Chapter 4Mendel’s WorkSection 1

Gregor Mendel Mid nineteenth century Priest named Gregor Mendel Worked in a garden at a monastery

Heredity: the passing of physical characteristics from parents to offspring

Why do pea plants have different characteristics?

Traits: physical characteristics Examples: stem height (tall or short),

seed color (green or yellow) Pea plant TRAITS were often similar to

those of their parents and sometimes they were different

His studies laid the foundation for GENETICS– the scientific study of heredity

Mendel’s Experiments The flower’s petals surround the pistils

and the stamens Pistil: produces female sex cells, or eggs Stamen: produce pollen, which contains

male sex cells, or sperm Fertilization: new organisms can form

when the egg and the sperm join in this process

Self-Pollinating Pea plants are usually self-pollinating Pollen from a flower lands on a pistil of the same

flower

Cross-Pollinating “crossed” Mendel removed pollen from a flower on one plant, then brushed the pollen onto a flower on a second plant

Crossing Pea Plants

How are you going to study the inheritance of traits in pea plants? Mendel decided to “cross” plants with

contrasting traits Example: tall plants with short plants

He started with purebred plants Purebred: an organism is the offspring

of many generations that have the same trait

The F1 and F2 Offspring

Mendel crossed purebred tall plants with purebred short plants---This is called the parental generation, or the P generation

F1 offspring are all tall---The offspring from this cross are the first filial generation, or the F1 generation

He then allowed for the F1 offspring to self-pollinate

F2 offspring are tall and short---The offspring from this cross are the second filial generation, or the F2 generation

Results of a Cross

P generation(one tall, one short)

F1 generation(both tall)

F2 generation(mix of tall and short)

What other traits did Mendel study? Pod color, seed shape, flower position

What were the two forms of the seed shape? Round and wrinkled

Genes and Alleles

Genes: factors that control a trait Alleles: different forms of a gene

An organism’s traits are controlled by alleles it inherits from its parents. Some alleles are dominant, while others are recessive

Dominant Alleles vs. Recessive Alleles

Dominant Allele: an allele whose trait always shows up in the organism when the allele is present

Recessive Allele: an allele that is masked when a dominant allele is present

Let’s do some math….Examples for Stem Height One recessive allele + one dominant

allele = dominant traits (tall stem)

One recessive allele + one recessive allele = recessive traits (short stem)

Alleles in Mendel’s Crosses Combination of Alleles:

Hybrid tall--- one allele for tall stems and one allele for short stems (one dom, one rec)

Purebred tall--- two alleles for tall stems (two dom)

Purebred short--- two alleles for short stems (two rec)

Symbols for Alleles Geneticists use letters to represent

alleles Dominant allele is represented by a

capital letter Example: tall stems = T

Recessive allele is represented by a lowercase letter Example: short stems = t

2 dominant alleles for tall stems

= TT

2 recessive alleles for short stems

= tt

1 recessive allele for short stems and 1 dominant allele for tall stems

= Tt

Significance of Mendel’s Contribution

The importance of Mendel’s discovery was NOT recognized during his lifetime

Then, in 1900, three scientists discovered his work

They quickly realized the importance of his work

Mendel is often called the Father of Genetics