1. Mendel’s workplace
Figure 2.5
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display

2. Prior history
To appreciate Mendels work, one must keep in mind the prevailing theories of inheritance
Preformationism: the idea that gamete contains an intact organism, was first proposed in the late 1600s.
A preformed human infant or homunculus contained within a sperm (Male centric view of the world)
Blending inheritance: essences of both sperm and egg mixed to form offspring intermediate between the parents Strains of plants and animals generated by blending
Mendel ignored development, focused solely on transmission of traits

3. Darwin and Heredity
Darwin: Among individuals of any species, there are differences (variations)
Evolution cannot occur unless there are differences among individuals
If individuals are identical and remain so generation after generation, there can be no evolution.
Variation is important
Variation must be transmitted from parent to offspring
Mechanism of inheritance is important for understanding evolution (1860s)
Mechanism of inheritance not understood
Certain traits suddenly appear in an individual
These traits are then transmitted to their progeny
Porcupine man- protuberances on body
Not due to the environment- only the man and his offspring have these traits

4. Pangenesis Pangenesis: (1860s) Whole organism reproduces itself
Gemmules determine characteristics (traits) of organism
Germ cells contain gemmules of all sorts and these are transmitted to the next generation.
Fertilization- gemmules unite and produce new cells of the types from which they are produced.

5. Cytology Cytology: 1600s Organisms are composed of cells
1830s The most distinct structure in a cell is the nucleus. ALL cells have a nucleus. Role of nucleus was controversial because it disappears during cell division.
1840s Cells are formed by the division of pre-existing cells (Mitosis)
1850s Sperm and Ovum are cells.
1873 Mitosis described in detail- nuclear and chromosome dynamics described
1883 Fertilization in sea urchin showed that sperm and ovum fuse. This links parents to offspring
Problem of ploid:. Nuclei of parents and progeny are diploids. Are nuclei of germ cells haploid?
1885 Meiosis described- Reductional division of chromosomes keeps number of chromosomes constant. No similar phenomenon seen in any other cellular organelle.

6. Cytology’s contribution to Mendel
Heredity is a consequence of genetic continuity
Germ cells are the vehicle of transmission from one generation to the next.
Germ cells contain half the number of chromosomes found in body cells.
Fertilization involves union of sperm and eggs. Fertilization involves union of nuclei.
Nucleus is crucial. During division it resolves into long chromosomes that split lengthwise.
Chromosomes do not lose their individuality. They are inherited intact.
Diploid embryo descends from maternal/paternal fusion of haploid chromosomes

7. The origin of genetics:
The study of genetics begins when Gregor Mendel, in 1865, addressed the question :
"How are characters passed on from one generation to the next?”
Mendel was the first to make a serious attempt of experimentally answering the question of heredity and not only were his answers correct, they were a complete and compelling proof.
Mendel published in 1866 but little attention was paid to his work until 1900, when it was simultaneously rediscovered by three scientists, one in Holland, one in Austria, and one in Germany.
There are often impressions that Mendel was removed from the scientific community, or that his papers were not well circulated. This was not true. Over 200 copies of Mendels papers have been discovered in different libraries.
All three of Mendel's rediscovers had read Mendel's work prior to publishing their own work.

8. Gregor Mendel was born on either 20th or 22nd July, 1822 in Heizendorf (today Hynice in the Czech Republic).
From 1851 to 1853, Gregor Mendel studied zoology, botany, chemistry, and physics at the University of Vienna.
He studied botany under Prof. Unger where he learned genetic crosses
He studied physics under Prof. Doppler where he learnt statistics
Mendel returned to Brno and began his experiments with the hybrid cultivation of pea plants in 1856.
After spending eight years carrying out experimental work in the monastery garden, he reported on the results of his observations at the meetings of the Association for Natural Research in Brno on the evenings of February 8th and March 8th, 1865
Why Peas?

