A. History of Heredity Studies

1) Gregor Mendel is considered the father of genetics.

2) He performed experiments on pea plants to find out why

living things possessed different traits.

3) Some traits of pea plants that he observed are as

follows:a) plant height (tall & short)

b) seed shape (round & wrinkled)

c) seed color (yellow & green).

B. Key Words:

1) Heredity: the passing on of traits from parents to offspring.

2) Genes: segments of DNA located on the

chromosomes that code for a particular trait.

3) Traits: physical or behavioral characteristics which are inherited from parents.

4) Alleles: alternative forms of a gene that govern the same

trait.

5) Recessive trait: a trait that is visible only when two

recessive alleles are inherited.

6) Dominant trait: the trait observed when at least one dominant allele is inherited.

7) purebred: an organism that inherits the same alleles for a particular trait (e.g. TT, tt, WW).

8) hybrid: an organism that inherits two different alleles

for a trait (e.g. RW, Tt).

10)phenotype: an organism’s inherited appearance.

11)probability: the chance of an event or combination of alleles

occurring and hence result in the organism’s inherited traits.

9) genotype: the inherited combination of alleles.

12)P Generation: parents of an organism.

13)F1 Generation: the offspring from the P Generation.

14) F2 Generation: the offspring from the F1 Generation or grandchildren of the

P Generation.

15)Punnett square: a tool used to visualize all the possible combinations of alleles from

parents.

C. Probability:

Some practice problems Some practice problems follow.follow.

1. Determining the chance of a trait appearing in an individual.

2. Use the Punnett Square to determine the chance of a

particular trait appearing.

A plant that produces only round seeds (RR), and a plant that produces only wrinkled seeds (rr), were crossed. Find the following:

The percent hybrid?

The percent purebred?

The percent having round seeds?

The percent having wrinkled seeds?

R R

r

r Rr Rr

RrRr

A plant that produces only round seeds (RR), and a plant that produces only wrinkled seeds (rr), were crossed. Find the following:

The percent hybrid?

The percent purebred?

The percent having round seeds?

The percent having wrinkled seeds?

100%0%

100%

0%R R

r

r Rr

Rr

Rr

Rr

Two hybrid plants with round seeds (Rr), are crossed. The dominant allele R, produces round seeds. The recessive allele r, produces wrinkled seeds. Find the following:

The percent hybrid?

The percent purebred?

The percent having round seeds?

The percent having wrinkled seeds?

R r

r

R RR

rr

Rr

Rr

Two hybrid plants with round seeds, Rr are crossed. The dominant allele R, produces round seeds. The recessive allele r, produces wrinkled seeds. Find the following:

The percent hybrid?

The percent purebred?

The percent having round seeds?

The percent having wrinkled seeds?

50%

50%

75%

25%

R r

r

R RR

rr

Rr

Rr

General Rules for filling in Punnett Squares:

1) Each letter fills in the two boxes immediately below or to the right of them.

2) lower case letters are written after UPPER CASE letters no matter which was on top or the side.

(e.g. Rr )

Four steps to solving probability problems:

STEP 1: Choose letters to represent the alleles of the parents being crossed.

STEP 2: Write the genotypes of the parents being crossed.

STEP 3: Construct a Punnett Square.

STEP 4: Answer the questions.

R R

r

r Rr Rr

RrRr

D. Complete Dominance vs. Incomplete Dominance genes:

Complete Dominance: a) condition in which

dominant genes completely hide or mask recessive genes.

b) the same letter is used to represent both the dominant and recessive forms of a gene.

c) but, capital letters are used to represent dominant genes and lower case letters represent recessive genes.

Incomplete Dominance: a) condition in which

dominant genes incompletely hide other genes and results in a “blending” of traits.

b) different capital letters are used to represent different forms of a gene (e.g. R = red, W = white).

c) a hybrid is represented by a mixture of capital letters (e.g. RW).

References:Holt, Rinehart & Wilson Science and Technology textbook, 2001, California Life Science, pp. 130-137.

Prentice Hall textbook, 1993, Heredity.