1. Patterns of Inheritance
Chapter 9
Patterns of Inheritance

2. HERITABLE VARIATION AND PATTERNS OF INHERITANCE
HEREDITY is the transmission of traits from one generation to the next.
Gregor Mendel
Worked in the 1860s
Was the first person to analyze patterns of inheritance
Described the fundamental principles of genetics

3. Figure 9.1
Figure 9.1 Gregor Mendel.

4. At the monastery garden
Mendel studied garden peas because they
Are easy to grow
Are easily manipulated (easy to cross them with each other)
Have many easily recognized trait variations
Tall or short plants (trait =
Puple or white flowers (trait =
Round or wrinkled seeds (trait =
Green or yellow seeds (trait =
Etc...
Please note: what your book calls “characters,” I will call “traits”

5. HYBRIDS are the offspring of two different true-breeding varieties.
Mendel
Created true-breeding varieties of plants
What does that mean?
Then he would cross different true-breeding varieties, collect the seeds, plant the seeds, and look at the results
The big new thing he did was to COUNT the results
HYBRIDS are the offspring of two different true-breeding varieties.
The parental plants are the P GENERATION.
Their hybrid offspring are the F1 GENERATION.
A cross of the F1 plants forms the F2 GENERATION.

6. Mendel performed many experiments. (1000s of plants)
He tracked the inheritance of traits that occur as two alternative forms (like yellow or green seeds).
White
Purple
Recessive
Dominant
Green
Yellow
Terminal
Axial
Wrinkled
Round
Seed shape
Seed color
Flower position
Flower color
Pod color
Pod shape
Stem length
Inflated
Constricted
Tall
Dwarf
Figure 9.4

7. Monohybrid Crosses
A monohybrid cross is a cross between parent plants that differ in only one trait.
Like flower color 
Purple flowers
F1 Generation
White flowers
P Generation
(true-breading
parents)
All plants have
purple flowers
F2 Generation
Fertilization
among F1 plants
(F1  F1)
of plants
have purple flowers
have white flowers
Figure 9.5-3

8. Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called ALLELES.
2. For each trait, an organism inherits two alleles, one from each parent.
An organism is HOMOZYGOUS for that gene if both alleles are identical.
An organism is HETEROZYGOUS for that gene if the alleles are different.

9. 3. The alleles come in two general forms:
The DOMINANT allele. If an organism gets either one or two copies of the dominant allele, the organism will “show” the dominant trait.
For example: homozygous dominant; RR or heterozygous; Rr
Dominant traits are not necessarily normal or more common
Example: having 5 fingers is recessive, 6 fingers (polydactyl) is dominant
The RECESSIVE allele. An organism has to get TWO copies of the recessive allele to show the recessive trait.
For example: homozygous recessive; rr

10. DOMINANT TRAITS RECESSIVE TRAITS Freckles Widow’s peak Free earlobe
No freckles
Straight hairline
Attached earlobe
Figure 9.12
Figure 9.12 Examples of inherited traits thought to be controlled by a single gene.

11. 4. Gametes carry only one allele for each trait.
The two members of an allele pair segregate (separate) from each other during the production of gametes (meiosis).
This happens during Metaphase I of meiosis
Example: you have TWO eye color genes, one from mom and one from dad. But when you make a sex cell, you don’t put both of those genes in one sex cell...they get separated. You put one of the eye color genes in one sex cell, and the other eye color gene in the other sex cell
This concept is the LAW OF SEGREGATION.

12. Geneticists distinguish between an organism’s physical traits and its genetic makeup.
An organism’s physical traits are its PHENOTYPE.
Blue eyes, brown hair, dark skin, tall, short, etc...
An organism’s genetic makeup is its GENOTYPE.
The letters we use to describe the organism’s genes.
Examples: AA, Rr, Jj, ff, cc, Dd, etc...