9. Genetic model organisms Amenable to genetic analysis
The genomes of many organisms have been sequenced.
A model organism is a species that has been widely studied, usually
because it is easy to maintain and breed in a lab and/or has particular
experimental advantages and are used to obtain information about species that are more difficult to study directly.
Remember: processes are conserved!!!!!
Genetic model organisms
Amenable to genetic analysis
Breed in large numbers
Short generation time (large-scale crosses over several generations.)
Many different mutants available
Detailed genetic maps
Baker's yeast (Saccharomyces cerevisiae), the fruit fly (Drosophila melanogaster) and the nematode (Caenorhabditis elegans)
Experimental model organisms
Long generation intervals and poor genetic maps
Produce robust embryos that can be studied and manipulated with ease.
Used in developmental biology.
Chicken, Zebrafish and Xenopus laevis
Genomic model organisms
Occupy a pivotal position in the evolutionary tree
Special quality of their genome.
An example is the puffer fish (Fugu rubripes) which has a similar gene repertoire to humans but a much smaller genome (400 million base pairs instead of 3000 million

10. Location in evolutionary tree Genome size, Ease of cloning a gene
Model organisms
Human model organisms
Relevance of model organisms to humans.
Over 60 per cent of the human disease genes have counterparts in the fly and worm. About 1500 gene families are conserved in all animals.
Genes affecting immune system etc, are less likely to have counterparts in simple animals. For these systems, we require closer models such as the mouse . Mice have been used to establish disease models by mimicking the gene defects seen in humans, and these models can be used to test the efficacy of new drugs.
E.coli
Yeast
Worm
Fly
Zebra fish
Mouse
Arabidopsis
Modern genetics:
Location in evolutionary tree
Genome size,
Ease of cloning a gene
Ability to insert DNA back into genome
Ability to monitor development
Xenopus, dog, tetrahymena

11. The Pea
Mendel chose the common garden pea to study patterns of inheritance. This was a excellent choice as a model system for the following reasons:
1 Many varieties
2 It can self pollinate and cross pollinate
3 Cheap
4 Short generation time
He identified over 20 traits and studied 7 (why 7?)
Seed shape: round versus wrinkled
Seed color: yellow versus green  
Flower color: red versus white
Pod shape: inflated versus pinched
Pod color: yellow versus green
Flower position: axial versus terminal
Stem length: long versus short

12. True breeding
The first two years of Mendel's work were devoted to selecting lines that breed true (pure lines) for a particular character or trait.
He identified over 20 traits that bred true and studied 7
Breeding True:
smooth seed |
Self cross
wrinkled seed
He identified plants that produced only smooth seeds and plants that produced only wrinkled seeds.
He identified plants that produced only purple flowers and plants that produced only white flowers.

13. P purple X white ----> All purple male anther female stigma
Mendel’s first cross
P purple X white ----> All purple male anther female stigma
F1 purple X purple -----> purple:white
705 purple:224 white
P purple X white ----> purple --> female male
Blending may not be correct
Reciprocal cross also gave the same result therefore preformationism also not correct

14. Mendel’s first cross
P yellow X green ----> All yellow
Mendel crossed pure breeding yellow pea plants to pure breeding green pea plants. All of the progeny were yellow pea plants. Next he selfed these yellow plants allowing the pollen to fall on its own stigma.
F1 yellow X yellow ----> yellow:green
yellow=6022 green=2001

15. Keys to success
He did these experiments with all seven traits!!!!
The ratios obtained were between 2.82:1 and 3.15:1
This cross involving only one character, seed color, is called a monohybrid cross.
Keys to success:
1 Generated a pure breeding line before starting the cross
2 Analyzed individual character
3 Counted progeny

16. Conclusions
Although others were doing crosses at the time, Mendel work was unique
Results and Conclusions:
1 In F1 all progeny showed only one of the two possible traits for all crosses
2 Males and females not important
3 Character missing in F1, reappeared in F2
Traits did not blend in the offspring but were transmitted in a discrete fashion and remained unchanged.
Reciprocal crosses produced the same results, this indicated that each parent makes an equal contribution to genetic makeup of the offspring. The sperm does not contain a homunculus.

17. Terms:
Phenotype and genotype:
The parental yellow pea plants are a pure line- they only produce yellow pea plants when selfed.
However the F1 yellow pea plants produce some green pea plants when selfed.
P YY X yy
(yellow) (green)
F1 Yy
(yellow)

18. The Yellow of the P generation is different from the yellow from the F1 generation
P YY x YY
(yellow) (yellow)
F1 YY (All yellow)
F1 Yy
(yellow)
F2 Yy (yellow) X
Yy (yellow)
1YY:2Yy:1yy
1 Yellow:2Yellow:1Green
Therefore it is necessary to make the distinction between the appearance of an organism and its genetic make-up.
Phenotype refers to the appearance of an organism
Genotype refers its genetic makeup
Would say the parental yellow pea plants and F1 yellow pea plants have the same phenotype but a different genotype.

