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.

Wednesday, October 6, 2010

Probability by Punnett

Sir Reginald Punnett was born at the turn of the 20th century in England.  While in medical school at Cambridge University he became interested in research and starting working on reproducing Mendel's experiments with Bateson a professor at the college.

He noted the need for a mathematical tool to predict phenotype and turned to his mathematician cricket partner to help devise a tool.  Thus the Punnett square was born!
The picture above shows Reginald Punnett (left) with his professor and then colleague William Bateson.  As their experiments progressed, they soon realized the vast body of research that lay before them and the rest of the scientific community.  The started the first genetics department at Cambridge University.

Oh Punnett squares!  How we love thee!

Punnett squares allow scientists to predict how traits will be passed along in generations of the same species.
As you can see above, in a monohybrid cross (only focusing on 1 trait) you place the possible alleles (form of gene; like for height: tall and short) on each side being sure to keep the alleles for each parent on separate sides.

Then you simulate all the possible combinations of gametes by writing the allele donated from each parent in the box.  Use the arrows above to review how to place each allele in the appropriate box.

The resulting genotypes are the probable genetic combinations for the offspring of the parents.  Punnett squares more so prove what is not possible.

In the square above, all genotypes are possible because each parent has a copy of both dominant and recessive alleles making all combinations doable.

In addition to using your flashcards we made today, visit the link below to experiment with the Punnett square calculator.  It not only shows a square for monohybrid crosses, but also allows you to view dihybrid and trihybrid crosses!

Punnett Square Calculator

As we learn more types of inheritance, you will find that while phenotype possibilities expand, Punnett square set up really only comes down to the 6 scenarios you translated into flashcards.

Get to practicing your Punnett squares my lovelies!

Mendel and His Peas

Pictured above is the father of modern genetics: Gregor Johann Mendel.  He was born in 1822 in what is today the Czech Republic.  He was the son of a farmer that due to financial issues joined a monastery so that his pursuit of math and science would be paid for in full.

Interestingly enough he wanted to be a teacher but after several failed attempts to pass his teaching certification exam, he resorted to a life a research and eventually became head of the monastery.

Over a 7 year course of time (1860s), he bred around 30,000 pea plants keeping detailed observations of their lineage while he tracked traits such as flower color and plant height. 

Unfortunately, his attempts to publish his works were not received well and it wasn't until he died that his experiments were recognized for their attention to detail and proof of genetic inheritance.

Review Mendel's pea plant experiments by following the hyperlink and go through the animation.

His laws are the basis for current genetic theory and research:

1. Law of segregation
    Think back to meiosis.  Remember that of each pair of chromosomes (DNA) separates in anaphase II so  
    each gamete only has 1/2 of alleles.  For example, if your normal body cells have a dominant and  
    recessive allele for a particular trait, then each of your gametes will have either a dominant or a recessive 
    allele for that trait because the pairs separate in the final stages of meiosis.

     Click this link for a review of meiosis and be sure to follow the chromatids on their journey to see how 
     the pair of gametes are split from their homolog in the final stages.

2. Law of Independent Assortment  (Click the title for a review animation)
     Genes located on separate chromosomes are inherited without relying on one another.  Much like 2 cars 
     driving down the highway are able to make turns without prompting the other vehicle to follow suit.

Mendel put into works what may have taken scientists far longer to have discovered in present day.  Read the next blog to hear about Sir Reginald Punnett and his miraculous tool to predict how offspring will inherit their traits.

Cell Division Test Images