13. How can we determine what children will result from a mating (a cross) of two parents?
First we figure out what gametes each parent could produce, then we set up a Punnett Square.
A PUNNETT SQUARE highlights the possible combinations of gametes and offspring that result from each cross.
What gametes could you produce if you had the genotype Ff?
F or f
What gametes could you produce if you had the genotype DD?
D or D

14. Figure 9.6-3 P Generation Genetic makeup (alleles) Purple flowers
White flowers PP pp
Alleles carried
by parents
Gametes
All P
All p
F1 Generation
(hybrids)
Purple flowers
Alleles
segregate
All Pp 1 2 1 2
Gametes P p
F2 Generation
(hybrids)
Sperm from
F1 plant P p P
Eggs from
F1 plant PP Pp p Pp pp
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.6-3
Figure 9.6 The law of segregation. (Step 3)

15. Mendel’s Law of Independent Assortment
A DIHYBRID CROSS is the crossing of parental varieties differing in two traits. Like seed color AND seed shape (two different traits)

16. Actual results (support hypothesis)
Independent assortment
P Generation
RRYY
rryy
Gametes RY ry
F1 Generation
RrYy
Sperm
F2 Generation RY rY Ry ry RY
RRYY
RrYY
RRYy
RrYy rY 16 9
Yellow
round
RrYY
rrYY
RrYy
rrYy
Eggs Ry 16 3
Green
round
RRYy
RrYy
RRyy
Rryy 16 3
Yellow
wrinkled ry
RrYy
rrYy
Rryy
rryy 1 16
Green
wrinkled
Actual results
(support hypothesis)
Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross.

17. Mendel’s dihybrid cross supported the hypothesis
that each pair of alleles segregates independently of the other pairs during gamete formation
known as LAW OF INDEPENDENT ASSORTMENT.
Inheritance of one character has no effect on the inheritance of another.
EXAMPLE: Independent assortment is seen in two traits in Labrador retrievers. Coat color AND eyesight (see next slide).

18. Mating of double heterozygotes (black coat, normal vision)
Blind dog
Blind dog
Phenotypes
Black coat,
normal vision
Black coat,
blind
Chocolate coat,
normal vision
Chocolate coat,
blind
Genotypes
B_N_
B_nn
bbN_
bbnn
(a) Possible phenotypes of Labrador retrievers
Mating of double heterozygotes
(black coat, normal vision)
BbNn
BbNn
Phenotypic
ratio of
offspring
9 black coat,
normal vision
3 black coat,
blind
3 chocolate coat,
normal vision
1 chocolate coat,
blind
(b) A Labrador dihybrid cross
Figure 9.9
Figure 9.9 Independent assortment of genes in Labrador retrievers.

19. Two trait cross (dihybrid cross) example
Let’s consider two traits, hairline and finger length
We’ll use:
W for widow’s peak, and w for straight hairline
S for short fingers, and s for long fingers
A person who is WwSs (widow’s peak and short fingers) and a person who is also WwSs have children
What are the gametes for each parent
_____ , _____ , _____ and _____
Make a big Punnett square as illustrated on the following slide
Figure the genotypes and phenotypes of the offspring

20. Dihbrid cross

21. Practice examples
For each of the following genotypes, give all possible gametes WW
WWSs Tt
Ttgg
AaBb
For each of the following, state whether the genotype or a gamete is represented D Ll Pw
LlGg
(No two letters in a gamete can be the same letter of the alphabet.)

22. Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between
An individual of dominant phenotype (but unknown genotype)
A homozygous recessive individual
© 2010 Pearson Education, Inc.

23. Two possible genotypes for the black dog:
Testcross
Genotypes B_ bb
Two possible genotypes for the black dog: BB or Bb
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate
Figure 9.10
Figure 9.10 A Labrador retriever testcross

24. Recessive Disorders Human genetic disorders are often recessive.
Meaning you have to get ____ copies of the allele to have the disorder.
Individuals who have the recessive allele but appear normal are CARRIERS of the disorder.
Parents
Offspring Dd
Hearing
Sperm
Eggs D d
(carrier) DD dd
Deaf
Figure 9.14

25. Genetic Testing
Today many tests can detect the presence of disease-causing alleles.
Most genetic testing is performed during pregnancy.
Example: Amniocentesis
Extracting fluid from around the fetus to check for genetic problems
Genetic counseling helps patients understand the results and implications of genetic testing.
Parents are given a choice as to what to do about the pregnancy or child.