19. Terms:
Dominant and recessive: All F1 seeds were yellow but when selfed the F2 produced some green seeds.
Mendel termed the trait that is expressed in the F1 as dominant
The trait that is hidden but re-expressed in the F2 as recessive.
The F1 plants must contain factors for green and yellow since both reappear in the F2.
From the fact the reciprocal crosses produce the same result, Mendel concluded that male and females contribute equally
With these assumptions the simplest model is that the F1 contains two hereditary factors
One for green and another for yellow
We will use the Uppercase Y to represent the dominant yellow factor and the lower case y to represent the recessive green.

20. The Principle of Segregation:
Mendel reasoned that without a mechanism to halve the number of factors in each generation, that factors would multiple with each generation and become unmanageable.
Mendel reasoned that during gamete formation the paired factors separate and each gamete receives one of the two factors.
Parent YY yy
Gametes Y Y y y
F Y y
Sperm and egg randomly combine to produce F2 progeny
Notice that while the parents have two factors, they produce gametes containing only a single factor and the progeny again have two factors

21. YY
Yellow
Grows into
A plant
Generates
Gametes Y Or yy
Green
Grows into
A plant
Generates
Gametes y Or
Gametes combine at Random Yy
Yellow

22. Mendel's assumption of two factors and segregation makes
a strong prediction concerning the genetic make-up of the of
F2 yellow pea plants
F1 Yy X Yy
Y y Y y Yy YY Y y F2 yy yY
Mendel's prediction:
Phenotype ratio: 3:1 (yellow:green)
Genotype ratio: 1:2:1
1/4YY 1/2Yy 1/4yy genotype
Yellow yellow green
3/ /4
Of the yellow F2 plants, 1/3 should be YY and 2/3 Yy
How would you test this prediction?

23. 120 green F2 were selfed: 120 green progeny seen
Selfing
Mendel selfed each of the green F2 plants
Green X Green
yy X yy
F3 yy
green
120 green F2 were selfed: 120 green progeny seen

24. Yellow Yellow YY X YY F3 YY yellow 1/3 of all yellow plants
Selfing
Mendel selfed each of the yellow F2 plants
Yellow Yellow
YY X YY
F3 YY
yellow
1/3 of all yellow plants
Yellow Yellow
Yy X Yy
YY:Yy:yy
3 yellow:1green
2/3 of all yellow plants
519 yellow F2 were selfed: 166 gave all yellow and 363 gave yellow/green
Therefore:
Phenotype ratio: 3:1 yellow:green
Genotype ratio: 1:2:1
1/4YY 1/2Yy 1/4yy genotype
Yellow yellow green
3/ /4
Of the yellow F2 plants, 1/3 should be YY and 2/3 Yy
That is what Mendel observed!
Is there another way to test this prediction?

25. Test cross
If instead of selfing the F2 plants, they are crossed to
pure breeding green plants, what are the expected outcomes: F2
YY x yy
------>all yellow (Yy)
Yy x yy
------>1:1 yellow (Yy):green (yy)
yy x yy
------>all green (yy)

26. Probability Lets cross a Yy with a Yy pea plant
What is the probability of obtaining a homozygous YY plant
Chance of a Y sperm uniting with a Y egg
Chance of sperm with Y allele 1/2
Chance of egg with Y allele 1/2
Chance of Y and Y uniting
½ x ½ = ¼
What is the probability of obtaining a heterozygous Yy plant
Chance of sperm with Y allele and egg with y allele 1/4
Chance of sperm with y allele and egg wit Y allele 1/4
Chance of Yy = (1/2x1/2) + (1/2x1/2)= 1/2

