23.1 Mendel’s Laws
• Gregor Mendel
– Augustinian Monk
– Around 1857, began
breeding garden peas to
– Performed crosses between
true breeding lines
of garden peas that differed
in a single trait.
• Pea plants have several advantages for genetics:
– Pea plants are available in many varieties with distinct
heritable features (characters) with different variants
• Mendel could control which plants mated with
– Each pea plant has male (stamens) and female
(carpal) sexual organs.
– In nature, pea plants typically self-fertilize, fertilizing
ova with their own sperm.
– However, Mendel could also move pollen from one
plant to another to cross-pollinate plants.
Stigma (receives pollen)
Ovules (produce female gametes)
1. Remove anthers
from one plant.
2. Collect pollen
from a different plant.
3. Transfer pollen
to a stigma of the
anthers have been
• In a typical breeding experiment, Mendel would cross-
pollinate (hybridize) two contrasting, true-breeding pea
– The true-breeding parents are the P generation and their hybrid
offspring are the F1 generation.
1. All offspring will be a blend of
the two colors (lavender)
2. All offspring will be some of
3. All offspring will be one color
or the other
• Mendel then allowed the F1 hybrids to self-pollinate to
produce an F2 generation.
• When Mendel allowed the
F1 plants to self-fertilize,
the F2 generation included
both purple-flowered and
– The white trait, absent in the
F1, reappeared in the F2.
• Based on a large sample size, Mendel recorded 705
purple-flowered F2 plants and 224 white-flowered F2 plants
from the original cross (a ratio of three purple to one white flowering
plant in the F2 offspring).
• Mendel reasoned that the heritable factor for
white flowers was present in the F1 plants, but it
did not affect flower color.
– Purple flower is a dominant trait and white flower
is a recessive trait.
• Mendel’s quantitative analysis of F2 plants
revealed the two fundamental principles of
– law of segregation
– law of independent assortment.
Law of Segregation
• Four related ideas:
1. Different factors or alternative versions of
genes (alleles) account for variations in inherited
– Different alleles vary somewhat in the sequence of
nucleotides at the specific locus (location) on paired
• The purple-flower
allele and white-flower
allele are two DNA
variations at the
2. For each character, an organism inherits two
alleles, one on each homologous chromosome
from each parent.
– Each diploid organism has a pair of homologous
chromosomes and therefore two copies of each locus.
– A diploid organism inherits one set of chromosomes
from each parent.
– These homologous loci may be identical, as in the
true-breeding plants of the P generation.
– Alternatively, the two alleles may differ
• In the flower-color example, the F1 plants inherited a purple-
flower allele from one parent and a white-flower allele from the
3. If two alleles differ, then one, the dominant
allele, is fully expressed in the the organism’s
– The other, the recessive allele, has no noticeable
effect on the organism’s appearance.
– Mendel’s F1 plants had purple flowers because the
purple-flower allele is dominant and the white-flower
allele is recessive.
4. The two alleles for each character segregate
(separate) during gamete production when the
homologous chromosomes are separated and
distributed to different gametes in meiosis.
– If an organism has identical alleles for a particular
character, then that allele exists as a single copy in all
– If different alleles are present, then 50% of the
gametes will receive one allele and 50% will receive
• The separation of alleles into separate gametes
is summarized as Mendel’s law of segregation.
Law of Segregation - Summary
• Each individual has alleles for each trait
• The alleles segregate (separate) during the
formation of gametes
• Each gamete contains only one allele from each
pair of alleles
• Fertilization gives each new individual two
alleles for each trait
Inheritance of a Single Trait
• Phenotype: physical appearance of the individual with
regard to a trait
• Genotype: alleles responsible for a given trait
– Two alleles for a trait
– A capital letter symbolizes a dominant allele (W)
– A lower-case letter symbolizes a recessive allele (w)
– Dominant refers to the allele that will mask the expression
of the alternate (recessive) allele
Example: Widow’s Peak
Single Trait Gamete Formation
• During meiosis, homologous chromosomes
separate so there is only 1 member of each pair
in a gamete
– There is one allele for each trait, such as hairline, in
– Example: if one parent’s genotype is Ww, then some
gametes from this individual will contain a W and
others a w
A homozygous man with a widow’s peak X a woman with a straight hairline
Two individuals who are both Ww
One-Trait Crosses and Probability
• The chance of 2 or more independent events occurring
together is the product of their chance of occurring
• In the cross Ww X Ww, what is the chance of obtaining
either a W or a w from a parent?
