Genetics
● Johann Gregor Mendel set the framework for genetics long before chromosomes or
genes had been identified
● Genetics is the study of heredity
● Because of Mendel’s work, the fundamental principles of heredity were revealed
● Mendel used pea plants as his primary model system and conducted experiments with
nearly 30,000 plants
● Mendel performed hybridizations, which involve mating two true-breeding individuals
that have different traits
● Plants used in first-generation crosses were called P0
● Offspring were called the F1
● Seeds from the F1 plants produce the F2
Basics of Heredity
● A trait is defined as a variation in the physical appearance of a heritable characteristic
● A reciprocal cross is a paired cross in which the respective traits of the male and
female in one cross become the respective traits of the female and male in the other
cross
● Dominant traits are those that are inherited unchanged in a hybridization
● Recessive traits become latent, or disappear, in the offspring of a hybridization
● The fact that the recessive trait reappeared in the F2 generation meant that the
traits remained separate (not blended) in the plants of the F1 generation
Genes vs. Appearance
● The seven characteristics that Mendel evaluated in his pea plants were each expressed
as one of two versions, or traits
● Gene variants that arise by mutation and exist at the same relative locations on
homologous chromosomes are called alleles
● Two alleles for a given gene in a diploid organism are expressed and interact to produce
physical characteristics
● The observable traits expressed by an organism are referred to as its phenotype
● An organism’s underlying genetic makeup, consisting of both physically visible
and non-expressed alleles, is called its genotype
● Diploid organisms that are homozygous at a given gene, or locus, have two identical
alleles for that gene on their homologous chromosomes
Determining Genotype from Phenotype
● When fertilization occurs between two true-breeding parents that differ in only one
characteristic, the process is called a monohybrid cross
● A Punnett square can be drawn that applies the rules of probability to predict the
, possible outcomes of a genetic cross or mating and their expected frequencies
● A test cross determines whether an organism that expressed a dominant trait was a
heterozygote or a homozygote
Law of Segregation
● This law states that paired unit factors (genes) must segregate equally into gametes
such that offspring have an equal likelihood of inheriting either factor
● Supports Mendel’s observed 3:1 phenotypic ratio
● The equal segregation of alleles is the reason we can apply the Punnett square to
accurately predict the offspring of parents with known genotypes
Law of Independent Assortment
● This law states that genes do not influence each other with regard to the sorting of
alleles into gametes, and every possible combination of alleles for every gene is equally
likely to occur
● Can be illustrated by the dihybrid cross, a cross between two true-breeding
parents that express different traits for two characteristics
Mendel’s Laws are Not Absolute
● We now know that some allele combinations are not inherited independently of each
other
● Each chromosome contains hundreds or thousands of genes, organized linearly
on chromosomes like beads on a string
● The segregation of alleles into gametes can be influenced by linkage, in which genes
that are located physically close to each other on the same chromosome are more likely
to be inherited as a pair
● When two genes are located in close proximity on the same chromosome, they
are considered linked, and their alleles tend to be transmitted through meiosis
together
● As the distance between two genes increases, the probability of one or more
crossovers between them increases, and the genes behave more like they are
on separate chromosomes
Incomplete Dominance
● The heterozygote phenotype occasionally does appear to be intermediate between the
two parents: this is described as incomplete dominance
● Flower color in snapdragons
Codominance
● A variation on incomplete dominance is codominance, in which both alleles for the
● Johann Gregor Mendel set the framework for genetics long before chromosomes or
genes had been identified
● Genetics is the study of heredity
● Because of Mendel’s work, the fundamental principles of heredity were revealed
● Mendel used pea plants as his primary model system and conducted experiments with
nearly 30,000 plants
● Mendel performed hybridizations, which involve mating two true-breeding individuals
that have different traits
● Plants used in first-generation crosses were called P0
● Offspring were called the F1
● Seeds from the F1 plants produce the F2
Basics of Heredity
● A trait is defined as a variation in the physical appearance of a heritable characteristic
● A reciprocal cross is a paired cross in which the respective traits of the male and
female in one cross become the respective traits of the female and male in the other
cross
● Dominant traits are those that are inherited unchanged in a hybridization
● Recessive traits become latent, or disappear, in the offspring of a hybridization
● The fact that the recessive trait reappeared in the F2 generation meant that the
traits remained separate (not blended) in the plants of the F1 generation
Genes vs. Appearance
● The seven characteristics that Mendel evaluated in his pea plants were each expressed
as one of two versions, or traits
● Gene variants that arise by mutation and exist at the same relative locations on
homologous chromosomes are called alleles
● Two alleles for a given gene in a diploid organism are expressed and interact to produce
physical characteristics
● The observable traits expressed by an organism are referred to as its phenotype
● An organism’s underlying genetic makeup, consisting of both physically visible
and non-expressed alleles, is called its genotype
● Diploid organisms that are homozygous at a given gene, or locus, have two identical
alleles for that gene on their homologous chromosomes
Determining Genotype from Phenotype
● When fertilization occurs between two true-breeding parents that differ in only one
characteristic, the process is called a monohybrid cross
● A Punnett square can be drawn that applies the rules of probability to predict the
, possible outcomes of a genetic cross or mating and their expected frequencies
● A test cross determines whether an organism that expressed a dominant trait was a
heterozygote or a homozygote
Law of Segregation
● This law states that paired unit factors (genes) must segregate equally into gametes
such that offspring have an equal likelihood of inheriting either factor
● Supports Mendel’s observed 3:1 phenotypic ratio
● The equal segregation of alleles is the reason we can apply the Punnett square to
accurately predict the offspring of parents with known genotypes
Law of Independent Assortment
● This law states that genes do not influence each other with regard to the sorting of
alleles into gametes, and every possible combination of alleles for every gene is equally
likely to occur
● Can be illustrated by the dihybrid cross, a cross between two true-breeding
parents that express different traits for two characteristics
Mendel’s Laws are Not Absolute
● We now know that some allele combinations are not inherited independently of each
other
● Each chromosome contains hundreds or thousands of genes, organized linearly
on chromosomes like beads on a string
● The segregation of alleles into gametes can be influenced by linkage, in which genes
that are located physically close to each other on the same chromosome are more likely
to be inherited as a pair
● When two genes are located in close proximity on the same chromosome, they
are considered linked, and their alleles tend to be transmitted through meiosis
together
● As the distance between two genes increases, the probability of one or more
crossovers between them increases, and the genes behave more like they are
on separate chromosomes
Incomplete Dominance
● The heterozygote phenotype occasionally does appear to be intermediate between the
two parents: this is described as incomplete dominance
● Flower color in snapdragons
Codominance
● A variation on incomplete dominance is codominance, in which both alleles for the
