1. CHAPTER 14
MENDEL & the GENE IDEA

2. 14.1 - Mendel’s Discovery The Law of Segregation
The 2 alleles of a gene separate (segregate) during gamete formation, so that a sperm or egg only carries 1 allele of each pair
Explains 3:1 ratio found in hybrid crosses
The Law of Independent Assortment
Each pair of alleles segregates into gametes independently of other pairs
Explains 9:3:3:1 ratio found in hybrid crosses

3. 14.1 – Mendel’s Discovery Vocabulary Character – heritable feature
Trait – variant of a character (heritable feature)
True Breed – Whatever traits the parent has are expressed in ALL subsequent populations
For example, self-pollinating a purple flowered plant produces a generation of only purple flowered plants.
Typically, need to do this for at least two (2) generations to ensure that the parent is a true-breed
Hybrid – Mating (crossing) of two (2) true-breeding varieties of true-breeds

4. 14.1 – Vocabulary (Continued)
P Generation – Parent generation
Two (2) true-breeding parents being crossed
F1 – First filial (child or son) generation
Hybrids
F2 – Second filial generation
Each member of F1 self-pollinates
Hybrids again
3:1 ratio and
9:3:3:1 ratio when 2 characters are considered

5. Dominant vs. Recessive Dominant trait Recessive trait
One that will mask the recessive trait if found together
Recessive trait
Trait that will be masked if found with the dominant trait

6. What Mendel Found… (stretched the truth about)
Only looked at “all-or-nothing” traits
Sometimes called binary traits – “yes” vs. “no”
Mendel took true breeding for 1 trait, and pollinated it with a true breed for another trait
What is the difference between trait & character?
Purple flowered + White flowered
F1 = All colored purple (all expressed only one trait)
F2 = 3:1 ratio of one trait to the other

7. Quick Review ___ Generation What type of plants? ____________

8. Mendel’s Model Particulate theory of inheritance
There are dominant and recessive forms of a particular character
In F1 the dominant character masked the recessive character
In F2 the recessive character reappeared, unaltered, from F1
Hence, no blending was occurring, just one trait (dominant) was masking the effects of the other (recessive).

9. Mendel’s Model (Page 2)
1. Alternative versions of genes account for variations in inherited characters
The alternative versions are called alleles
One plant had the allele for purple flower color while the other had alleles for white flower color
Chromosomal Relationship
Each gene resides on a chromosome
DNA at that locus, however, varies in its nucleotide sequence (C, G, T, or A [U for RNA]) at that locus
So the alleles are different nucleotide sequences on a chromosome
Even though the sequences are the same locus

10. Alleles – Alternative Versions of a gene

11. Mendel’s Model (Page 3)
2. For each character, an organism inherits two (2) alleles, one from each parent
A locus is represented twice in a diploid cell
Once on the chromosome from Mom,
Once on the chromosome from Dad
The two alleles may be the same or different
If the 2 alleles are the same = Homozygous
Different = Heterozygous
If the alleles differ, then the dominant allele determines the organism’s appearance.
If the other allele, recessive allele, has no noticeable effect on appearance

12. Law of Segregation
The 2 alleles for a heritable character separate (segregate) during gamete formation & end up in different gametes
This makes sense in terms of chromosome, we already know that homologous chromosomes assort independently during meiosis into gametes
However, Mendel did his research far before the discovery or even the mere conception of chromosomes

13. Some more vocab… Homozygous Dominant Homozygous Recessive Heterozygous
2 copies of dominant allele
Homozygous Recessive
2 copies of recessive allele
Heterozygous
One copy of dominant allele, one recessive
Genotype
Genetic Makeup
Phenotype
Appearance

14. Phenotype vs. Genotype

15. Punnett Squares P p Pp You know ‘em, & you love them!!
Put one parent along the top, the other parent along the left side:
Let us assume that we cross a homozygous dominant (PP) & homozygous recessive (pp): P p Pp

16. Figure 14.5 (Pp. 255)

17. Your Turn…. Cross all 6 combinations: Heter x Heter Cross Genotype
Phenotype
HomoD x HomoD
HomoR x HomoR
Heter x Heter
HomoD x HomoR
HomoD x Heter
HomoR x Heter

18. Pattern recognition…
What pattern did you find in phenotypes for each of the crosses?
What about the genotypes?
What must be the parental genotypes?
If all have recessive phenotype
If there is a 3:1 ratio of phenotype
If you get half with dominant, and half with recessive
If you get all dominant (careful with this one!)

19. Test Cross Def - Cross a recessive phenotype & Dominant phenotype
We know the genotype of the recessive phenotype (must be HomoR) but we don’t know the genotype of the Dominant.
If the cross produces:
ALL Dominant = ???
½ Dominant, ½ Recessive =???

