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Gregor Mendel

1822-1884

• Priest who did scientific research

• Father was a farmer, mother was the daughter

of a gardener

• Crossed pea plants

• Had no knowledge of DNA, genes, or

chromosomes

• Described the units of inheritance and how

they pass from one generation to the next

• Not recognized during his lifetime

Why Peas?

• Easy to grow.

• Develop quickly.

• Traits have only two

distinguishable forms.

–Ex- tall or short

• Pea plants use parts of their

flowers to reproduce.

• Male part produces pollen,

female part produces eggs.

• When pollen fertilizes an egg,

a seed for a new plant is

formed.

• The original plants crossed are called the P (parent) generation.

• The offspring are called the F1 (first filial) generation.

–Example: Mendel crosses a tall pea plant with a short pea plant. What do you think the F1 generation will look like?

Monohybrid Cross

Parental generation (P1)

Tall X Short

F1

All Tall

• All of the offspring looked

like only one of the

parents. The

characteristics of the

other parent seemed to

disappear.

–Did it really disappear?

–No! The short trait was

hidden by the tall trait.

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Monohybrid Cross

Parental generation (P1)

Tall X Short

F1

All Tall

F2

1/4 Short : 3/4 TallFigure 4.3

Figure 4.2

• Mendel determined that there is some factor being passed from parent to offspring.

• Scientists now know that these factors are called genes found on chromosomes.

• The different forms of a gene are called alleles.

Principle of Dominance• Some alleles are dominant, some are recessive.

–A dominant allele for a certain trait will always be

expressed over a recessive allele.

–A recessive allele will only be expressed when no

dominant allele is present.

Representation of Dominant and Recessive Alleles

• A capital letter symbolizes

a dominant allele.

• The lower case form of the

same letter symbolizes the

recessive allele.

–Example: The allele for

tallness (T) is dominant and

the allele for shortness (t) is

recessive.

• Genotype- genetic makeup of the organism

–If both alleles acquired by the offspring are the

same, it is said to be homozygous. (ex- TT or tt)

–If both alleles acquired are different, it is said to be

heterozygous or hybrid. (ex- Tt)

• Phenotype- physical appearance of an organism

–Ex- tall or short

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Figure 4.4

Punnett Square

A Punnett square is used to represent possible combinations

of genes in gametes and how they may combine in offspring

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BB

Bb

Bb

bb

25%

25%50%

75%25%

18

Bb

Bb

Bb

Bb

B B

b

b

0%

0%100%

100%0%

19

Bb

Bb

bb

bb

B b

b

b

0%

50%

50%

50%

50%