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Snustad, D.P. & M.J. Simmons. (2009) Principles of Genetics, 5th ed. Chapter 2: Mendelism: The Basic Principles of Inheritance Lecture 2, Professor Michael Pierce

Theories of Inheritance through the ages

homunculus, inheritance of acquired characteristics; blending model of inheritance, particulate model of inheritance.


spermists and ovists; preformation; idea of preformed human in the sex cell

inheritance of acquired characteristics

introduced in the 18th century by LaMarck; theory that characteristics were developed through use and disuse.

blending model of inheritance

developed in the 19th century; the idea that the genetic material of both parents were mixed, or blended.

problems with blending model

-parental contribution cannot be recovered
- populations will become homogenous
-characteristics can skip generations and blending cannot explain this.

particulate model of inheritance

the "gene idea" developed by Mendel in the 1860s; explains all the blending problems and introduces a discrete unit of inheritance in offspring, parental contributions coexist independently and retain their individual properties and can be recovered.

Why did Mendel pick the garden pea, or Pisum satirum?

- It's a dicot, a plant that sprouts 2 leaves or a cotyledon.
- The petals of the flower close down tightly, preventing pollen grains from entering or leaving, which enforces self-fertilization. Peas, as a result, are highly inbred, displaying minimal genetic variation, or true breeding.
- Had contrasting traits which helped him determine inheritance of characteristics of the peas.
- singular differences allowed him to study one trait at a time.
- easy to grow, control
- clearly distinct traits

7 traits identified and used by Mendel

height, texture, seed color, flower color, pod shape, pod color, flower position


sperm and egg from different individuals; male organs removed before matured; Mendel used tall and dwarf pea plants to investigate how height was inherited.

mono-hybrid crosses

a cross between parents differing in only one trait or in which only one trait is being considered.


an offspring of 2 homozygous parents that differ from one another by the alleles present at only one gene locus.

steps of a monohybrid cross

1) Tall (dominant) and dwarf varieties are cross-fertilized
2) all the hybrid progeny are tall.
3) the hybrid progeny are self-fertilized.
4) Tall and dwarf plants appear among the offspring of hybrids approximately in a ratio of 3 tall :1 dwarf.

parental (P): dwarf x tall (cross-pollinated) -->First filial (F1): tall = monohybrids (self-pollination) --> Second Filial (F2): 3Tall + 1Dwarf

reciprocal cross

crosses between different strains with the sexes reversed; for example: female A x male B and male A x female B.


genetic makeup or identity of the alleles.


physical appearance


2 alleles are identical


2 alleles are different

dominant & recessive alleles

dominant: expressed factor in a cross; recessive: latent factor

The Principle of Dominance

when one allele conceals the presence of the other allele; Mendel inferred that for all characteristics, there's 1 dominant and 1 recessive.

The principle of Segregation

In a heterozygote, two different alleles segregate from each other during the formation of gametes; reduces the number of alleles; Mendel predicts meiosis with this principle.


backcross to the recessive parental type or a cross between genetically unknown individuals with a fully recessive tester to determine whether an individual in question is heterozygous or homozygous for a certain allele.


an individual that is heterozygous for two pairs of alleles; the progeny of a cross between homozygous parents differing in two respects.

dyhybrid cross

following the inheritance for two traits; Mendel used seed color and texture; uses the principle of independent assortment

P: yellow, round x green, wrinkled
F1: Yellow, round (self-fert.)
F2: 9:3:3:1 ratio of yellow, round:green, round:yellow, wrinkled: green wrinkled.

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