Transcript Lecture: Mendelian Genetics

CELLULAR BIOLOGY &
MENDELIAN GENETICS
Cellular Biology
THE CELL



Smallest functional unit of living
organisms
Each cell maintains its own
homeostasis
Each cell contributes to the
homeostasis of the tissue, organ
and/or organism
THE CELL

All living organisms consist of cells

Humans have trillions of cells (Yeast one cell)


Cells are of many different types (blood,
skin, nerve), but all arose from a single
cell (the fertilized egg)
Each cell contains a complete copy of
the genome (the program for making
the organism), encoded in DNA
SOME DEFINITIONS
Chromosomes = made up of a protein core and strands of
DNA in the nucleus of a cell (46 chromosomes make up 1
human cell)
DNA (deoxyribonucleic acid) = Molecule that carries the
genetic code, ladder with rungs made of base pairs
(“letters”: A,C, T, G)
Codon = Sequence of 3 bases. Each 3-letter word stands for
an amino acid. (“word”: “GCA” = Alanine)
Genes = Portions of the DNA molecule that code for specific
proteins (“sentence”)
Protein = chain of amino acids (see p. 54 for protein
synthesis)
Replication = Copying of genetic code during cell division
The Question of Inheritance
Darwin was very concerned with the question:
How are traits inherited?
•
Variation within populations was the raw material
that powered the evolutionary process
•
Could not explain, however, the source of this
variation or how it was distributed across
generations
•
Towards the end of his life, Darwin admitted that
the greatest challenge to his theory was its
inability to explain the mechanism of inheritance
Theories of Inheritance
During Darwin’s Time
Blending Inheritance
•
Belief of how inheritance operated: offspring
received a combination of all characteristics of
each parent through the mixture of their
“bloods”
•
If true, unique traits would be diluted, and…
•
Individuals would become more uniform
If not, how are individual adaptive traits maintained
and selected for within populations?
Czech monk, Gregor Mendel, working in obscurity,
came up with the answer: particulate inheritance
Gregor Johann Mendel (1822 - 1884)
•
1865: Mendel published the results of his
experiments with garden peas in which he
crossed parents and examined offspring,
became the “father” of genetics.
•
Concluded that each parent contributes
“particles” or genetic units to their
offspring
•
The particulate nature of inheritance and
the laws of inheritance = final pieces in the
puzzle that fulfilled Darwin’s vision of
evolution by natural selection
•
Mendel’s contribution DID NOT become
known until early in the 20th century
Some Definitions
Allele = Variants of a gene. Every gene
has 2 alleles with different instructions
for developing a certain phenotype
Monogenic = Trait coded for by a single
gene (ex. Taster trait for “PTC”)
Polygenic = Trait coded for by more than
one gene (ex. Skin color)
Characteristics of Good
Model Organisms
Mendel’s work with garden peas was successful
because they had:
•
•
•
•
•
a well-known genetic history
short generation time
large number of progeny
small size and easy to handle
wide availability of mutants - genetic
variation within garden peas
Mendel had true-breeding varieties of peas for 7
different traits including:
One of Mendel’s Monohybrid Crosses
Gametes =Cells of sexual
reproduction (sperm & egg),
contain only ½ the
chromosomes of normal cell
Genotype: alleles of an
organism
1/4 SS : 1/2 Ss : 1/4 ss =
1 SS : 2 Ss : 1 ss
Phenotype: physical
expression of genetic code
3/4 Smooth seeds :
1/4 wrinkled seeds
Mendel’s Model
• Genes do not blend
together
• Pea genes have two
versions or alleles
• Each gamete gets one allele
• Males & females contribute
equally
• Some alleles are dominant,
while others are recessive
What Accounts for
Species Variation?
• Independent Assortment: Alleles
segregate into gametes independently
(Mendel’s Law)
• Crossing Over: Sections of chromosomes
switch between chromosome pairs during
meiosis
• Recombination: Exchange of genetic
material between pairs of chromosomes
during meiosis