4.3 Theoretical Genetics

Genotype: the allele combination that expresses a specific characteristic
Phenotype: The observable characteristics as a result of the genotype (physical description of the genotype)
Dominant Allele: The characteristic of this allele is always present whether it is homozygous or heterozygous
Recessive Allele: The characterisitc of this allele is only portrayed when it is homozygous.
Codominant Allele: Alleles that share the dominance so that both of the characteristics are present
Incomplete dominance: neither allele is entirely expressed resulting in a mixture of the two traits. (A red allele and a white allele that are have incomplete dominance can result in a pink phenotype)
Locus: The position of a gene on a homologous chromosome
Homozygous: two identical alleles of a gene
Heterozygous: two different alleles of a gene
Carrier: Genotype consists of only one recessive allele that causes a genetic disease. Offspring can develop the genetic disease when possessing alleles that are homozygous.
Test Cross: Crossing an organism with a dominant genotype with a known homozygous recessive

Punnet Grid:
A genetic cross is often used to determine the genotype of the offspring using the known genotypes of the parents. A monohybrid cross can be used to find out what the genotype, for one specific gene, will look like.
A punnet grid is used to predict monohybrid crosses

1. Designate one character that defines the alleles using a capital letter for the dmominant allele and the lower case version of the character as the recessive allele
2. Determine the genotype and phenotype of the parent generations (P generation)
3. Determine the genotype of the parental gametes as a result of meiosis
4. Use the punnet square to determine the potential combinations.  
5. Record the phenotype and genotype ratios of the offspring (F1 generation, first generation)

Some genes have multiple alleles such as blood type which possess three alleles. Alleles can be codominant, show incomplete dominance, or have . a dominance order (eg. allele A > Allele B > Allele C)

The alleles that determine blood type show codominance and multiple alleles. Codominace is usually written with an identical character to determine the recessive and dominant alleles and other superscript characters to show the codominant alleles.

     For blood types:

           I represents immunoglobin, a protein in blood

           A and B are used to name the co-dominant variables

There are three alleles in the blood typing gene.: I^A, I^B and i. The I^A  and I^B alleles are co-dominant and the i allele is recessive. 

Out of the 23 pairs of chromosomes that each human possess, 22 of them are autosomal; that is they are not sex chromosomes. The remaining pair will determine the gender. Females will have two XX chromosomes and males will have a X chromosome and a shorter Y chromosome. The genes for the male characteristics are found in the Y chromosome that can only be passed on by the father. If the spermatozyes contain the X chromosome then the offspring will be a girl, if it's a Y chromosome then the offspring will be a boy. The mother will always pass on a X chromosome.

The different sizes of the two chromosomes will prevent any crossing over or recombination from occuring during meiosis so that there will always be a 50% chance of a boy and a girl. The size differentials also means that the chromosomes are not homozygous and that genes will be absent on the shorter Y chromosome.  The Y chromosomes only contains a few genes such as the SRY sex determination gene and the genes for haemophilia and red-green color blindness are not present on the Y chromosome. 

In human females, only one X chromosome is active while the other is condensed as heterochromatin  that forms a condensed Barr body. 

Sex Linkage:

Sex linked genes refers to characteristics that are found on the sex chromosomes. These are usually found on the X chromosomes as the Y chromosome carries very few genes. Since males only have one X chromosome they are known as hemizygous, while females can still be heterozygous or homozygous because of the presence of two X chromosomes. 

Some recessive diseases that are found on the X chromosomes will be displayed in males because the gene loci is only present on the X chromosome and non-existant on the Y chromosomes. Therefore, there will not be any gene on the Y chromosomes of males. Any allele found on the X chromosome without a pair on the Y chromosome will be displayed whether it is dominant or recessive. Hemophilia and color blindness are examples of diseases only found on the X chromosome. Since males can only get this X chromosome from their mother then the mother must have been a carrier for the disease, if the mother was heterozygous for this trait, or have displayed the trait if the mother was homozygous for the trait. 

H - hemophilia negative, h - hemophilia positive

Carriers are individuals that possess a recessive allele for a disease that is covered by a healthy dominant allele.These heterozygous individuals (females) will not experience the condition but have a chance of passing on the alleles to the next generation. Males cannot be carriers for sex linked genes because the disease will be shown, but they can still pass on the trait to a daughter.