27. Chance of Y sperm uniting with a Y egg
Probability
Cross Yy xYy pea plants.
Chance of Y sperm uniting with a Y egg
(1/2) chance of sperm with Y allele
(1/2) chance of egg with Y allele
Chance of Y and Y uniting = 0.5 x 0.5 = (1/4)
Chance of Yy offspring
1/4 chance of sperm with y allele and egg with Y allele
1/4 chance of sperm with Y allele and egg with y allele
Chance of Yy = (0.5x0.5) + (0.5x0.5) = 0.5

28. More terms:
Mendel's factors are now known as genes
Alternative forms of a gene that determine different traits are known as----
_alleles__
Individuals with two identical alleles are said to be
_homozygous_
Individuals with two different forms of alleles are said to be—
_heterozygous__

29. The dihybrid cross and the principle of independent assortment:
In the second set of experiments Mendel investigated the
pattern of inheritance for two sets of characters simultaneously.
A cross involving two sets of characters is called a dihybrid cross.
Pea shape: smooth, wrinkled (Smooth is dominant to wrinkled)
Cotyledons color: yellow, green (Yellow is dominant to green)
P Smooth yellow x wrinkled green
F1 smooth yellow
selfed
F2 315 smooth yellow 9
101 wrinkled yellow 3
108 smooth green 3
32 wrinkled green 1

30. 9:3:3:1 ----> 3:1
If we examine seed shape only (smooth, wrinkled) and ignore cotyledon color (yellow, green), in the F2, we expect to find:
3/4 smooth and 1/4 wrinkled:
Take all the smooth peas from the four classes and add them up
# Smooth = = 423 
# wrinkled = = 133
423:133 is close to the 3:1 expected 
Now, if we only examine cotyledon color and ignore seed shape
we expect 3/4 Yellow to 1/4 green. # Yellow =
# green =
416:140 is also close to the 3:1 expected ratio
The wrinkled and green phenotypes INDIVIDUALLY behave as a standard recessives in a monohybrid cross.

31. The 9:3:3:1 ratio.
The 9:3:3:1 ratio appears a lot more complex than the 3:1 ratios of the monohybrid cross.
Mendel's insight was to realize the 9:3:3:1 ratio is nothing more than two 3:1 ratios combined at random. That is if one examined the traits individually they formed a 3:1 ratio.
To determine the mode of inheritance of the two genes in this
dihybrid cross Mendel examined each of the traits separately:

32. Each trait behaves as a standard recessive found in a
monohybrid cross.
They do not affect one another
Genes segregate independently!!!!
GeneS in a gamete does not affect the segregation of geneY in that gamete
SS YY
ss yy
Parent
Gamete SY sy F1
Ss Yy
Self cross
Ss Yy x Ss Yy

33. In a heterozygous individual (Self cross)
SsYy x SsYy
Genes line up in two ways during gamete formation S s Y y S s y Y or SY sy or Sy sY
Gamete
SY sy Sy sY

34. SsYy SY sy Sy sY In a heterozygous individual with two traits
SsYy x SsYy
Genes line up in two ways during gamete formation giving rise to 4 different gametes
SsYy SY sy or Sy sY
Gamete
SY sy Sy sY
25% 25% 25% 25%
Mendel concluded that SsYy individuals produced
gametes in a 1:1:1:1 ratio ---SY Sy sY sy

35. Independent assortment
If independent assortment is occurring, four different kinds of
gametes will be produced in equal frequencies.
The only rule is that S and s segregate to separate gametes and
Y and y segregate to separate gametes
(that is one does not get an Ss gamete or a Yy gamete)
SsYy males and SsYy females can produce four types of gametes
in equal frequencies:
SY, Sy, sY, sy
The male and female gametes randomly combine to restore diploidy

36. Independent assortment of gene pairs
YYSS x yyss
YySs x
9Y-S-:3Y-ss:3yyS-:1yyss
Different gene pairs assort independently during gamete formation.
The presence of a Y in a gamete does not influence the
probability of a S or s being in that gamete.

37. Calculation
A heterozygous Red eyed curly winged female fly is crossed to a heterozygous Red eyed curly winged male fly.
What is the expected ratio for a Red eyed flat winged male fly?
A heterozygous smooth yellow pea is crossed to a heterozygous smooth yellow pea.
What is the expected ratio for a smooth green pea?

