This is a comprehensive and detailed note on Chapter 12; inheritance,genes and chromosomes for Biol 105.
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Chapter 12:
Inheritance, Genes and Chromosomes
What Are the Mendelian Laws of Inheritance?
● Gregor Mendel was an Austrian monk. His studies in physics and mathematics
were a strong influence on his use of quantitative (e.g., over seven years, he
made crosses with 24,034 plants) experimental methods.
● Early studies of inheritance worked under two assumptions about how
inheritance works:
● Each parent contributes equally to offspring (correct).
● Hereditary determinants blend in the offspring (incorrect). It was thought that
once hereditary elements had blended they could never be separated;
Mendel’s studies refuted this.
● Mendel’s new theory of inheritance was published in 1866, but was largely
ignored.
● By 1900, meiosis had been observed, and it was realized that chromosomes
and meiosis provided a physical explanation for Mendel’s results.
● Mendel worked with the garden pea.
● He could control pollination and fertilization—and be sure of the parents of
offspring.
● Mendel worked with the garden pea, which has both male and female sex
organs and normally self-fertilizes.
● Mendel could control pollination and fertilization by removing the male organs
and manually pollinating the flowers.
Why the Garden Pea?
● Rapid growth; produces a lot of seeds; requires little space.
● Naturally self-pollinating, but pollination and fertilization can be controlled
(done by Mendel) to be sure of offspring and parents.
● Has distinct characteristics: observable physical features (e.g., flower color) and
traits: form of a character (e.g., purple flowers or white flowers, wrinkled or
smooth seeds). Heritable traits are passed from parents to offspring.
Traits Chosen by Mendel
● Mendel looked for well-defined, true-breeding traits—the observed trait is the
only one present for many generations.
● True-breeding strains were isolated by inbreeding and selection.
● He concentrated on seven characteristics.
Mendel’s Terminology
● Pollen from one parent was transferred to the pistil of the other parent.
Parental generation = P.
● Resulting offspring = first filial generation or F1.
● If F1 plants self-pollinate, produces second filial generation or F2.
● He first crossed plants differing in just one trait; the F1 generations are called
monohybrids.
, ● The monohybrids were then allowed to self-pollinate to form the F2 generation:
a monohybrid cross.
● Mendel repeated this for all seven characters.
Mendel’s Methods --- a Monohybrid Experiment
● One trait of each pair disappeared in the F1 generation and reappeared in the
F2—these traits are recessive.
● The trait that appears in the F1 is the dominant trait.
● The ratio of dominant to recessive in the F2 was about 3:1
● Mendel proposed:
○ that heritable units were
discrete particles—the
particulate theory.
○ each plant has two particles
for each character, one from
each parent (we call this
diploid).
○ During gamete formation only
one of these paired units is
given to the gamete (we call
this haploid)
● The true-breeding plants in the P
generation had two identical copies
of the particle (gene) for each
character.
● Example: Spherical SS; wrinkled ss
gametes from SS will have one S
gametes from ss will have one s
offspring (F1) will be Ss
○ S is dominant; s is not
expressed in F1.
Terminology
● Alleles: different forms of a gene
● Homozygous: an individual having
two copies of the same allele of a
gene in the genome. These
individuals are true-breeding. (SS ss )
● Heterozygous: An individual carrying
two different alleles of the same
gene. (Ss)
● Dominant allele: an allele that is preferentially expressed in an organism that
also carries another allele of the same gene.
● Recessive allele: an allele that is not expressed or weakly expressed.
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