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23.1 Mendel’s Laws. Gregor Mendel Augustinian Monk Around 1857, began breeding garden peas to study inheritance. Performed crosses between true breeding lines of garden peas that differed in a single trait. Pea plants have several advantages for genetics:

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23 1 mendel s laws
23.1 Mendel’s Laws
  • Gregor Mendel
    • Augustinian Monk
    • Around 1857, began breeding garden peas to study inheritance.
    • 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 (traits).



Seed shape



Seed color



Pod shape



Pod color



Mendel could control which plants mated with which.
    • 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)


(produce pollen

grains, which

contain male


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

individual whose

anthers have been


mendel s experiment
Mendel’s Experiment
  • In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.
    • The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.
  • Hypotheses:
  • All offspring will be a blend of the two colors (lavender)
  • All offspring will be some of each color
  • All offspring will be one color or the other

When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants.

    • The white trait, absent in the F1, reappeared in the F2.
  • Mendel then allowed the F1 hybrids to self-pollinate to produce an F2 generation.
  • 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 s conclusions
Mendel’s Conclusions
  • 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 heredity:
    • law of segregation
    • law of independent assortment.
law of segregation
Law of Segregation
  • Four related ideas:

1. Different factors or alternative versions of genes (alleles) account for variations in inherited characters.

    • Different alleles vary somewhat in the sequence of nucleotides at the specific locus (location) on paired chromosomes.
      • The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus.
2. For each character, an organism inherits twoalleles, 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 other.
3. If two alleles differ, then one, the dominantallele, is fully expressed in the the organism’s appearance.
  • 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 gametes.
    • If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other.
  • The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.
law of segregation summary
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
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

single trait gamete formation
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 each gamete
    • Example: if one parent’s genotype is Ww, then some gametes from this individual will contain a W and others a w
one trait cross
One-Trait Cross

A homozygous man with a widow’s peak X a woman with a straight hairline

punnett square
Punnett Square

Two individuals who are both Ww

one trait crosses and probability
One-Trait Crosses and Probability
  • The chance of 2 or more independent events occurring together is the product of their chance of occurring separately
  • 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 follows
    • 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
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 heterozygous
inheritance of two traits
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 gametes
two trait crosses dihybrid cross26
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
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

pedigree analysis
Pedigree Analysis
  • 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
Beyond Simple Inheritance Patterns
  • Incomplete Dominance
    • Occurs when the heterozygote shows a distinct intermediate phenotype not seen in the two homozygotes
    • Offspring of a cross between heterozygotes will show three phenotypes: each parent and the heterozygote.
    • The phenotypic and genotypic ratios are identical, 1:2:1.
A clear example of incomplete dominance is seen in flower color of snapdragons.
    • A cross between a white-flowered plant 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 offspring.
  • Occurs when alleles are equally expressed in a heterozygote
  • 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 cells.
  • 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
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 codominant
abo blood types
ABO Blood Types

Phenotype Genotype




O ii

  • Both IA and IB are dominant over i, IA and IB are codominant
  • The Rh factor is inherited separately from ABO blood types.
sex linked inheritance in humans
Sex-Linked Inheritance in Humans
  • 22 pairs of autosomes, 1 pair of sex chromosomes
    • 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 homologous
    • Traits controlled by genes in the sex chromosomes are called sex-linked traits
    • X chromosome has many genes, the Y chromosome does not
sex linked alleles
Sex-Linked Alleles
  • Red-green colorblindness is X-linked
    • The X chromosome has genes for normal color vision
      • XB = normal vision
      • Xb – colorblindness

Genotypes Phenotypes

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

polygenic inheritance
Polygenic Inheritance
  • 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.
environmental influences
Environmental Influences
  • Environmental factors can influence the expression of genetic traits.
  • Example:
    • Siamese cats and Himalayan rabbits are darker in color where body heat is lost to the environment.
inheritance of linked genes
Inheritance of Linked Genes
  • All the alleles on one chromosome form a linkage group.
  • Recall that during meiosis crossing over sometimes occurs
  • 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 chromosome
  • 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.