1. Patterns of Inheritance
Chapter 9:
Patterns of Inheritance

2. Fundamentals of Genetics
Gregor Mendel – “ The Father of Genetics”
A. History
1. he was a monk who tended a garden in the 1850’s
2. he was educated in math and statistics
3. he observed pea plants and noticed trends in the inheritance of certain traits
a. looked at 7 traits (seed color and shape, flower color and position, pod color and shape, plant height)
b. based on his observations he conducted experiments with certain plants.
c. Self-fertilization – fusion of a sperm (pollen) and egg made on the same plant
d. Cross-fertilization – fusion of a sperm (pollen) and egg made on different plants

3. Experimental design:
Worked with plants until he knew he had true breeding
Terms:
Cross
Hybrid
Hybridization
P generation
F1 generation
F2 generation

4. B. Conclusions Mendel made from his experiments
1. Principle of Dominance and Recessiveness
a. genes are in pairs (think homologous chromosomes)
b. Dominant gene – masks over the other gene in the pair
c. Recessive gene – gene in pair that is “hidden”
2. Principle of Segregation
a. the paired genes separate during the formation of egg and sperm (think meiosis…Metaphase 1 to Anaphase 1)
3. Principle of Independent Assortment
a. genes for different traits are distributed to egg and sperm independent from each other. (think how chromosomes line up in Metaphase 1 and 2…independent from one another)

5. II. Terms
A. Gene – segment of DNA that codes for a protein
B. Allele – variety of a gene
1. Dominant Allele – use CAPITAL letters to symbolize
2. Recessive Allele – use lower case letters to symbolize
C. Phenotype – Physical appearance of a trait
D. Genotype – Genetic makeup of the phenotype
1. Homozygous Dominant: TT
2. Heterozygous: Tt
3. Homozygous Recessive: tt
E. Homologous chromosomes – chromosomes that carry genes coding for the same characteristics
1. You get 1 from your mom and 1 from your dad

6. F. Autosomes – chromosomes that do not code for gender (44)
G. Sex Chromosomes – chromosomes that code for gender
1. Female – XX
2. Male -- XY

7. III. Genetic Crosses: Punnett Squares – used to find the probability of types of offspring certain parents may obtain
A. Monohybrid cross: Crossing of one trait
1. write down cross
2. write possible egg and sperm combinations (if there are two letters, then egg and sperm will have 1 letter each)
3. make Punnett square (4 boxes)
a. egg possibilities go on top
b. sperm possibilities go along the side
4. complete the cross
5. write the genotypic ratio (the chance that one of the genotypes would be present in an offspring)
a. homozygous dominant : heterozygous : homozygous recessive
6. write the phenotypic ratio (the chance that one of the phenotypes would be present in an offspring)
b. dominant phenotype : recessive phenotype

8. t t t t T T t
T T T t T t T t

9. B. Dihybrid cross: crossing of two traits 1. write down cross
2. write all possible egg and sperm combinations (think meiosis…principle of segregation and principle of independent assortment) (if there are 4 letters, then egg and sperm will have 2 letters…one of each kind)
3. make Punnett square (16 boxes)
4. complete the cross
5. write the genotypic ratio: TTRR TTRr TTrr TtRR TtRr Ttrr ttRR ttRr ttrr
6. write the phenotypic ratio: Dom/Dom : Dom/Rec : Rec/Dom : Rec/Rec

10. C. Testcross – mating between an individual of unknown
C. Testcross – mating between an individual of unknown genotype and a homozygous recessive individual.
1. Mrs. Barton has brown eyes (BB or Bb) ?
2. Mr. Barton has blue eyes (bb)
B__ bb
I would be BB if my kids were I would be Bb if my kids were
brown eyed blue eyed

11. Tricks Rule of Multiplication (aka Product Rule) Example: TtRr X TtRr
how do we determine the chance that two or more independent events will occur together in a specific combination?
Example: TtRr X TtRr
What is the probability of getting a short smooth offspring?
Short?
Smooth?

12. Example: TtRr X TtRr What is the probability of getting a tall wrinkled offspring?
The probability of getting a tall offspring is _______
The probability of getting a wrinkled offspring is _______
The probability of getting a tall wrinkled offspring is:

13. Example:TtrrGG X TtRrgg What is the probability that you would get a Tall plant with Wrinkled Green seeds?
“T’s” = Tt X Tt = “R’s” = rr X Rr = “G’s” = GG x gg = Product is:

14. Another Trick Rule of Addition
used to find the probability of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways
Use the Product Rule and Rule of Addition to calculate the results of complex crosses rather than work out the Punnett Squares.

15. What is the probability that offspring will exhibit recessive phenotypes for at least 2 of the 3 traits? TtRrGg X Ttrrgg
Write the possibilities
Solve for probability for each possibility (rule of multip)
Add probabilities together to get answer.

16. IV. Exceptions to Mendel’s Principles
A. Sex Linked traits – there are traits (genes) that are on the sex chromosomes and are therefore linked to gender.
1. X chromosome carries most of these genes
a. colorblindness b. hemophilia c. muscular dystrophy d. baldness
2. most of these are recessive traits
3. Genotypes you would need to have to express these traits are: XhXh and XhY
4. A carrier of these traits (has the gene, but doesn’t express it) would be: XHXh (this can also be written XhX)

17. B. Codominance – both alleles are expressed when a
B. Codominance – both alleles are expressed when a heterozygous genotype is present
1. “Co” means “with, together, jointly”
2. Black chicken (BB) x White chicken (bb)
100% Black and white checkered chicken (Bb) (some feathers are black and some are white)
C. Incomplete dominance – the dominant allele doesn’t completely mask the recessive allele, so you get a “blending” appearance when a heterozygous genotype is present.
1. Red flowers (RR) x White flowers (rr)
100% Pink flowers (Rr)

18. Phenotype Genotype Type O Type A Type B Type AB iOiO IAIA or IAiO
D. Multiple Alleles
1. when a trait has more than 2 alleles
2. each organism still ends up with 2 alleles, but there are more alleles to choose from
3. Blood type – there are three alleles: iO IA IB
Phenotype
Genotype
Type O
Type A
Type B
Type AB
iOiO
IAIA or IAiO
IBIB or IBiO
IAIB
a. this is an example of codominance as well

19. E. Pleiotropy
1. When a single gene controls more than one phenotypic characteristic
2. Cystic fibrosis and sickle cell disease.

20. F. Polygenic inheritance
1. The additive effects of two or more genes on a single phenotypic character.
2. human skin color and height

22. V. Pedigrees – a record that tracks the inheritance of a trait
V. Pedigrees – a record that tracks the inheritance of a trait through several generations of a family
A. Helps us to determine how and why certain traits are inherited.
B. Used as a form of genetic counseling
1. keeping records of an individual’s relatives with a genetic disorder enables a genetic counselor to determine an individual’s chances of being a carrier for that disorder and therfore make family planning decisions.
C. Common symbols you should know: (go to next page)

23. I.
Male
Female
II.
Mating between individuals
X-linked carrier
Affected individual or