26. VARIATIONS ON MENDEL’S LAWS
Why do some of you have dark skin or light skin or some shade in between?
Why do some plant flowers come in red, white or pink in color?
Some patterns of genetic inheritance are not explained by Mendel’s laws. (Incomplete dominance and Codominance)
Incomplete Dominance (in Plants and People)
In INCOMPLETE DOMINANCE, F1 hybrids have an appearance in between the phenotypes of the two parents.

27. P Generation Red White RR rr Gametes R r F1 Generation Pink Rr Gametes
Sperm R r R RR Rr
Eggs r Rr rr
Figure
Figure 9.18 Incomplete dominance in snapdragons. (Step 3)

28. Example of Incomplete Dominance in Humans
Hypercholesterolemia
(Review: what does hyper mean? __________)
Is characterized by dangerously high levels of cholesterol in the blood.
Is a human trait that is incompletely dominant.
Heterozygotes have blood cholesterol levels about twice normal.
Homozygotes for the trait have blood cholesterol levels about five times normal.

29. HH Hh hh
Homozygous dominant
for ability to make
LDL receptors
Heterozygous
GENOTYPE
Homozygous recessive
for inability to make
LDL receptors
LDL
LDL
receptor
PHENOTYPE
Cell
Normal
Mild disease
Severe disease
HDL is “good” cholesterol, and LDL is “bad” cholesterol. If you are homozygous dominant (normal) your cells bind and remove LDL. If you are heterozygous or homozygous recessive, you are less able to remove bad (LDL) cholesterol.
Figure 9.19 Incomplete dominance in human hypercholesterolemia.

30. ABO Blood Groups: An Example of Multiple Alleles and Codominance
The A B O blood groups in humans are an example of codominance.
More than 2 possible alleles exist in the population.
People can have A or AB or O blood types. The AB blood type is the example of codominance.
© 2010 Pearson Education, Inc.

31. Given the following genotypes, what would the phenotype (blood type) be?
AA = blood type
AB = blood type
BB = blood type
OO= blood type
AO= blood type
BO= blood type

32. If you are given the wrong blood, your immune system produces proteins called antibodies that can bind specifically to the wrong blood cells.
The antibodies “know” your blood cells, and can detect the wrong blood cells.
If the wrong blood is given, your antibodies will stick to the “invading” blood cells and cause the cells to clump together.
This clumping would cause blood clots, which would lead to heart attacks and strokes.
That is why you MUST have your blood type checked before being given blood!

33. Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
Most traits (and most diseases) are polygenic in humans!!!
Example: Skin pigmentation in humans

34. Fraction of population
Skin pigmentation
Figure 9.22b
Figure 9.22b A model for polygenic inheritance of skin color.

35. The Role of Environment
Many human characters result from a combination of heredity and environment.
Example: If you inherit the gene (or genes) that make you likely to develop lung cancer, how does your environment play a role in this?
If you don’t smoke:
If you do smoke:
If you work in a factory or mine that produces toxic gasses:
If you work at a computer in an office:

36. The dark color we see in these cats is a result of increased melanin production (the same thing that gives human skin color).
Do dark colors absorb more heat or less heat?

38. SEX CHROMOSOMES AND SEX-LINKED GENES
Any gene located on a sex chromosome is called a SEX-LINKED GENE.
MOST sex-linked genes are found on the X chromosome.
Few genes are found on the Y chromosome.
RED-GREEN COLOR BLINDNESS is a common human sex-linked disorder.
It is a trait found on the X chromosome.
Since the Y chromosome is NOT homologous, it does not carry any gene related to color vision

39. Y X Colorized SEM Figure 9.29
Figure 9.29 The chromosomal basis of sex determination.

40. XNXN XnY XNXn XNY XNXn XnY Xn XN Y Xn Y Y XN XN XNXn XNY XNXN XNY XN
Sperm
Sperm
Sperm Xn XN Y Xn Y Y XN XN
XNXn
XNY
XNXN
XNY XN
XNXn
XNY
Eggs
Eggs
Eggs XN Xn Xn
XNXn
XNY
XNXn
XnY
XnXn
XnY
Carrier female 
colorblind male
Normal female 
colorblind male
Carrier female 
normal male
Key
Unaffected individual
Carrier
Colorblind individual
Figure 9.31
Figure 9.31 Inheritance of colorblindness, a sex-linked recessive trait.