1. More of Mendel

2. What we know…
1. Inheritance of biological characteristics
is determined by genes
2. Principle of Dominance
– When there are two or more alleles for a
gene, some are dominant while others are
recessive
3. Law of Segregation
– In sexually reproducing organisms, adult cells
have two copies of each gene—one from each
parent; these genes segregate when
GAMETES are formed

3. Does the segregation of 1
pair of alleles affect the
segregation of another pair
of alleles?

4. • Let’s look at Pea shape and pea color
– Round (R) vs. wrinkled (r)
– Yellow (Y) vs. green (y)
• Remember, there are 4 possible gametes
• If a parent is homozygous dominant (true-
breeding dominant) for both shape and color,
what will all possible gametes be?
• If a parent is homozygous recessive (true-
breeding recessive) for both shape and
color, what will all possible gametes be?

5. Make a test cross of:
•True breeding Round Yellow Peas
•Genotype (RRYY)
•True breeding Wrinkled Green peas
•Genotype (rryy)
ry ry ry ry
RY
RY
RY
RY

6. • What is the phenotype of the F1
offspring?
• What is the genotype of the F1
offspring?

7. What this first cross
told us…
• All F1 offspring were heterozygous for
seed shape (round) and seed color (yellow)
RrYy
• The F1 plant was made from fusing a
gamete carrying RY and a gamete carrying
ry
• Will the dominant alleles stay together or
separate when making the F2 offspring?

8. Now cross these hybrid
(RrYy) plants on a new 4X4
Punnett Square

10. What does the F2 cross
tell us?
• Are there combinations of alleles that we
did not see in either of the parents?
• This means that the alleles for seed color
separated independently than the alleles
for seed shape
• Genes that segregate separately do NOT
influence each other’s inheritance

11. Principle of Independent
Assortment
• Genes for different traits can
segregate independently during the
formation of gametes. This principle
helps account for many genetic
variations in plants, animals and other
organisms.

12. Mendel’s 4 Principle’s
1. Inheritance of biological
characteristics is determined by
genes
2. Principle of Dominance
3. Law of Segregation
4. Principle of Independent
Assortment

13. Exceptions to Mendel
• Genetics more complicated
• Some alleles are neither dominant
nor recessive
• Many traits are controlled by
multiple alleles or multiple genes

14. Incomplete Dominance
• When red flowered (CRCR) plants were
crossed with white flowered (CWCW) plants
they made…pink flowers (CRCW)
• Which allele is dominant?
– neither
• Incomplete dominance:
– Case in which one allele is not dominant over
another
– The heterozygous phenotype is somewhere
between the two homozygous phenotypes

16. Codominance
• Both alleles contribute to the phenotype
• Chickens
– Allele for black feathers is codominant with allele for
white feathers
– Chicken looks speckled with black and white feathers
– Not like the blending of dominant phenotypes…
– BOTH dominant phenotypes show up
• In humans
– Gene for protein that controls cholesterol levels in the
blood
– People with heterozygous form make both types of protien

18. Multiple Alleles
• When a gene has more than two alleles
• NOT more than 2 alleles for a person but MORE than 2 alleles for the
trait exist
• Coat color in rabbits
– A single gene for coat color
– At least 4 different alleles
– Simple dominance and make 4 possible coat colors
• Genes for human blood type
– 3 different alleles: IA, IB, I
• You can get different genotypes:
– IAIA
– IAi
– IAIB
– IBIB
– I bi
– ii
• You can get different Phenotypes:
– Type A (dom)
– Type B (dom)
– Type AB (dom)
– Type O (recessive)

23. Polygenic Traits
• ―Poly‖ many
• ―-genic‖ genes
• Traits controlled by two or more genes
• Several genes interact to produce a trait
• Wide range of phenotypes
• Skin color
– Four different genes
• Fruit Fly eye color
– Three genes make the reddish brown pigment

25. Epistasis
• When the expression of one gene effects the expression of
another gene
• Ex. Fur color in mice…controlled by 2 separate genes
– Gene 1
• Brown fur pigment (BB or Bb) is dominant over gray fur
pigment (bb)
– Gene 2
• Coat Pigment depositing gene
• Dominant gene (CC or Cc) means fur will get pigment and
this is determined by gene one
• Recessive gene (cc) means that no pigment will be
deposited on fur…whether the they have the gene for
black or brown fur
• The gene for Pigment Deposition is the EPISTATIC gene
because it alters the Phenotypic ration
• Sd

26. Homework
• Complete a Dihybrid cross for 2 mice
that are heterozygous for both
Brown fur (Bb) and Pigment
deposition (Pp)
• Predict your phenotypic ratio…
• What are you final phenotypic ratios?
• Write a paragraph explaining your
results.