38. Punnet diagram of a dihybrid cross
male
Female
1/4 Sy
1/4 SY
1/4 sY
1/4 sy
1/16
SY/SY
Smooth
yellow
1/16
SY/Sy
Smooth
yellow
1/16
SY/sY
Smooth
yellow
1/16
SY/sy
Smooth
yellow
1/4 SY
1/16
Sy/SY
Smooth
yellow
1/16
Sy/Sy
Smooth
green
1/16
Sy/sY
Smooth
yellow
1/16
Sy/sy
Smooth
green
1/4 Sy
1/16
sY/Sy
smooth
yellow
1/16
sY/SY
Smooth
yellow
1/16
sY/sY
wrinkled
yellow
1/16
sy/sY
wrinkled
yellow
1/4 sY
1/16
sy/SY
Smooth
yellow
1/16
sy/Sy
Smooth
green
1/16
sy/sY
wrinkled
yellow
1/16
sy/sy
wrinkled
green
1/4 sy
9 smooth yellow: 3 smooth green: 3 wrinkled yellow: 1 wrinkled green
Therefore the 9:3:3:1 ratio is a natural outcome of applying
Mendel's two laws

39. Monohybrid phenotype--->dihybrid phenotype
If you combine the monohybrid ratios for two PHENOTYPES
you get:
3/4 Yellow
1/4 Green
3/4x3/4= 9/16
3/4 Smooth
1/4 Wrinkled
3/4x1/4= 3/16
1/4x3/4= 3/16
1/4x1/4= 1/16

40. Probability
Yellow is dominant to green
Smooth is dominant to wrinkled
Heterozygous yellow
Heterozygous smooth X
Homozygous wrinkled
Probability of yellow wrinkled?
1/2 smooth
3/4 yellow
1/2 wrinkled
1/4 green
Probability of yellow wrinkled?
Heterozygous yellow
Homozygous smooth X
Homozygous wrinkled
3/4 yellow
1 smooth
0 wrinkled
1/4 green

41. More probabilities
Yellow is dominant to green
Smooth is dominant to wrinkled
Tall is dominant to short
Heterozygous yellow
Heterozygous smooth
Heterozygous tall
Heterozygous yellow
Homozygous wrinkled
Heterozygous tall X
Probability of green wrinkled short?
smooth
green
tall
wrinkled
short

42. More probabilities
Yellow is dominant to green
Smooth is dominant to wrinkled
Tall is dominant to short
Heterozygous yellow
Heterozygous smooth
Heterozygous tall
Heterozygous yellow
Homozygous wrinkled
Heterozygous tall X
Probability of green wrinkled short?
1/2 smooth
1/4 green
3/4 tall
1/2 wrinkled
1/4 short

44. Rules of Probability
Independent events: The probability of two events occurring together
What is the probability that both X and Y will occur together?
Answer: First determine the probability of each event
Then multiply them together.
Mutually exclusive events: The probability of one or another event occurring.
What is the probability of X or Y occurring?
Then add them together.

45. Probability and Mendel’s Results
Cross Yy xYy pea plants.
What is the chance of Y sperm uniting with a Y egg
½ chance of sperm with Y allele
½ chance of egg with Y allele
Therefore chance of Y and Y uniting = ½ x ½ = 1/4
What is the chance of Yy offspring
½ chance of sperm with y allele and egg with Y allele or
½ chance of sperm with Y allele and egg with y allele
Therefore chance of Yy – (½ x ½) + (½ x ½) = 2/4, or 1/2

46. What is the probability of obtaining the genotype RrYyTtss in the F2?
Multiple genes
P RRYYTTSS  rryyttss
F RrYyTtSs  RrYyTtSs
What is the probability of obtaining the genotype RrYyTtss in the F2?

47. F2 What is the ratio of different genotypes
Multiple genes
P RRYYTTSS X rryyttss
gametes
RYTS
ryts
F RrYyTtSs X RrYyTtSs
gametes
RYTS
RyTS
rYTS
RYTs
RyTs
rYTs
RYtS
RytS
rYts
Ryts
RYts
ryTs
rYtS
ryts
RYTS
RyTS
rYTS
RYTs
RyTs
rYTs
RYtS
RytS
rYts
Ryts
RYts
ryTs
rYtS
ryts
F2 What is the ratio of different genotypes
and phenotypes?

