Monday, January 24, 2011

Gel Electrophoresis....For What!?!

Another way our gene code can be useful takes advantage of the sequences of our nucleotides.


Nucleotides are composed of 3 parts: a phosphate group, a 5 carbon sugar, and a nitrogenous base.

In DNA, there are 4 nitrogenous bases (Adenine, Thymine, Guanine, and Cytosine).

In RNA there are similar bases, but one is different: (Adenine, URACIL, Guanine, and Cytosine).
DNA is double stranded meaning that there are 2 nucleotides connected by their nitrogenous bases that when combined in succession make a double helix.  This shape almost looks like a ladder.  The rungs would be the nitrogenous bases and the outside would be the phosphate group and the 5 carbon sugar.

Monday, October 18, 2010

Karyotypes!! Say what!

Karyotype practice and vocabulary

The link above will take you to the second website we visited in class today.  There you can find practice terms and revisit how to make and read a karyotype.

The karyotype above is a normal male.  A technician preparing a karyotype would first arrange the chromosomes by length from longest to shortest.  Then they would match up pairs using banding patterns and centromere placement.

Just think of the way you would match up your socks when folding laundry.  You wouldn't pair the ankle sock with a tube sock.  Nor would you match a pink ankle sock with a white one.  Socks are matched by length and pattern just like chromosomes are matched by length and banding patterns.

Once the karyotype is arranged, then we make sure all the chromosome are normal:
   1. Correct length
   2. Correct banding pattern
   3. Correct # of chromosomes in each pair (none missing and no extra)
If any of these characteristics aren't met, then they seek to diagnose the disorder.

For example, above is a karyotype from a human breast cancer cell.  In a normal cell, there are 46 total chromosomes.  As you can see, the cell division process has been altered and the chromosomes are duplicating without any checks and balances.

In the above karyotype, this female is missing a whole piece of her chromosome.  So even though there are 46 chromosomes present, there is still a disorder because information is missing.

This patient has monosomy 7 because there is only 1 chromosome at pair 7.

Basically it boils down to Goldilocks again!  Too much information = problem.  Too little information = problem.

When meiosis goes wrong, the zygote's chromosomal make-up will suffer!

Sex-Linked Traits

In addition to determining the sex of a child, our sex chromosomes have traits on them that code for a variety of characteristics.  If there are recessive alleles at some of these traits, then the offspring can express those phenotypes.

Because females and males differ in their sex chromosomes, they also differ in the type of genes found on these chromosomes and the frequency at which they are expressed.

First things first, females have 2 X chromosomes (called X because during meiosis and mitosis they look like x's)  and males have XY (the Y gets its name from its shortened appearance in comparison to the X during cell division).

Every time a sperm and egg combine to form a zygote, there are the same 50/50 chance of having a boy or a girl.

To notate that a trait is found on a chromosome, we place our trusty upper and lowercase letters as superscripts to the right of the X or Y (it depends on whether the trait is X or Y linked as to where the letter will be placed).

Some other common sex-linked traits: color blindness and male pattern baldness.

Multiple Alleles

When there are more than 2 choices of allele at one gene locus, there are more phenotypes possible.  This pattern of inheritance is known as multiple alleles because where we have only seen 2 options of alleles (dominant and recessive or 2 different dominants), now we have more!

Human blood type is the classic example of multiple alleles that we studied in class.  There are 2 dominant alleles and 1 recessive making for 6 possible genotypes and 4 possible phenotypes.

Codominance - Roan Cows and Erminette Chickens

In some traits, there are dominant alleles that fight for expression.  Because they are both dominant, neither one wants to give up expression -- there will be NO blending of phenotype here.

Here are some classic examples!


These cows have possibilites of several colors of hairs that aren't a blend of the dominant and recessive phenotypes because the traits for each phenotype are dominant.
RR = red cow hairs
RW = red and white cow hairs
WW = white cow hairs

These chickens have feathers that are both white and black, but not grey.

BB = black feathers
BW = black and white feathers
WW = white feathers

The alleles for A and B are codominant so that if both are in the genetic code then they are expressed as blood type AB.

Wednesday, October 13, 2010


Be sure to check out the following link to practice pedigrees.

Make sure you open it in Internet Explorer (Mozilla Firefox doesn't work)!

Drag and Drop Pedigree Practice

Monday, October 11, 2010

Incomplete Dominance

So far according to Mendel's rules.....

Genes have two options (aka alleles): dominant and recessive.

Dominant traits always outweigh or mask the recessive traits.

Phenotype (outward appearance) is only dominant or recessive.


As we have pursued genetics as a science further, we quickly realized that not all traits are inherited in such a simple pattern.

Patterns that follow a similar pattern as the one Mendel put in place, but have some special circumstances are known as Non-mendelian patterns of inheritance.

This blog entry will focus on the pattern known as incomplete dominance.
In the picture above you should see 3 different colored snapdragon flowers: red, pink, and white.  The color of this flower is determined by a similar set of alleles as simple dominance, but the way in which those genes expressed differs.

In Punnett squares before we saw fewer phenotypes than genotypes because Aa and AA would reflect the same dominant phenotype.

In snapdragons, inheritance is expressed with a slight twist.  Homozygous dominant flowers are red (RR) and homozygous recessive flowers are white (rr), but the heterozygote (Rr) is a blend of dominant and recessive or in this case pink.