BIOL 1500 Lecture Notes - Pea, Gregor Mendel, Mendelian Inheritance

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BIOL 1500
June 12, 2012
Single gene trait
Traits that are determined by the instructions a person carries on one gene.
There are more than 9000 human traits that exhibit straightforward patterns of
Gregor Mendel conducted studies to help us understand how traits are inherited.
1) Chose garden pea to studypea plants are relatively easy to fertilize and
produce hundreds of offspring from a single cross and could conduct enough
experiments that lasted multiple generations
2) Chose easily observable and unambiguously identifiable traits (flower colour, pea
shape and pea colour)
3) Repeatedly bred similar plants until he had many populations, unvarying for a
particular traittrue breeding
a. True breeding round pea plants always produced plants with round peas
when crossed.
b. True breeding purple-flowered pea plants always produced purple-
flowered offspring
4) Cross breeding true breeding purple-flowered plants with true breeding white-
flowered plants produce ALL purple offspring.
5) Then two of the purple-flower offspring (no longer true breeding) were crossed
and produced mostly purple flowers but some white flowers.
a. Therefore the dominant trait would be the purple-flower trait
b. Therefore the recessive trait would be the white-flower trait
Generally speaking, a dominant trait masks the effect of a recessive trait when an
individual carries both the dominant and the recessive version of the instructions for the
The existence of traits could disappear for a generation and then show up again down
the line.
Mendel’s hypothesis for explaining inheritance incorporated three ideas to make
predictions about crosses he hadn’t done yet:
1) Each parent puts into every sperm or egg it makes a single set of instructions for
building the trait (a gene)
2) Offspring receives two copies of the instructions for any trait
a. Sometimes each parent contributes a slightly different allele for the trait.
The pea plants would have two alleles for flower colours, purple and white.
3) The trait observed depends on the two copies of the gene it inherits from its
a. When an individual inherits the same two alleles for this gene, the
genotype for the gene is homozygous and the individual shows the trait
specified by instructions embodied in those alleles
BIOL 1500
June 12, 2012
b. When an individual inherits a different allele from each parent, the
individual’s genotype for that gene is said to be heterozygous
Dominant and recessive alleles are defined by their action why they are in a
heterozygous state
Explain the concept of a single-gene trait
Discuss Mendel’s contributions to the field of genetics
Define the terms gene, allele, dominant, recessive, homozygous, and heterozygous
Describe the difference between an organism’s genotype and its phenotype. Discuss
how phenotypes are determined.
Demonstrate the ability to perform Punnett squares to predict the offspring of parents
with particular genotypes
Use the rules of probability to predict the inheritance of specific traits
Explain how a test-cross can be used to determine the genotype of an organism
Be able to predict patterns of inheritance based on pedigree analysis
Explain how each of the following genetic “rules” works: incomplete dominance,
codominance, multiple alleles, polygenic inheritance, and pleitropy
Describe how sex-linked traits are inherited
Explain why linked genes do not assort independently
Discuss how evolution can be observed in various populations
Describe Charles Darwin’s impact on evolution and the study of biology
Describe Darwin’s most important observations
Explain the four ways evolutionary change can take place
Identify the difference between evolution and natural selection
Explain the five different lines of evidence for the occurrence of evolution
Describe ways evolution can be observed today