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4.1 Foundations of Genetics
heredity
allele
genetics
phenotype
dominant
genotype
recessive
homozygous
gene
heterozygous
4.1 Foundations of Genetics
Early Ideas About Heredity
• Combined genetic material from a sperm
and an egg determines the traits or
features of an offspring.
• Heredity is the passing of traits from
parents to offspring.
law of segregation
law of independent
assortment
4.1 Foundations of Genetics
4.1 Foundations of Genetics
Early Ideas About Heredity (cont.)
Gregor Mendel and His Experiments
• The idea of blending inheritance is
offspring are a blend of genetic material
from both parents.
• Gregor Mendel was the first to record
evidence that traits are determined by
factors passed from parents to offspring.
– The genetic material mixed or blended
like colors of paint.
• Mendel established the basic laws of
heredity.
– Over many generations, populations
would eventually look alike.
• Genetics is the study of how traits of
organisms are passed from parents to
offspring.
– Blending inheritance cannot explain why
some traits skip a generation.
4.1 Foundations of Genetics
4.1 Foundations of Genetics
Mendel’s Experimental Methods
Controlled Experiments
• Mendel conducted breeding experiments
by studying seven traits of pea plants and
each traits had only two variations.
• Mendel controlled fertilization in the pea
plants, allowing him to see how traits pass
from one generation to another.
• Mendel allowed some flowers to
self-fertilize.
• He also performed cross-fertilization by
transferring pollen from one pea flower to
another.
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4.1 Foundations of Genetics
4.1 Foundations of Genetics
Mendel’s Unique Methods
Mendel’s Experimental Results
• Used true-breeding plants for each trait—
plants that always produce offspring with
that trait when they self-pollinate
• Mendel concluded that two factors control
each inherited trait.
• Recorded the inheritance of traits for
several generations
• When organisms reproduce, each gamete—
sperm or egg—contributes one factor for
each trait.
• Used a mathematical approach
4.1 Foundations of Genetics
4.1 Foundations of Genetics
Dominant Factors
Recessive Factors
• A genetic factor that blocks another
genetic factor is called dominant.
• A genetic factor that is hidden by the
presence of a dominant factor is recessive.
• A dominant trait is observed when offspring
have one or two dominant factors.
• A recessive trait can be observed only when
two recessive genetic factors are present in
offspring.
4.1 Foundations of Genetics
4.1 Foundations of Genetics
Mendel’s Laws of Heredity
Modern Definitions of Mendel’s Ideas
• Law of segregation: the two factors for
each trait segregate—separate from each
other—during meiosis when gametes form
• Mendel did not know about DNA or how cells
reproduce, but his ideas about inheritance
are still true today.
• Law of independent assortment: the
factors for one trait separate independently
of how factors for other traits separate
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4.1 Foundations of Genetics
4.1 Foundations of Genetics
Genes and Alleles
Phenotype and Genotype
• A gene is a section
of DNA that has
information about a
trait in an organism.
• The observable traits and all characteristics
of an organism make up the organism’s
phenotype.
• The alleles that make up an organism is
the organism’s genotype.
• Each form of a gene
with different
information is called
an allele.
• The alleles of a particular gene is that
gene’s genotype.
4.1 Foundations of Genetics
Homozygous and Heterozygous
Genotypes
4.1 Foundations of Genetics
Homozygous and Heterozygous
Genotypes (cont.)
• Because eukaryotes have pairs of
chromosomes, a genotype for a gene has
two alleles.
• If the two alleles have the same information,
the genotype is homozygous.
• If the two alleles have different information,
the genotype is heterozygous.
4.1 Foundations of Genetics
Law of Segregation Explained
• The movement of chromosomes during
meiosis explains Mendel’s law of
segregation.
– Each set of chromatids separates into
different gametes during meiosis II.
– Each gamete receives only one allele.
4.1 Foundations of Genetics
Law of Independent Assortment
Explained
• The daughter cells produced by meiosis
receive only one chromosome from each
pair of homologous chromosomes.
– A daughter cell might receive the A or a
chromosome from pair 1 and the B or b
chromosome from pair 2.
– This results in four possible allele
combinations for two homologous pairs
of chromosomes.
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4.1 Foundations of Genetics
Importance of Mendel’s
Genetic Studies
• In the 1860s, no one knew about
chromosomes or meiosis so it was hard to
understand Mendel’s discoveries.
• All the research of modern genetics is
based on Mendel’s conclusions from his
work with pea plants.
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