20. Monohybrid vs. Dihybrid Cross
Monohybrid Cross – Take pure breeds for 1 character and cross (AA x aa)
Dihybrid Cross – Take pure breeds for 2 characters and cross (AABB x aabb)
Dihybrid crosses =
F1 all hetero for both characters
F2 9:3:3:1 for phenotype

21. Dihybrid Cross

22. How to explain the results?
We already know that the alleles contained on individual chromosomes segregate from each other in meiosis
BUT what about the traits themselves?
Do the traits segregate independently of each other or does their distribution depend on the other trait being crossed?
The answer now is complicated, but Mendel chose (lied about them but who cares? ) traits that behaved independently

23. Law of Independent Assortment
States that each pair of alleles segregates independently of other pairs of alleles during gamete formation
This rule really only pertains to genes (allele pairs) on different chromosomes
That is, on non-homologous chromosomes
If they are on homologous chromosomes, they are considered to be linked genes
Cannot do Mendelian genetics, so we save this for next chapter (CH 15)

24. 14.2 – Laws of Probability govern Mendelian Genetics
The Multiplication & Addition rules apply to monohybrid crosses
Multiplication rule – probability of a compound event is equal to the product of the individual probabilities of the independent events
Addition rule – If an event that can occur in two or more independent, mutually exclusive ways, its probability is the sum of the individual probabilities
In multi-character crosses, consider each character separately and then multiply the probabilities together

25. 14.2 – Laws of Probability Probability Basics
Probabilities range from 0 to 1
0 = NOT gonna happen 1 = It will happen
P(event) = Probability of an event occurring
P(Tails on a fair coin) = ½
P(king of spades) = 1/52
P(king) = 4/52 = 1/23
Sum of probabilities of all possible outcomes must sum to 1
P(heads) + P(tails) = 1

26. Multiplication Rule
If two (2) events occur but are independent of each other, then we can multiply their individual probabilities to get the compound probability P(both events).
Example: P(Dolphins will beat Bills) = 0.5 & P(Jets will beat Steelers) = 0.5, THEN
P(both events) = 0.5x0.5 = 0.25
*** NOTE: these events MUST be independent

27. Multiplication Rule (AND rule)
P(head) = ½
So if I flip a coin twice, what is the P(heads on the first flip AND the second)?
Are coin flips independent of each other?
In other words, does getting heads on the first flip affect the other coin flip?
P(Both heads) = P(heads) x P(heads) = ½ x ½
P(Both heads) = ¼

28. Addition Rule (OR rule)
When there are 2 or more ways independent & mutually exclusive ways to get some result, you can add up the individual probabilities for the compound result.
Example: P(Pats win Superbowl) = 0.5 &
P(Fins win Superbowl) = 0.1, THEN
(Pats OR Fins win Superbowl) =
= 0.6

29. Find P(aa) & P(Aa) P(aa) = P(“a” from mom) * P(“a” from dad) = ½ * ½
= ½ * ½
= ¼
P(Aa) = P(Aa) + P(aA)
= (½ x ½) + (½ x ½)
= ¼ ¼
= ½

30. Solving Complex Probability Questions with these rules
What is the probability of only two recessive phenotypes in a trihybrid cross of
One parent is heterozygous for all 3 characters
Other is hetero for flower color, but HomoR for the other 2 characters

31. Your Turn…
What is the probability of getting 1 recessive phenotype in a dihybrid cross between 2 heterozygotes (hetero for both characters)
Characters: Y = yellow seed color, y = green seed color
R = round seed shape, r = wrinkled seed shape
Answer the problem both ways:
1. Use compound probabilities to calculate this value
2. Use Punnett square to determine the number directly

32. AaBbCc x Aabbcc
P(Recessive Phenotype for at least two traits)?
P(aabbCc) = ¼ * ½ * ½ = 1/16
P(aabbcc) = ¼ * ½ * ½ = 1/16
P(aaBbcc) = ¼ * ½ * ½ = 1/16
P(Aabbcc) = ½ * ½ * ½ = 2/16
P(AAbbcc) = ¼ * ½ * ½ = 1/16
6/16 = 3/8

33. Discuss Detail at least 2 limitations of Mendelian Inheritance
Give an example of each of these limitations
How are these limitations accommodated or accounted for?
Format
Use complete sentences
Type up in Word or other Word processing program
Send it to your

34. 14.3 – Extensions of Mendelian Genetics
Mendelian genetics seems to be relevant to only a small set of heritable features
For only a few characters there are…
Only 2 versions of an allele (green or yellow)
1 gene codes for a single external character
1 allele is completely dominant to the other
The basic patterns of segregation & independent assortment apply to more complex patterns of inheritance

35. Incomplete Dominance vs. Codominance
Phenotype of heterozygote & HomoD are indistinguishable
Incomplete dominance
Phenotype of heterozygote is in between the 2 Homo phenotypes
Example: pink snapdragons
Codominance
Phenotype of heterozygote is separate &
distinguishable from HomoD & HomoR.
Example: AB blood type

36. Example of Incomplete Dominance

37. Doesn’t incomplete dominance provide evidence for “blending” theories?
So incomplete dominance does NOT provide evidence for “blending” theories

38. Dominance & Phenotype
The observed dominance/recessiveness of alleles depends on the level of the investigation
Consider Tay-Sachs disease
Brain cells of the baby do not metabolize certain lipids
As lipids accumulate, seizures, blindness, and mental degeneration
Death occurs within a few years of conception

39. Tay-Sachs disease example (Page 2)
@ the Organismal level, the disease is recessive
Only children with 2 copies of the recessive trait will have the malady
Heterozygote is not afflicted – they produce some lipid- metabolizing enzyme, though not as much as in HomoD
So intermediate enzyme production
This suggests the biochemical level, the disease is an example of incomplete dominance
Which is Tay-Sachs: dominance or incomplete dominance?

