Practice problems in Genetics and the topic of Sex Linkage

Here's a worksheet for Monohybrid crosses : http://www.biologycorner.com/worksheets/genetics_practice.html
And here is its answer key: http://www.biologycorner.com/worksheets/genetics_practice_key.html
try some sex linkage questions here: http://www.k-state.edu/biology/pob/genetics/intro.htm

SEX LINKED GENES

The sex chromosomes are XX and XY in humans. Some alleles are located on the X chromosome. But very few on the Y chromosome.  Remember that females have the genotype XX and males have XY.

The Y chromosome carries very little information, only enough to influence the embryo on the path towards male primary and secondary characteristics.

Meanwhile, many RECESSIVE ALLELES are located on the X chromosome, including
1. hemophilia, the inability to clot blood
2. colourblindness inability to tell the difference between red and green colour
3.  baldness

These characteristics are far more common in men than women because men have only one X. And if that X contains the recessive allele, they will show the phenotype.  Meanwhile, women may have the recessive and their "normal" dominant X will protect them.  For example

An example of using a punnet square to calculate probability is here:

Some things to note about sex linkage:  Signs of sex linked alleles are:
1. more males get the phenotype, but women can be carriers
2.  men can inherit from mother but not father (because men get their X from mom)
3.  Women have an extra X and this protects them.  Women must be homozygous to show the sex linked phenotype.
Further reading on sex linkage here http://learn.genetics.utah.edu/content/pigeons/sexlinkage/

FYI if you are a guy and you want to know if you're colourblind:  Test yourself here: Apparently most men find out they are colourblind from their science teacher...
http://colorvisiontesting.com/home.html

codominant alleles: Blood type

What if the heterozygous trait exhibited codominance?  in this case, two homologous alleles can code for co-dominant traits.  That is neither is dominant over the other.  Blood type is an example of this:
A is codominant to B
A is dominant over O
B is dominant over O

Possible genotypes:

heterozygous AB blood gives both type A and B on the erythrocytes
homozygous AA gives blood type A  protein on the erythrocytes
homozygous BB is blood type B protein on the erythrocytes
Heterozygous AO is gives blood type A
Heterozygous BO is blood type B

homozygous recessive OO gives blood type O or neither A or B on erythrocytes.

You have heard that blood type is important for blood transfusions. That is because the immune system will always attack an unknown protein.  Thus

type A person can receive  type A blood, and will reject type B blood
type B person can receive type B blood and will reject type A
type AB person can receive type A blood and Type B blood
Type O person can receive nobody's blood except from another type O

Everyone can receive type O, No-one can receive type AB

Blood type is heritable and that means you can sometimes deduce genotype by looking at phenotype. Here is a sample question:

Dominant, Recessive alleles and using Punnet Squares.

Alleles come in pairs and a GENOTYPE IS a pair of alleles which code for a PHENOTYPE.  Phenotype refers to the observed characteristic.
For example:

B= brown eyed allele   and    b = blue eyed allele
Genotype   Bb will give the PHENOTYPE of brown eyes because B is dominant over b.

GENOTYPE BB is homozygous dominant = brown eyes
GENOTYPE Bb is heterozygous = brown eyes
bb is homozygous recessive = blue eyes.

This is a case of COMPLETE DOMINANCE and it results in grandparents passing traits to grandchildren:

If a BB person mates with a bb person, we can calculate probability of their offspring traits using a punnet square:

When Baby grows up, she mates with this guy who happens to have the same GENOTYPE!

Note that a recessive allele can be hidden inside a HETEROZYGOUS genotype.

Introduction to Genetics

Gregor Mendel, monk, was the father of Genetics.

Last day we had an introduction to Genetics and we defined these terms:
Gene
Allele
Chromatin, Chromatid, Chromosome
Homologous pair of Chromosomes
diploid number vs haploid number or 2n vs n
Human diploid number = 46 chromosomes or 23 pairs, here they are below for a male human

Courtesy National Human Genome Institute http://www.genome.gov/glossary/resources/karyotype.pdf

Each of the HOMOLOGOUS CHROMOSOMES above  can contain many pairs of alleles.  To understand this, we considered an analogy of shoes...

A review of Gene, Chromatin, Chromosome and inheritance

INHERITANCE is the ability to pass on traits from one generation to another.
The first experiments on inheritance was from farmers.  They made PUREBRED and HYBRID organisms to help them control the breeding of useful traits.  Every population has variation and by selecting certain traits to breed, one can get a purebred characteristic.  For example.  Suppose there is a population of bunnies:  white, brown, black, grey.  If you wanted to get only grey baby bunnies, you can SELECTIVELY BREED the grey ones.

The first experiments to look at the LAWS OF INHERITANCE was conducted by GREGOR MENDEL, a monk.  He called purebreds HOMOZYGOUS.  He called hybrids HETEROZYGOUS.

A Gene is a section of DNA which codes for a  protein. Note how it looks in Chromatin form and also Chromatid form.

A chromosome usually refers to a  duplicated chromatid.  It is a double copy of the DNA for the purpose of asexual or sexual reproduction.  Chromosomes become visible only during reproduction.

Chromatids are HOMOLOGOUS, that is, they come in pairs. In Eukaryotes, a PAIR OF GENES located on a PAIR OF CHROMATIDS  codes for one trait.

quiz next day: define the following words and download some summary notes

Adaptation
Fitness
Natural selection -
Transcription -
Translation -
mRNA
tRNA
rRNA -

CODON -
Anti CODON -

Promoter -

What are two forms of protection for the mRNA? -
What are the two parts of a ribosome -
How does the ribosome attach the amino acids? -
What is the start CODON and what amino acid does it code for? -
What are the stop CODONS?

If the DNA is this?  
Then what is the mRNA and what is the amino acid?

SUMMARY NOTES:
DNA and Protein Synthesis, a summary of DNA replication (a review), and the Central Dogma of DNA, RNA and protein synthesis
DNA and Protein Synthesis worksheet
Fun converting DNA to RNA and RNA to Amino Acids

our diagrams, note the correction in red