Theories of Inheritance through the ages
homunculus, inheritance of acquired characteristics; blending model of inheritance, particulate model of inheritance.
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.
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
crosses between different strains with the sexes reversed; for example: female A x male B and male A x female B.
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.