48. Punnet Square method for multiple genes
-Four loci
Each loci is heterozygous i.e. 2 types of gametes
- 24 = 16 possible gamete combinations for each parent
Thus, a 16  16 Punnet Square giving rise to 256 genotypes
Big Punnet Square
Alternatively –
Loci Assort Independently
Look at each locus separately.

49. Probability of obtaining individual with Rr and Yy and Tt and ss.
Branched diagram
P RRYYTTSS  rryyttss
F RrYyTtSs  RrYyTtSs
What is the probability of obtaining the genotype RrYyTtss in the F2?
Rr  Rr
1RR:2Rr:1rr
2/4 Rr
Yy X Yy
1YY:2Yy:1yy
2/4 Yy
Tt  Tt
1TT:2Tt:1tt
2/4 Tt
Ss  Ss
1SS:2Ss:1ss
1/4 ss
Probability of obtaining individual with Rr and Yy and Tt and ss.
2/4  2/4  2/4  1/4 = 8/256 (or 1/32)

50. What is the probability of obtaining a completely homozygous
Branched diagram
P RRYYTTSS  rryyttss
F RrYyTtSs  RrYyTtSs
What is the probability of obtaining a completely homozygous
genotype in the F2?
Genotype could be RRYYTTSS or rryyttss
Rr  Rr
1RR:2Rr:1rr
1/4 RR
1/4 rr
Yy  Yy
1YY:2Yy:1yy
1/4 YY
1/4 yy
Tt  Tt
1TT:2Tt:1tt
1/4 TT
1/4 tt
Ss  Ss
1SS:2Ss:1ss
1/4 SS
1/4 ss
(1/4  1/4  1/4  1/4) + (1/4  1/4  1/4  1/4)
= 2/256

52. Significance of Ratios
What is the biological significance of the 9:3:3:1 ratio? This
ratio is only produced if TWO DIFFERENT GENE PAIRS assort
independently of each other during gamete formation.
The presence of one gene in a gamete does not influence
the probability of another gene being found in that gamete
Principle of segregation: for one gene, each individual has two copies. These two copies segregate from one another during gamete formation.
Independent assortment: Segregation of one gene pair is independent of the segregation of any other pair of gene.

53. Mendel tested seven different traits and all assorted independently!
The pea plant has seven chromosomes
(The seven traits are not on different chromosomes)
Mendel had generated 20 true breeding lines
What about the other 13 traits?
Did he do experiments with these and find that they do not assort independently?
Why did he so thoroughly study only the seven traits that assorted independently.
He probably only used data he understood and it is possible that the other data did not fit any statistical understanding and he therefore ignored/ suppressed this data.
Fire destroyed his original notes, so we will never know!

54. Mendels laws
The principle of segregation: Each individual carries two
copies of a given gene and these segregate from one another
during gamete formation.
2. The principle of independent assortment: The segregation of
one pair of genes is independent of the segregation of any
other pair of genes during gamete formation
(as we will find their are important exceptions to this rule)!!!!!
By applying these rules Mendel concluded that SsYy individuals
produced the following gametes in a 1:1:1:1 ratio
SY Sy sY sy
As described above, he inferred these gamete ratios by selfing
SsYy individuals and looking at phenotypes of progeny.
He could also have inferred these gamete ratios by crossing
SsYy individuals to ssyy individuals.
Crossing to the homozygous recessive individuals is known as a
test cross

55. A test cross is easier that a self cross for the F2
SsYy x ssyy
Gamete:
1/4 SY 1 sy
1/4 Sy
1/4 sY
1/4 sy
What are the expected genotypic and phenotypic ratios of the progeny produced from this cross? 1 sy
1/4 SY sY Sy
What are the expected genotypic and phenotypic ratios of the
progeny produced from this cross?
To answer this, first describe the gamete classes and their
frequencies produced from the SsYy and ssyy individuals.
Use these to construct a Punnett square or branched diagram
1/4
SY/sy
Smooth
Yellow
1/4
sy/sy
Wrinkled
Green
1/4
sY/sy
Wrinkled
Yellow
1/4
Sy/sy
Smooth
green

