Introduction to Genetics

Or, the things that made you who you are!

What is Genetics?

The study of heredity.

Heredity is the passing of traits from parents to offspring.

How was Genetics “discovered?”

Gregor Mendel• Austrian Augustinian Monk

• Grew up on a farm

• Studied bee keeping and gardening while in the monastery

Known as the “Father of Genetics”• Discovered how traits were inherited

• Used pea plants

Mendel and His Peas

While working with the pea plants in his garden, he noticed that certain traits would “disappear” and return from generation to generation.

Mendel and His Peas (con’t.)

Mendel also noticed that he could get certain traits to breed true• True-breeding plants

always create plants that look like themselves

• Mendel used 6 true-breeding traits

Mendel and His Peas (con’t.)

Mendel liked working with pea plants because the plants could be self-fertilized (same plant pollinating itself) or cross-fertilized (one plant pollinating another plant)

Mendel and His Peas (con’t.)

Mendel would cross-fertilize a true breeding plant of a certain trait, like height, with another true breed plant of the opposite trait• He would breed a tall plant with a short plant

• This created hybrids: offspring of two different true breeding plants of the same trait

Mendel and His Peas (con’t.)

Mendel then cross the offspring and noticed that traits disappeared• For instance, when he

crossed a true breeding purple flowered plant with a true breeding white flowered plant, the offspring were all purple flowered…he expected some to be white and he didn’t see this.

Mendel and His Peas (con’t.)

Mendel performed this with all six true breeding traits he found and always got the same results, one of the traits disappeared

Mendel and His Peas (con’t)

Mendel then decided to self-fertilize the offspring (hybrids)

The white colored flowered reappeared!

This is what happened!

Mendel and His Peas (con’t.)

Mendel performed this same experiment with all six true breeding traits and saw similar results: for example

Mendel and His Laws

From the pea plant experiments, Mendel concluded that• Rule of Unit Factors: Each organism has two factors that control

each trait• Rule of Dominance: Recall what happened when a true breeding

purple plant was crossed with a true breeding white plant, and the white flower color “disappeared” in those offspring. However, the white reappeared when the offspring self-pollinated. Mendel called the purple color dominant and the white color recessive

• Law of Segregation: Every organism has two alleles of each gene and when gametes are produced, each gamete receives only one of these alleles.

Phenotype vs. Genotype

Genotype is the make up of alleles that create the trait (gene)• TT vs. Tt (tall); tt (short)

• PP vs Pp (purple flower); pp (white flower)

• RR vs Rr (round pea); rr (wrinkled pea)

Phenotype is how an organism looks• Tall or short, purple or white flower, round or

wrinkled pea

Punnett Squares

In 1905, Reginald Punnett, an English biologist devised a method of finding possible genotypic outcomes in crosses.

Punnett Squares: Monohybrid Cross

Monohybrid crosses involve only one trait• In this case it is height

• T: tall (dominant)

• t: short (recessive)

• Female tt• Across the top

• Male Tt• Along the side

Punnett Squares: Monohybrid Crosses (con’t.)

1)Separate alleles 2) Filling in the top-left box:

Punnett Squares: Monohybrid Crosses (con’t.)

3) Filling in the bottom-left box:

Filling in the top-right box:

Punnett Squares: Monohybrid Crosses (con’t.)

Filling in the bottom-right box:

Results:• Genotype

• Percentage TT (homozygous dominant)?

• Percentage Tt (heterozygous)?

• Percentage tt (homozygous recessive)?

• Phenotype • Percentage tall?

• Percentage short?

Punnett Squares: Monohybrid Crosses

Practice Problem A• In pea plants, purple

flowers (P) are dominant to white flowers (p). Cross 2 heterozygous purple flower plants

Practice Problem B• Widow’s peak (W) is

dominant to a smooth hairline (w). Cross a woman who is homozygous for widow’s peak with her husband, who has a smooth hairline.

Punnett Squares: Dihybrid Crosses

Dihybrid crosses involve two traits

RR: homozygous dominant for round

rr: homozygous recessive for round (looks wrinkled)

YY: homozygous dominant for yellow

yy: homozygous recessive for yellow (looks green)

Cross two heterozygous

RrYy X RrYy

Punnett Squares: Dihybrid Crosses (con’t.)

Results of Genotype• RRYY: /16• RRYy: /16• RRyy: /16• RrYY: /16• RrYy: /16• Rryy: /16• rrYY /16• rrYy /16• rryy: /16

Results of Phenotype• Round/Yellow /16• Round/Green /16• Wrinkled/Yellow /16• Wrinkled/Green /16

• Ratio: 9:3:3:1

Punnett Squares: Dihybrid Crosses (con’t.)

Dihybrid practice problem: In garden peas, tallness (T) is dominant to shortness (t) and axillary flowers (A) are dominant to terminal flowers (a). What are the expected ratios for the genotypes and phenotypes of the offspring if a heterozygous tall, heterozygous axillary plant is crossed with a heterozygous tall, terminal plant?

Punnett Squares: Dihybrid Crosses (con’t.)

What Mendel learned from dihybrid crosses are is that each trait are inherited independently from each other.• For example, height is inherited independently

from seed color.

• Law of Independent Assortment

Recap

Who is Gregor Mendel? Why is Mendel’s work important to know?Explain what a trait is. Explain what an allele is.What are Mendel’s four laws and explain them.What is a Punnett Square, and how is it used?What is the difference between monohybrid crosses and dihybrid crosses? What did the monohybrid crosses tell Mendel (hint: look back at the four laws)?What did the dihybrid crosses tell Mendel?