Chapter 10 - Patterns of Inheritance
10.2 Tracking Traits
Early Thoughts about Heredity
● In the nineteenth century, people thought that hereditary material must be some
type of fluid, with fluids from both parents blending at fertilization like milk into
coffee.
● However, the idea of “blending inheritance” failed to explain what people could
see with their own eyes.
● Children sometimes have traits such as freckles that do not appear in either
parent, for example. A cross between a black horse and a white one does not
produce gray offspring.
● At the time, no one knew that hereditary information is divided into discrete units
(genes), an insight that is critical to understanding how traits are inherited.
● Around 1850, Gregor Mendel began an extended series of experiments breeding
pea plants, which vary in traits such as flower color, height, and so on.
● Mendel, an Austrian monk, crossed thousands of plants, and kept careful records
of the traits of parents and offspring.
● Through these experiments, he gained insight into the nature of inheritance.
Mendel’s Pea Plant
● Mendel cultivated the garden pea.
● This species is naturally self-fertilizing, which means its flowers produce male
and female gametes that form viable seeds when they meet up.
● In order to study inheritance, Mendel had to carry out controlled matings
(crosses) between individuals with specific traits.
● First, he removed the pollen-bearing parts (anthers) from pea flowers.
● Removing anthers from a pea flower prevents it from self-fertilizing.
● Second, he cross-fertilized the flowers by brushing their egg-bearing parts
(carpels) with pollen from other plants.
● Third, he collected seeds that formed from the cross-fertilized flowers, planted
them, and recorded the traits of the resulting pea plant offspring .
● Many of Mendel’s experiments started with plants that “breed true” for particular
traits such as white flowers or purple flowers.
● Breeding true for a trait means that, new mutations aside, all offspring have the
same form of the trait as the parent(s), generation after generation.
● For example, all offspring of pea plants that breed true for white flowers also
have white flowers.
● As you will see in the next section, Mendel cross-fertilize pea plants that breed
true for different forms of a trait, and discovered that the traits of the offspring
often appear in predictable patterns.
● Mendel’s meticulous work breeding pea plants and tracking their traits led him to
, conclude (correctly) that hereditary information passes from one generation to
the next in distinct units.
● He published his work in 1866, but apparently it was read by few and understood
by no one at the time.
● In 1871 he was promoted, and his pioneering experiments ended.
● When he died in 1884, he did not know that his work with pea plants would be
the starting point for modern genetics.
Inheritance in Modern Terms
● Today, we know that Mendel’s “hereditary units” are genes. Individuals of a
species share certain traits because their chromosomes carry the same genes.
● Each gene occurs at a specific location on a particular chromosome
● Diploid cells have pairs of homologous chromosomes, so they have two copies of
each gene; in most cases, both copies are expressed at the same level.
● The two copies of any gene may be identical, or they may be different alleles.
Homozygous and Heterozygous
● An individual with the same allele of a gene on both homologous chromosomes
is homozygous for the allele (homo- means “the same”).
● Organisms breed true for a trait because they are homozygous for alleles
governing the trait.
● By contrast, an individual with different alleles of a gene is heterozygous for the
allele (hetero- means “different”).
● A hybrid is a heterozygous individual produced by a cross or mating between
parents that breed true for different forms of a trait.
Genotype and Phenotype
● Homozygous and heterozygous describe genotype, the particular set of alleles
that an individual carries.
● Genotype is the basis of phenotype, which refers to the individual’s observable
traits.
● “White-flowered” and “purple-flowered” are examples of pea plant phenotypes
that arise from differences in genotype.
Dominant and Recessive
● The phenotype of a heterozygous individual depends on how the products of its
two different alleles interact.
● In many cases, the product of one allele influences the effect of the other, and
the outcome of this interaction is reflected in the individual’s phenotype.
● An allele is dominant when its effect masks that of a recessive allele paired with
it.
● A dominant allele is often represented by an uppercase italic capital letter such
as A; a recessive allele, with a lowercase italic letter such as a.
Overall Message
● Genotype refers to the particular set of alleles that an individual carries.
