Fundamentals of Genetics

I. 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

Experimental design:

• Worked with plants until he knew he had true breeding

•Terms:•Cross•Hybrid•Hybridization•P generation•F1 generation•F2 generation

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)

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

F. Autosomes – chromosomes that do not code for gender (44)

G. Sex Chromosomes – chromosomes that code for gender

1. Female – XX

2. Male -- XY

III. Genetic Crosses: Punnett Squares – used to find the probability of types of offspring certain parents may obtain

A. Monohybrid cross: Crossing of one trait1. 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 side4. 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

B. 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 werebrown eyed blue eyed

C. 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:TTRRTTRrTTrrTtRRTtRrTtrrttRRttRrttrr

6. write the phenotypic ratio:Dom/Dom : Dom/Rec : Rec/Dom : Rec/Rec

Tricks• Rule of Multiplication

– how do we determine the chance that two or more independent events will occur together in a specific combination?

• Rule of Addition– used to find the probabilility of an event that

can occur in two or more different ways is the sum of the separate probabilities of those ways

Example: TtRr X TtRrWhat is the probability of getting a tall wrinkled offspring?

• The probability of getting a tall offspring is ¾.

• The probability of getting a wrinkled offspring is ¾.

• The probability of getting a tall wrinkled offspring is ¾ x ¾ = 9/16

Example:TtrrGG X TtRrggWhat 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:

Using the Rule of additionTtRrGg X Ttrrgg

• Use the Product Rule and Rule of Addition to calculate the results of complex crosses rather than work out the Punnett Squares.

• What is the probability that offspring will exhibit recessive phenotypes for at least 2 of the 3 traits?

Solution to question on previous slide TtRrGg X Ttrrgg

(at least 2 of the 3 traits recessive)

Write the possibilities

Solve for probability for each possibility (rule of multip)

Add probabilities together to get answer.

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)

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)

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

a. this is an example of codominance as well

iOiO

IAIA or IAiO

IBIB or IBiO

IAIB

E. Pleiotropy1. When a single gene controls more than one phenotypic characteristic2. Cystic fibrosis and sickle cell disease.

F. Polygenic inheritance1. The additive effects of two or more genes on a single phenotypic character. 2. human skin color and height

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)

Male

Female

Mating between individuals

X-linked carrier

I.

II.

1 2 3 4

or Affected individual