Download Quick Review of Genetics

So what is genetics anyway?
Simply put, genetics is the study of heredity. Heredity is the passing on of
characteristics from parents to their children. Therefore, genetics is the study of
the process in which characteristics are passed along through the generations.
The basic foundation of genetics was made by the monk Gregor Mendel in the
19th century, and since then, the study of genetics has exploded. Mendel began
his experimentation by mating pea plants; we are now to the point where we
manipulate genes and can clone an animal or even a human.
Mendel, the Father of Genetics, and his Experiments.
Gregor Mendel (1822-84) was an Austrian monk. His experiments are the basis
for inheritance. In his experiments he used garden pea plants (Piscum Sativum).
Why Pea Plants?
First: this plant has many contrasting traits that Mendel could easily study
Second: pea plants can easily be cross pollinated
Third: a relative large number of seeds are produced
Mendel’s Assumptions
Mendel assumed that every trait an organism contains is written down in some
material factors. These factors are transferred during the process of
reproduction.
Mendel’s Experiment: http://library.thinkquest.org/18258/noframes/mendelsteps.htm
Mendel’s Observations: http://library.thinkquest.org/18258/noframes/mendelobs.htm
Mendel's Conclusions are Known as Mendel's Laws
#1
Mendel's first law of genetics - the law of segregation:
During the formation of gametes (sex cells) in the process of
meiosis, a pair of genes located on
homologous
chromosomes, which are tightly coiled strands of genes located in
the nucleus of every eukaryotic cell. Each chromosome has a
fixed number of genes, and every species has a characteristic
number of chromosome pairs - humans have 23 pairs, mice have
19 and pea plants have 7. During the formation of gametes, these
homologous chromosomes are separated from each other, which is
what Mendel's law of segregation states.
#2
Mendel's second law of genetics - the law of independent
assortment:
When gametes are formed the alleles (one of the possible forms of
a gene) for one trait segregate independently of the alleles of a
gene for another trait.
Are all of Mendel’s assumptions and observations correct?
No, they are not. Scholars have some restrictions:
Incomplete Dominance
Incomplete dominance is basically just a blending of traits—the heterozygous
form is a blend of the homozygous dominant and the homozygous recessive
forms.
Codominance
Codominance is an interesting case where there is no real dominant or recessive
allele. Instead, both alleles are fully expressed in the heterozygous form (the
homozygous forms act normally).
A good example of codominance is a person's blood type. A person with blood
type AB is the result of having both the IA and IB codominant genes. The AB
blood type expresses the characteristics of both blood types A and B; therefore,
the alleles for blood type must be codominant.
Multiple Alleles
An allele is a possible form of a gene. Normally, for a given gene, there are two
alleles: the dominant and the recessive. However, with multiple alleles, there
can be more than two forms of the allele: there can be 3, 4, 5, or even more
different forms of the gene!
The crucial thing to understand about multiple alleles is that you still will only
have two of these alleles. A good demonstration for this is the good ol' blood
type again. There are 3 different alleles for blood: IA (Type A), IB (Type B), and
IO (Type O). You can only have two of these three alleles, and they combine to
form interesting genotypes that result from codominance. Below is a table of the
different genotypes of blood, along with the corresponding phenotypes.
Punnett Squares
Ever been curious of what determines the color of fruit flies? How about why
baldness is almost exclusively a male affliction? Can't figure out why some
flowers in a patch are tall, and others short, yet all are from the same species?
Utilizing Punnett squares can make these seemingly elusive facts very apparent.
Why not Punnett Circles?
A Punnett square is a mathematical device used by geneticists to show
combinations of gametes and to predict offspring ratios.
Before jumping right into Punnett squares, there are a few fundamental concepts
that must be understood for them to predict correctly.
1) Each trait is controlled by two genes
2) Due to the binary nature, genes can be represented by letters
3) An uppercase letter denotes the wild type (usual, dominant) form of the gene
4) A lowercase letter denotes the mutant (unusual, recessive) form of the gene
5) In monohybrid crosses (Punnett square configurations) there are three distinct
combinations; e.g. TT,Tt, and tt
6) If a dominant gene is paired with a recessive gene, the dominant's trait will take
effect
To set up a Punnett square, draw a large square, and then divide it into 4 equal sections
(also squares).
Time to Talk About "The Birds and the Bees"
Meiosis is a very important process that creates the cells necessary for sex: egg and sperm
cells. These cells used for reproduction are called gametes. If a normal cell was used for
reproductive purposes, we would have lots of problems! Picture this: A normal human has 46
chromosomes. If a father and mother each contribute a cell that contains 46 chromosomes to
their offspring, this child would have 92 chromosomes, and the next generation would have a
whopping 184 chromosomes! Meiosis solves this problem: it creates cells that have half the
regular number of chromosomes. These haploid cells only contain one set of chromosomes, as
compared to regular body cells that are diploid, containing two sets of chromosomes. Meiosis is
divided into two stages of division: Meiosis I and Meiosis II. Before meiosis actually starts, a
diploid cell goes through interphase (the growing phase of the cell cycle), forming doublestranded chromosomes.
Prophase I
During prophase I, the nuclear membrane that surrounds the nucleus of the cell disappears, the
chromosomes become visible, and the centrioles move to opposite poles of the nucleus. Then,
the chromosomes line up side-by-side with their counterparts. This process of the chromosomes
pairing up is called synapsis.
Metaphase I
Now the chromosome pairs (tetrads) line up along the center of the cell. These pairs of
chromosomes are what we use in karyotyping.
Anaphase I
Here each chromosome within the tetrad separates and moves to opposite poles of the cell,
towards the centrioles. The chromosomes do not separate at the centromere.
Telophase I
During telophase I, the cells begin to split as a nuclear membrane forms around each set of
chromosomes. The cells then split in two by the process of cytokinesis. We now have two
diploid daughter cells, each of which enters meiosis II.
Meiosis II
During the second division of meiosis, both daughter cells divide, finally resulting in four gametes.
During prophase II, the chromosomes condense and become visible. Next in metaphase II, the
chromosomes move towards the center of each cell, lining up in single file instead of in pairs.
Then, in anaphase II, the chromatids of each chromosome split at the centromere and move
towards each end of the cells. Finally, in telophase II, a nuclear membrane forms around each
set of chromosomes, forming four haploid cells.
How Can I Remember All This?
No problem... here's a simple way to remember the order of the stages of meiosis: I've Probably
Mixed All This Up!