, Genotype is the basis of phenotype, which refers to the individual’s observable
traits.
● A homozygous individual has two identical alleles of a gene. A heterozygous
individual has two nonidentical alleles.
● A dominant allele masks the effect of a recessive allele paired with it in a
heterozygous individual.
10.3 Mendelian Inheritance Patterns
Segregation of Genes into Gametes
● Meiosis separates the homologous chromosomes of a pair and packages each in
a different gamete.
● Thus, alleles on the homologous chromosomes end up in different gametes.
● Let’s use our pea plant alleles for purple and white flowers in an example.
● A plant homozygous for the dominant allele (PP) can only make gametes that
carry the dominant allele P.
● A plant homozygous for the recessive allele (pp) can only make gametes that
carry the recessive allele p.
● If the two homozygous plants are crossed (PP × pp), only one outcome is
possible: A gamete carrying allele P meets up with a gamete carrying allele p.
● All offspring of this cross will have both alleles—they will be heterozygous (Pp).
● A grid called a Punnett square is helpful for predicting the outcome of crosses
like this one.
Monohybrid Crosses
● Mendel did not know what alleles were, but he discovered that they segregate
into gametes and recombine in offspring.
● Experiments called monohybrid crosses were key to this discovery.
● A monohybrid cross is a cross between individuals that are identically
heterozygous for alleles of one gene (Aa × Aa, for example).
● The experiment begins with a cross between individuals that breed true for
different forms of a trait.
● The cross produces F1 (first-generation) hybrid offspring.
● A cross between two of these F1 individuals is the monohybrid cross, and it
produces F2 (second-generation) offspring.
● The frequency at which the two forms of the trait appear among the F2 offspring
offers information about a dominance relationship between alleles governing the
trait.
● A cross between two purple-flowered heterozygous plants (Pp × Pp) offers an
example of a monohybrid cross. Each of these plants makes two types of
gametes: gametes that carry a P allele, and gametes that carry a p allele.
● The two types of gametes can meet up in four possible ways at fertilization.
● Three of the four possible combinations include the dominant allele P. In other
10.2 Tracking Traits
Early Thoughts about Heredity
● In the nineteenth century, people thought that hereditary material must be some
type of fluid, with fluids from both parents blending at fertilization like milk into
coffee.
● However, the idea of “blending inheritance” failed to explain what people could
see with their own eyes.
● Children sometimes have traits such as freckles that do not appear in either
parent, for example. A cross between a black horse and a white one does not
produce gray offspring.
● At the time, no one knew that hereditary information is divided into discrete units
(genes), an insight that is critical to understanding how traits are inherited.
● Around 1850, Gregor Mendel began an extended series of experiments breeding
pea plants, which vary in traits such as flower color, height, and so on.
● Mendel, an Austrian monk, crossed thousands of plants, and kept careful records
of the traits of parents and offspring.
● Through these experiments, he gained insight into the nature of inheritance.
Mendel’s Pea Plant
● Mendel cultivated the garden pea.
● This species is naturally self-fertilizing, which means its flowers produce male
and female gametes that form viable seeds when they meet up.
● In order to study inheritance, Mendel had to carry out controlled matings
(crosses) between individuals with specific traits.
● First, he removed the pollen-bearing parts (anthers) from pea flowers.
● Removing anthers from a pea flower prevents it from self-fertilizing.
● Second, he cross-fertilized the flowers by brushing their egg-bearing parts
(carpels) with pollen from other plants.
● Third, he collected seeds that formed from the cross-fertilized flowers, planted
them, and recorded the traits of the resulting pea plant offspring .
● Many of Mendel’s experiments started with plants that “breed true” for particular
traits such as white flowers or purple flowers.
● Breeding true for a trait means that, new mutations aside, all offspring have the
same form of the trait as the parent(s), generation after generation.
● For example, all offspring of pea plants that breed true for white flowers also
have white flowers.
● As you will see in the next section, Mendel cross-fertilize pea plants that breed
true for different forms of a trait, and discovered that the traits of the offspring
often appear in predictable patterns.