– Chance of W = ½ and the chance of w = ½
• Therefore the probability of having these genotypes is as
– Chance of WW= ½ X ½ = ¼
– Chance of Ww = ½ X ½ = ¼
– Chance of wW= ½ X ½ = ¼
– Chance of ww = ½ X ½ = ¼
One-Trait Test Cross
• Breeders of plants and animals may do a test
cross to determine the likely genotype of an
individual with the dominant phenotype
– Cross with a recessive individual - the recessive has
a known genotype (ww)
– If there are any offspring produced with the recessive
phenotype, then the dominant parent must be
Inheritance of Two Traits
The Law of Independent Assortment:
• Each pair of factors assorts independently (without
regard to how the others separate)
• All possible combinations of factors can occur in
The Inheritance of Two Traits
Two-Trait Crosses (Dihybrid Cross)
Two-Trait Crosses (Dihybrid Cross)
• WwSs (X) WwSs
– Phenotypic Ratio:
• 9 widow’s peak, short fingers
• 3 widow’s peak, long fingers
• 3 straight hairline, short fingers
• 1 straight hairline, long fingers
Two-Trait Crosses and Probability
– Probability Laws
• Probability of widow’s peak = ¾
• Probability of short fingers= ¾
• Probability of straight hairline= ¼
• Probability of long fingers= ¼
– Using the Product Rule
• Probability of widow’s peak and short fingers =
¾ X ¾ = 9/16
• Probability of widow’s peak and long fingers =
¾ X ¼ = 3/16
• Probability of straight hairline and short fingers =
¼ X ¾ = 3/16
• Probability of straight hairline and long fingers =
¼ X ¼ = 1/16
• Information about the presence or
absence of a particular phenotypic trait is
collected from as many individuals in a
family as possible and across as many
generations as possible.
• The distribution of these characters is then
mapped on the family tree.
• Example: If an individual in the third generation lacks a
widow’s peak, but both her parents have widow’s peaks, then
her parents must be heterozygous for that gene.
• If some siblings in the second generation lack a widow’ peak
and one of the grandparents (first generation) also lacks one,
then the other grandparent must be heterozygous and we can
determine the genotype of almost all other individuals.
Beyond Simple Inheritance Patterns
• Incomplete Dominance
– Occurs when the heterozygote shows a distinct
intermediate phenotype not seen in the two
– Offspring of a cross between heterozygotes will show
three phenotypes: each parent and the heterozygote.
– The phenotypic and genotypic ratios are identical,
• A clear example of incomplete dominance
is seen in flower color of snapdragons.
– A cross between a
and a red-flowered
plant will produce all
pink F1 offspring.
– Self-pollination of the
F1 offspring produces
25% white, 25% red,
and 50% pink
– Occurs when alleles are equally expressed in
– Example: the M, N, and MN blood groups of
humans are due to the presence of two
specific molecules on the surface of red blood
– People of group M (genotype MM) have one
type of molecule on their red blood cells,
people of group N (genotype NN) have the
other type, and people of group MN (genotype
MN) have both molecules present.
Multiple Allele Inheritance
• A trait is controlled by multiple alleles, the gene
exists in several allelic forms.
– Each person has only two of the possible alleles.
– ABO Blood Types
• IA = A antigens on red blood cells
• IB = B antigens on red blood cells
• i = has neither A nor B antigens on red blood cells
• Both IA and IB are dominant over i, IA and IB are
ABO Blood Types
A IAIA or IAi
B IBIB or IBi
Both IA and IB are dominant over i, IA and IB are
The Rh factor is inherited separately from ABO
Inheritance of Blood Types
Sex-Linked Inheritance in Humans
• 22 pairs of autosomes, 1 pair of sex
– X and Y
• In females, the sex chromosomes are XX
• In males, the sex chromosomes are XY
• Note that in males the sex chromosomes are not
– Traits controlled by genes in the sex
chromosomes are called sex-linked traits
– X chromosome has many genes, the Y
chromosome does not
• Red-green colorblindness is X-linked
– The X chromosome has genes for normal color vision
• XB = normal vision
• Xb – colorblindness
XBXB female with normal color vision
XBXb carrier female with normal color vision
XbXb colorblind female
XBY male with normal color vision
XbY colorblind male
Cross involving an X-linked Allele
• Occurs when a trait is governed by two or more
sets of alleles.
• Each dominant allele codes for a product
• The effects of the dominant alleles are additive.
• The result is continuous variation.
• Examples of traits include size or height, shape,
weight, and skin color.
Polygenic Inheritance – Skin Color
• Environmental factors can influence the expression
of genetic traits.
darker in color
heat is lost to
Inheritance of Linked Genes
• All the alleles on one chromosome form a
• Recall that during meiosis crossing over
• If crossing over occurs between two alleles of
interest, then four types of gametes are formed
instead of two
• The occurrence of crossing-over can help
determine the sequence of genes on a
• Crossing-over occurs more often between
distant genes than genes that are close together
• In the example below, it is expected that
recombinant gametes would include G and z
more often than R and s.