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Gregor Mendel. Pea Plants and Inheritance Patterns. Who is Gregor Mendel?. Mendel was born in 1822 in Austria His father was a peasant farmer, tenanted to a local aristocrat who was very interested in scientific crop improvement The family was very poor

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Gregor Mendel

Pea Plants and Inheritance Patterns

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Who is Gregor Mendel?

  • Mendel was born in 1822 in Austria

  • His father was a peasant farmer, tenanted to a local aristocrat who was very interested in scientific crop improvement

  • The family was very poor

  • Mendel’s early education was with local priest and teacher

  • Showed considerable academic capability and so was sent to larger town with more opportunities

  • Financial problems plagued him, affecting his health, so Mendel decided to enter monastery as means of support

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At the Monastery…

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At the Monastery…

  • Mendel taught 6th and 7th grade age children

  • He also had access to the monastery’s library

  • Fr. Napp, head of the monastery, had a shared interest in botany and agriculture

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Mendel’s Model Organism –

The Garden Pea

  • Self-Fertilizing

  • Matured Quickly

  • Several Easily Identifiable Traits

  • Used Consistent Methods:

  • Opened flower & placed

  • pollen from one type onto

  • the stigma

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Pea Plant Characteristics

  • Seed Shape

  • Seed Color

  • Pod Shape

  • Pod Color

  • Flower Color

  • Flower Position

  • Stem Length

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Mendel’s First Experiment

  • Crossed Pure Tall Pea Plant (TT) x Pure Short (Dwarf) Pea Plant (tt)

  • Hypothesis:

    • The offspring would be:

      • All tall

      • All short

      • All intermediate

      • Some would be talls and some short

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1st Experiment:

Crossed Pure TallxPure Short

All offspring (F1)tall

2nd Experiment:

Bred F1

Ratio of 787 tall to 277 short (3:1)

Similar to chance events from flipping 2 pairs of coins

The Results?

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Monohybrid Crosses and the Law of Segregation

A cross between individuals differing in single character is a monohybrid cross.

The analysis of monohybrid crosses allowed Mendel to deduce the Law of Segregation…

Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization).

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Staying the Course –Crosses to the F2

(The Grandchildren)

  • Crossed one of the F1 tall plants with its dwarf parent:

    F1 Tall x Dwarf

  • Possible Outcomes:

    • All would be tall

    • Mixture of Tall & Dwarf

    • All would be intermediate

  • Experimental results 

The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells.

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Monohybrid Crosses are Consistent!

Therefore, the Law of Segregation indeed is a general principle of genetics.

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Mendel’s Experiments – The Next Generation



  • Mendel recognized that it is not always possible to tell what offspring will be like by inspecting the parent

  • Mendel could test if tall plants were pure-breeds (homozygotes) or hybrid (heterozygotes) by the “back-cross” or “test-cross”

  • What % would you predict for each genotype?

t t

t t

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Mendel’s Hypothesis

  • There are alternative forms for genes, the units that determine inheritable characteristics (AA or Aa or aa)

  • For each inherited characteristic, an organism has two alleles, one inherited from each parent.

  • A sperm or egg carries only one allele (A or a)for each inherited characteristic, because allele pairs separate from each other during meiosis. At fertilization, the sperm and egg unite and restore the gene to the paired condition.

  • When the two alleles of a pair are different, one is fully expressed (dominant) and the other is completely masked (recessive). The members of the pair may be identical (homozygous) or non-identical (heterozygous).

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Revisiting Meiosis

Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

The alignment of one pair of homologs is independent of any other.

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Incomplete Dominance

  • Incomplete dominance is a blending of colors

  • Dominance relationships may differ, but the Principle of Segregation is the same

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Polygenic Inheritance:When a Single Trait is Influenced by Many Genes

Height is a polygenic trait

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Multiple Alleles

  • Many genes are present in three or more versions (alleles) – this is multiple alleles

  • The human ABO blood group is determined by three alleles (IA, IB, and I) of a single gene

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  • The human ABO blood group also exhibits codominance – another genetic phenomenon

  • Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote

The AB phenotype (genotype IA IB) is an example of codominance

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male / sperm






female / eggs




Sex-Linked Traits

2 normal parents,

but mother is carrier




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Sex-Linked Traits

  • Sex chromosomes have other genes on them, especially the X chromosome

    • Hemophilia in humans

      • Blood doesn’t clot

    • Duchenne muscular dystrophy in humans

      • Loss of muscle control

    • Red-green color blindness

      • See green & red as shades of grey

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  • Individuals are born with extra fingers and toes

  • The allele for 6+ fingers and toes is dominant,while the allele for 5 digits is recessive

  • Recessive is far more common! (1:500 have polydactyly)

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Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?

Mendel performed dihybrid crosses to find out.

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Dihybrid Crosses Together or Inherited Independently?

Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.