56. Gene number
Power of Mendel phenotype ratios:
The ratio tells you the number of genes involved in determining a phenotype
3;1 ratio (selfing) = 1 gene
9:3:3:1 ratio (selfing) = 2 genes
27:9:9:9:3:3:3:1 ratio (selfing) = 3 genes
Say seed color is controlled by two genes- GeneS and GeneT
Green seeds are sstt
All others are yellow (if either S or T are present, the seed is yellow)
True breeding yellow x true breeding green
SSTT sstt
F1 SsTt
(yellow)
Self
SsTt x SsTt
How many green will you get out of this cross:
9 S-T- yellow
3 S-tt yellow
3 ssT- yellow
1 sstt green
These conclusions about gene number become very important when applied to human traits like size, behavior, temperament etc.

57. Take two individuals
They both have blue eyes
They mate and produce 16 progeny
12 have blue eyes and 4 have black eyes
There is ________ gene/s for eye color
If instead you get 15 with blue eyes and 1 with black eyes
There are _______ gene/s for eye color

58. Take two individuals
They both have blue eyes
They mate and produce 16 progeny
12 have blue eyes and 4 have black eyes
There is ___1___ gene/s for eye color
If instead you get 15 with blue eyes and 1 with black eyes
There are ___2___ gene/s for eye color
Both individuals must be heterozygous! Why? Because black eye appears. If only one was heterozygous and the other homozygous blue then no black should appear.
1st scenario: Bb x Bb-----> 3B- (blue) : 1bb (black)
2nd scenario BbCc x BbCc ----->
9 B-C-(blue): 3B-cc(Blue): 3bbC- (blue):1bbcc (black)

59. Dog genome
The Dog Genome Project is an example of how basic Mendelian
principles are being used to identify genes that control
morphology and behavior.
Dr. Jasper Rine at U.C. Berkeley crossed
Newfoundlands to Border Collies.
These dogs differ extensively in size and behavior.
Newfoundlands are vigorous swimmers and weight about pounds
Border Collies are herders and weigh about 50 pounds
By performing a series of Mendelian crosses one can begin to
determine how many and what kinds of genes are involved in
determining their behavior!!!!

60. Dogs have been breed for specific traits for 10,000 years
About 150 breeds have been generated through selective breeding
Diversity in
Physical makeup:
Coat color
height
mass
muscle
Behavior:
herding
tracking
retrieval
Guarding
Intelligence:
The individual breeds can mate with one another and produce viable fertile offspring
They can also mate with Cayotes and wolves
Border collies do not like water, Newfoundlands love water.
In the cross swimming is dominant!
This complex trait is likely mediated by a small number of genes (~2)

61. Coat hair phenotypes
Coat Variation in the Domestic Dog Is Governed by Variants in Three Genes
2 OCTOBER SCIENCE VOL 326 p150

62. Specific breeds of dogs are also associated with specific diseases
Dobermans: narcolepsy
Scotties: Haemophilia
Terriers: copper metabolism (menke disease)
Labrador: hip dysplasia
Beagle: seizure risk
The ratio from a doberman cross suggests that narcolepsy is most likely mediated by a single gene! (3:1 ratio)
Why are Mutts healthier than true breeds?

63. Hybrid Vigor
Hybrid vigor:
The first cross between two purebred lines is often healthier than either parent
Breed1 Breed2
a-B-C-d-E A-b-c-d-E
F1 a-B-C-d-E
A-b-c-d-E
If GeneA causes narcolepsy and geneC causes haemophilia
F1 will be normal.

64. Corn
Commercial corn is a F1 hybrid because of hybrid vigor
Two inbred lines are mated to generate a F1 that is sold
AAbb x aaBB
F1 AaBb
hybrid vigor
1930’s increase in corn output because of these hybrid varieties
Line1 -disease resistant- rust and mold resistance but low yield
Line2- large crop but susceptible to disease
Hybrid- disease resistant and large yield----Sold to farmer
The F1 hybrids do not breed true!
The farmer cannot plant the seeds he gets from the F1 hybrid in the next season. They will not be disease resistant with high yields!
Gametes AB Ab aB ab
The seeds will generate plants with
A-B- high yield, disease resistant
A-bb high yield, susceptible to disease
aaB- low yield, disease resistant
aabb low yield, susceptible to disease