● Mendel’s meticulous work breeding pea plants and tracking their traits led him to
, conclude (correctly) that hereditary information passes from one generation to
the next in distinct units.
● He published his work in 1866, but apparently it was read by few and understood
by no one at the time.
● In 1871 he was promoted, and his pioneering experiments ended.
● When he died in 1884, he did not know that his work with pea plants would be
the starting point for modern genetics.
Inheritance in Modern Terms
● Today, we know that Mendel’s “hereditary units” are genes. Individuals of a
species share certain traits because their chromosomes carry the same genes.
● Each gene occurs at a specific location on a particular chromosome
● Diploid cells have pairs of homologous chromosomes, so they have two copies of
each gene; in most cases, both copies are expressed at the same level.
● The two copies of any gene may be identical, or they may be different alleles.
Homozygous and Heterozygous
● An individual with the same allele of a gene on both homologous chromosomes
is homozygous for the allele (homo- means “the same”).
● Organisms breed true for a trait because they are homozygous for alleles
governing the trait.
● By contrast, an individual with different alleles of a gene is heterozygous for the
allele (hetero- means “different”).
● A hybrid is a heterozygous individual produced by a cross or mating between
parents that breed true for different forms of a trait.
Genotype and Phenotype
● Homozygous and heterozygous describe genotype, the particular set of alleles
that an individual carries.
● Genotype is the basis of phenotype, which refers to the individual’s observable
traits.
● “White-flowered” and “purple-flowered” are examples of pea plant phenotypes
that arise from differences in genotype.
Dominant and Recessive
● The phenotype of a heterozygous individual depends on how the products of its
two different alleles interact.
● In many cases, the product of one allele influences the effect of the other, and
the outcome of this interaction is reflected in the individual’s phenotype.
● An allele is dominant when its effect masks that of a recessive allele paired with
it.
● A dominant allele is often represented by an uppercase italic capital letter such
as A; a recessive allele, with a lowercase italic letter such as a.
Overall Message
● Genotype refers to the particular set of alleles that an individual carries.
, Genotype is the basis of phenotype, which refers to the individual’s observable
traits.
● A homozygous individual has two identical alleles of a gene. A heterozygous
individual has two nonidentical alleles.
● A dominant allele masks the effect of a recessive allele paired with it in a
heterozygous individual.
10.3 Mendelian Inheritance Patterns
Segregation of Genes into Gametes
● Meiosis separates the homologous chromosomes of a pair and packages each in
a different gamete.
● Thus, alleles on the homologous chromosomes end up in different gametes.
● Let’s use our pea plant alleles for purple and white flowers in an example.
● A plant homozygous for the dominant allele (PP) can only make gametes that
carry the dominant allele P.
● A plant homozygous for the recessive allele (pp) can only make gametes that
carry the recessive allele p.
● If the two homozygous plants are crossed (PP × pp), only one outcome is
possible: A gamete carrying allele P meets up with a gamete carrying allele p.
● All offspring of this cross will have both alleles—they will be heterozygous (Pp).
● A grid called a Punnett square is helpful for predicting the outcome of crosses
like this one.
Monohybrid Crosses
● Mendel did not know what alleles were, but he discovered that they segregate
into gametes and recombine in offspring.
● Experiments called monohybrid crosses were key to this discovery.
● A monohybrid cross is a cross between individuals that are identically
heterozygous for alleles of one gene (Aa × Aa, for example).
● The experiment begins with a cross between individuals that breed true for
different forms of a trait.
● The cross produces F1 (first-generation) hybrid offspring.
● A cross between two of these F1 individuals is the monohybrid cross, and it
produces F2 (second-generation) offspring.
● The frequency at which the two forms of the trait appear among the F2 offspring
offers information about a dominance relationship between alleles governing the
trait.
● A cross between two purple-flowered heterozygous plants (Pp × Pp) offers an
example of a monohybrid cross. Each of these plants makes two types of
gametes: gametes that carry a P allele, and gametes that carry a p allele.
● The two types of gametes can meet up in four possible ways at fertilization.
● Three of the four possible combinations include the dominant allele P. In other