28. Genes and the
Environment
• Genes provide the plan for development
• How the plan unfolds depends on the
environment
• Example:
– Sunflower has genes for height and color of
flowers
– But these traits are also influenced by climate,
soil conditions and water availability

30. Genes

31. Applying Mendel’s
Principles…
• Apply Mendel’s Principles to many
organisms, including humans
• Thomas Hunt Morgan (1900’s)
– American geneticist
– Common fruit fly
• Drosophilia melanogaster
• Produced offspring very quickly
• Single pair of flies=100 offspring
• Mendel’s principle’s were tested with
Drosophilia and many other organisms and
they applied to all of them as well

32. • Would genes on the same
chromosome be inherited together?
• Thomas noticed that almost every
single time he crossed two flies that
each had red eyes and mini wings, the
offspring almost always inherited
BOTH red eyes and mini wings
– This went against Mendel’s Principle of
Independent Assortment…(review!)

33. Gene Linkage
• Thomas Hunt Morgan gave us the answer
– 1910 PRINCIPLE of LINKAGE
– 50 Drosophilia genes
– Seemed to contradict Principle of Independent
Assortment b/c certain genes were always
inherited together
– He grouped the fly’s genes into linkage groups

34. • Linkage groups are made up of genes
that seem to be inherited together
• Linkage groups assort independently
but all genes in one group are
inherited together
• Drosophilia
– 4 linkage groups
– 4 chromosomes
• What can be concluded?

36. Conclusions
1. Each chromosome is actually a group of linked
genes
2. Mendel’s Principle of Independent Assortment
holds true but an adjustment needs to be
made…
IT IS THE CHROMOSOMES THAT
ASSORT INDEPENDENTLY, NOT THE
INDIVIDUAL GENES

39. How Mendel missed it…
• What 7 genes did he study?
• 6 of the 7 genes were on different
chromosomes
• Two genes were on the same
chromosome but they were so far
apart on the same chromosome that
they assorted independently

40. • If two genes are on the
same chromosome, does
that mean they are linked
for ever?
– No, chromosomes cross
over during meiosis so
they can separate
– Crossing over produces
new combinations of
alleles
– Important for genetic
diversity

41. Lucky Student
• Alfred Sturtevant 1911
• Columbia University
• Worked in Morgan’s Lab
• Hypothesis
– If two genes are farther apart on a chromosome, the
they are more likely to be separated during meiosis
• Experiment
– Measured the rate at which linked genes were separated
and recombined to make a ―map‖ of distances between
genes
• Conclusion
– Recombination rates could be used to make gene maps
– Gene maps showed the location of a gene on a
chromosome

42. Gene Linkage and
Crossing Over
• The farther apart 2 genes are on a
chromosome….
– The more likely they are to ―cross-over‖
• The closer two genes are on the same
chromosome…
– The less likely they are to be separated

47. 1.
Steps for Dihybrid cross
Make a Key
1. Trait 1: height
1. Dominant phenotype: Tall TT or Tt (ways to get
it)
2. Recessive phenotype: Short tt
2. Trait2: color
1.
2.
Dominant phenotype: Purple
Recessive phenotype: White pp
PP or Pp
• Dominant- capital letter D
3. Write out genotypes for each parent • Recessive- lower case d
3. ____x____
2. Write Out Gametes for each Parent (use
• Homozygous-
arrows) – 2 of the same size letter
1. 4 gametes for each parent (_ _) – If its two little letters
3. Make Punnett Square (16) boxes recessive dd
1. Label parent one and write gametes along top
2. Label Parent 2 and write their gametes on side – If its 2 big letters it is
3. Fill in each box (should have 4 letters) dominant DD
4. Tally genotypes • Heterozygous-
5. Write out 4 possible phenotypic combinations
1. Dominant trait 1 and dominant for trait
– 2 different size letters
2:______ (capital and lowercase) Dd
2. Dominant trait 1 and recessive trait 2:_______ – ALWAYS dominant
3. Recessive trait 1 and Dominant trait 2:_______
4. Recessive trait 1 and recessive trait 2:________
6. Tally Phenotypes (should =16)
7. Write phenotypic ratio
1. __dd__:__dr__:__rd__:__rr__