MENDEL’S WORKKey Concepts

What were the results of Mendel’s experiments or crosses?

What controls the inheritance of traits in organisms?

Key Terms

Heredity Trait Genetics Fertilization Purebred

Gene Alleles Dominant allele Recessive allele hybrid

Mendel’s Work Mendel experimented with thousands

of Pea plants looking at their different traits to understand the process of heredity. His discoveries form the foundation of genetics.

Heredity –the passing of physical characteristics from parents to

offspring Trait- each different form of a

characteristic Genetics – the study of heredity

Mendel’s Experiments

Flowering Plant anatomy Pistil – produces the female sex cells or eggs Stamens – produce pollen which contains

the male sec cells or sperm Fertilization is when the egg and sperm join

forming a new organism In plants the pollen must reach the pistil for

fertilization to occur. This is called pollination

Pollination

Pea plants usually self-pollinate. The pollen from their stamens lands of the same plants pistils.

Mendel developed a method to cross-pollinate pea plants. He took pollen from one pea plant and brushed it onto the pistil of another.

Crossing Pea Plants

Mendel crossed plants with contrasting traits

Ex. Tall plants with short plants

Started with purebred plants – a purebred organism is one who is the offspring of many generations of that have the same trait

The F1 Offspring

Mendel crossed purebred tall with purebred short Parental (P) generation

Tall x short Offspring from the cross are called F1

(filial) All F1 offspring were tall

F2 offspring

When F1 were full grown, Mendel allowed them to self-pollinate

F2 were a mix of tall and short

¾ were tall and ¼ were short

Experiments with OtherTraits

Mendel crossed pea plants with other contrasting traits such as seed shape, seed color, seed coat color, etc.

In all crosses the F1 generation had only 1 form of the trait

In the F2 generation the “lost” form reappeared in ¼ of the plants.

Dominant and Recessive Alleles Mendel’s Conclusion factors control the

inheritance of traits in peas. They exist in pairs The female parent

contributes one factor and the male parent contributes the other factor

One factor in a pair can mask or hide the other factor

Genes and Alleles Genes – factors that control a trait Alleles – different forms of a gene An organisms traits are controlled by the

alleles it inherits from its parents. Some alleles are dominant, while others

are recessive Dominant allele – trait always shows Recessive allele –trait is hidden whenever

dominant allele is present

Alleles in Mendel’s Crosses

Stem Height Cross P generation tall tall x short short F1 generation all were tall short- Look

tall F2 generation ¼ tall tall - look tall ¼ tall short + ¼ short tall

– look tall ¼ short short – look

short

Symbols for alleles

Letters are used to represent alleles

Capital letters are used for dominant alleles

Lowercase letters are used for recessive alleles

Purebred tall = TT Purebred short = tt Hybrid (one of each) =Tt

Significance of Mendel’s Contribution Before Mendel most people thought that the

traits of an individual were a blend of their 2 parents.

If they blended the Tt plants should be medium height

Mendel found out that traits are determined by individual alleles some of which are dominant and some recessive.

Recessive traits may seem to disappear in the offspring only to reappear in the next generation

Mendel’s work was not recognized during his lifetime, but was rediscovered in 1900. He is now

considered the Father of Genetics.

Sources

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nary_cross_pollination.htm http://academic.kellogg.edu/herbrandsonc/bio111/genetics.htm http://www.bioinformatics.nl/webportal/background/

mendelinfo.html http://www.absoluteastronomy.com/topics/

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