1. 11-1 The Work of Gregor Mendel
photo credit: W. Perry Conway/CORBIS
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Gregor Mendel’s Peas
Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity.
Mendel carried out his work with ordinary garden peas.
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Gregor Mendel’s Peas
Mendel knew that
the male part of each flower produces pollen, (containing sperm).
the female part of the flower produces egg cells.
To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen from another flower.
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Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join in a process called fertilization.
Fertilization produces a new cell.
Pea flowers are self-pollinating.
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Gregor Mendel’s Peas
Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.
Cross-pollination
Mendel was able to produce seeds that had two different parents.
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Gregor Mendel’s Peas
Mendel made three key decisions in his experiments.
use of purebred plants
control over breeding
observation of seven “either-or” traits
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Genes and Dominance
Genes and Dominance
A trait is a specific characteristic that varies from one individual to another.
Mendel studied seven pea plant traits, each with two contrasting characters. (pea shape, pea color, flower color, pod shape, pod color, flower position, and plant height)
A cross is the mating of two organisms. He crossed plants with each of the seven contrasting characters and studied their offspring.
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Genes and Dominance
Each original pair of plants is the P (parental) generation. Mendel used purebred plants for the P generation: for example, he crossed purebred plants with purple flowers with purebred plants with white flowers.
The offspring are called the F1, or “first filial,” generation. For example, Mendel’s F1 all had purple flowers. Mendel allowed this generation to self pollinate.
The offspring from the first filial generation is the F2, or “second filial”, generation. For example, the offspring were 3/4 purple and 1/4 white.
The offspring of crosses between parents with different traits are called hybrids.
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Genes and Dominance
When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive.
Purebreds are organisms with the same alleles for a particular trait
Hybrids are organisms with the different alleles for a particular trait.
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Genes and Dominance
Mendel’s Seven F1 Crosses on Pea Plants
When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive.
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Genes and Dominance
Mendel's first conclusion
was that biological inheritance is determined by factors that are passed from one generation to the next.
Today, scientists call the factors that determine traits genes.
Genes are pieces of DNA that tells a cell to make a certain protein.
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Genes and Dominance
Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.
The different forms of a gene are called alleles.
Alleles are any of the possible forms in which a gene for a specific trait can occur. There are usually 2 forms of a gene, one comes from mom, the other comes from dad. (For example, blue eyes or brown eyes)
Mendel’s second conclusion
is called the principle of dominance.
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Genes and Dominance
The principle of dominance states that some alleles are dominant and others are recessive.
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14. Mendel’s F1 Crosses on Pea Plants
Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive.
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Segregation
Segregation
Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.
The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
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Segregation
Mendel's F2 Generation
P Generation
F1 Generation
F2 Generation
When Mendel allowed the F1 plants to reproduce by self-pollination, the traits controlled by recessive alleles reappeared in about one fourth of the F2 plants in each cross.
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short
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Segregation
The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness.
The law of segregation-Organisms inherit two copies of each gene but donate only one copy of gene in gametes (sex cells).
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Segregation
Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the sex cells, or gametes.
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Segregation
Alleles separate during gamete formation.
During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant.
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20. Traits, genes, and alleles
Genes encode proteins that produce a diverse range of traits.
A gene is a piece of DNA that directs a cell to make a certain protein.
Each gene has a locus, a specific position on a pair of homologous chromosomes.

21. Traits, genes, and alleles
An allele is any alternative form of a gene occurring at a specific locus on a chromosome.
Each parent donates one allele for every gene.
Organisms that have two identical alleles for a particular trait are said to be homozygous.
Organisms that have two different alleles for the same trait are heterozygous.

22. Traits, genes, and alleles
Genes influence the development of traits.
All of an organism’s genetic material is called the genome.
A genotype refers to the makeup of a specific set of genes.
A phenotype is the physical expression of a trait. Phenotypes are influenced by genotype and environmental factors.

23. Traits, genes, and alleles
Alleles can be represented using letters (or symbols).
Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters.
A dominant allele is expressed as a phenotype when at least one allele is dominant.
A recessive allele is expressed as a phenotype only when two copies are present.

24. Traits, genes, and alleles
Both homozygous dominant and heterozygous genotypes yield a dominant phenotype.
Most traits occur in a range and do not follow simple dominant-recessive patterns.

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11-1
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Gametes are also known as
genes.
sex cells.
alleles.
hybrids.
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11-1
The offspring of crosses between parents with different traits are called
alleles.
hybrids.
gametes.
dominant.
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11-1
In Mendel’s pea experiments, the male gametes are the
eggs.
seeds.
pollen.
sperm.
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In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F1 generation consists of
all short plants.
all tall plants.
half tall plants and half short plants.
all plants of intermediate height.
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If a particular form of a trait is always present when the allele controlling it is present, then the allele must be
mixed.
recessive.
hybrid.
dominant.
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