40. How common are dominant alleles?
Polydactyly
Extra fingers or toes
1 of 400 in the US
The allele for polydactyly is dominant, but rarely present
Recessive homozygotes (HomoR) are found 399 out of 400 instances

41. Multiple Alleles
Only 2 alleles existed for Mendel’s peas, but this is not typical for most traits
Consider ABO blood group in humans
A refers to the “A” carbohydrate & type A blood
B refers to (seriously, I’m not writing this down)
O means neither A or B carbohydrate is found
AB means both A & B are found
BUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles

42. Blood Types

43. Clumping

44. Nobel Prize, so it has to be good!?!
re.html The Blood Type Game – Will u kill the patient? ml

45. Pleiotrophy Single gene has multiple effects
Should be unsurprising given intricate molecular and cellular interactions for development of an organism

46. Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus
Example: Mouse fur color
Bb or BB = Black bb = brown
If HomoR for (C) gene [cc], then no fur color (albino or white)
Regardless of fur color specified by brown-black gene
If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)

47. Epistasis Continued What is the phenotype of BBcc? BbCc? bbCC? Bbcc?

48. Epistasis - Punnett Square

49. Polygenic Inheritance
The additive effect of 2 or more genes on 1 phenotypic character
Called quantitative characters since there is a continuum of gradations
Normal curve of phenotypes
Example: human skin pigmentation is determined by at least 3 separately inherited genes
AABBCC = Dark skin colored
aabbcc = Light skin colored
AaBbCc = Intermediate skin colored

50. Polygenic Trait – Skin Color

51. Environmental Impacts
Skin pigmentation, and every other character, is affected by environmental factors as well
For example, sun tanning
Hence, we say that a genotype is associated with (or codes for) a range of phenotypic possibilities
The phenotypic range is called the norm of reaction for a genotype
For example, red blood cell count (part of standard screening protocol - CBC, Chem7[BMP7], UA)
Exercise = higher RBC count
Altitude, presence of infectious agents, race, age = differential RBC

52. 14.4 – Humans & Mendelian Inheritance
Pedigree Analysis
Family tree describing the interrelationships of parents & children across the generations

53. Pedigree Analysis

54. Genetic Disorders Due Friday (11/13/09)
For each of the following disorders
What are the major characteristics of this disorder?
What is the genetic basis of the disorder?
Relate disorderly structure to dysfunction
Sickle Cell Anemia
Tay-Sachs
Huntington’s disease
SCID
Trisomy 211

55. Mendelian Genetics? Due Wednesday (11/11/09)
For each of the following inheritance patterns
How does each differ from typical Mendelian inheritance?
How does Mendelian genetics explain this pattern of inheritance?
Give an example
Relate genetic structure to the phenotype for each
Epistasis
Codominance
Incomplete Dominance
Multiple alleles

56. END

57. Recessively Inherited Traits
Albinism to Cystic Fibrosis
Only show up in individuals who have 2 copies of the recessive trait
Heterozygotes are called carriers
Normal phenotype, but may transmit disease to offspring

58. Cystic Fibrosis Lethal disease Common for those of European descent
1 of 2,500 affected
1 of 25 are carriers
Affects Chloride ion transport between a cell and extracellular fluid
If untreated, most die before 5th birthday
Typically, patients live until their 20s or 30s with efficacious treatment

59. Sickle-Cell Disease Recessive autosomal disease
Affects African-Americans and those of African descent
Affects Hemoglobin protein in RBCs
Low blood oxygen = hemoglobin molecules clump together forming sickle shaped RBCs
Sickled RBCs may clump together creating chronic vascular occlusion of small vessels
Example of incomplete dominance
Heterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation

60. Consanguineous Matings
Mating of close relatives
Dramatically increases the probability of passing on recessive traits
Most deleterious alleles have such severe effects that spontaneous abortion occurs long b4 birth

61. Dominantly Inherited Disorder
Most harmful alleles are recessive, but some human diseases are due to dominant alleles
Achondroplasia – form of dwarfism
Heterozygous individual = dwarf
1 in 25,000 have achondroplasia, so 99% of the population are HomoR
Hypothesis: if there is a lethal disease carried on a dominant allele, it would have burned out its carriers by now.
UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced age
Like Huntington’s disease