arrange themselves independently during meiosis

17. 11-3 Exploring Mendelian Genetics
Going Beyond Simple Dominance:
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
X- linked (or sex linked) Traits

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

19. Example: Snapdragons
RR = Red
WW = White
All RW = Pink

20. 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
C1- gene for black feathers, C2- gene for white feathers:
C1C1- black, C2C2- white,
C1C2- “erminette” Black and White!

21. Multiple Alleles
Many genes have more than just two alleles for a trait
Remember, you can still only have 2 alleles at a time.
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

23. Practice Cross cchch x cc
What are the phenotypes of the parents?
light gray x albino
Fill in the Punnett Square.
What is the probability that an offspring will be albino (albino = cc)?
0 out of 4 = 0%
What is the probability that an offspring will be himalayan (himalayan = chch, chc)?
2 out of 4 = 50%
cch ch c
Light gray
Himalayan
chc
cchc
Light gray
Himalayan c
cchc
chc

24. 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

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

27. Gradation of Human Skin Color

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

29. Human Blood Types Exhibit 3 special situations
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

32. What does your blood type actually mean?

33. 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?

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

35. Crossing Over Video

36. Gene Maps Where actual genes are located on chromosomes.
Discovered by a student working in molecular lab at Columbia in 1931.
By looking at statistical inheritance patterns, you can calculate the “recombination frequency” of alleles.
If genes are far apart, crossing over is more likely

38. Sex-linked Traits Often called X- 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

39. Examples of X-linked traits
Colorblindness
Hemophilia
Duchenne
Muscular
Dystrophy
Female: Carries hemophilia gene
Male: Has hemophilia
Red = hemophiliac gene