The Origins of Genetics

I. Gregor Mendel studied the garden pea plant

• Why are these good subjects?

1. Many traits with 2 forms (purple or white flowers, for example)

2. Can either self-pollinate (flower fertilizes itself) or cross-pollinate

3. Small, easy to grow, with a large number of offspring

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

II. Mendel’s Experiments and Results

1. P generation are the parents

2. F1 generation are the first generation offspring

3. F2 generation are the second generation offspring

4. Some traits disappeared in the F1 generation, but reappeared in some of the F2 generation

Mendel’s Theory1. For each trait, an individual has 2 genes,

one from the mother, and 1 from the father

2. There are alternate versions of genes. Each version is called an allele.

3. When two different alleles occur together, one trait may be expressed

(the dominant trait). The trait not expressed is the recessive trait.

purple white

4. Gametes carry only one allele for each trait.

• If both alleles or genes are the same, the individual is said to be homozygous

• If one allele is dominant and the other is recessive, the individual is heterozygous

• In heterozygous individual, the dominant allele is expressed

• The physical appearance (what you see) is called the phenotype

• The set of alleles (or genes) of an individual is called the genotype

Mendel’s Laws

The Law of Segregation: The two alleles for a trait separate when gametes form

Law of Independent Assortment: the alleles of different genes separate independently of

one another during gamete formation

Probability

1. What is the probability of flipping a coin and having it come up heads 3 times in a row?

½ x ½ x ½ = 1/8HHH TTTHHT TTHHTH THTHTT THH

2. What is the probability of a family having four sons in a row?

½ x ½ x ½ x ½ = 1/16

A couple is expecting twins.

What is the percent likelihood that they will have at least one daughter?

Two sisters are each expecting twins.

• What is the percent likelihood that they will each have a girl and a boy?

Monohybrid crosses: involve one pair of contrasting traits