Genetics and Inheritance

Traits that determine and our physical and some of our behavioral make-up are inherited on the chromosomes we receive from our parents.

A chromosome is composed of many genes and not all genes are alike.  We receive 23 homologous pairs of chromosomes from our parents.  On each chromosome in the pair is gene that codes for a particular trait, however, they may not code for the same version of the trait.  Different versions of a gene are called alleles. 

For example, let’s say that on pair number 1 there is a gene that codes for eye color.  However, on one of the chromosomes, the gene calls for brown eyes, and on the other chromosome, the gene calls for blue eyes.  These are two different alleles.

When two different alleles are inherited, one may be dominant over the other.  These dominant alleles hide the presence of the other allele.  The allele that is hidden is called the recessive allele.  The variation expressed by the recessive allele will only be seen if both of the inherited alleles are recessive.

When we discuss an organism’s genetic make-up, we refer to it as the genotype of that individual.  When we discuss that organism’s physical appearance, we are referring to the phenotype for that individual.

Possible alleles for gene that codes for eye color:

B – brown eyes = dominant

b – blue eyes = recessive

Multiple alleles:

The information above shows normal “Mendelian” genetics.  In that case there are only 2 possible alleles for each gene and one is always dominant, and the other is always recessive.

Some genes however have more than 2 alleles.  We can only possibly inherit 2 of them because we only get one chromosome from each parent, but there are many possibilities for alleles we get.

An example of this can be seen when looking at blood type. 

The red blood cells have markers on their cell membrane called glycoproteins.  We may also refer to these as antigens because they can be recognized by the immune system as either belonging to that individual or as a foreign object.

The type of glycoprotein present on an individual’s red blood cells is determined by which alleles of a particular gene that they inherit.

There are 3 possible alleles for blood type: A, B, O.  A and B are dominant alleles and O is a recessive allele.  When an organism gets 2 different alleles that are dominant, their physical trait will be some combination or intermediate of the two traits.  This is referred to as co-dominance.  Review the genotypes and phenotypes given in the chart below.

Another protein found on the surface of red blood cells is the Rh factor.  There are only 2 possible alleles for this.  Rh+ is dominant and the phenotype would be to have the Rh factor on the cell membrane of the red blood cells.  Rh- is recessive and would call for no Rh protein (the absence of Rh) on the cell membrane.

Knowing an individuals blood type is important when that individual needs a blood transfusion.  The blood type must be matched so as not to cause a massive immune response when foreign blood is introduced into the body.  The rejection is because a person's body will produce antibodies (defensive proteins) that will attack any glycoproteins (antigens) that are different from the antigens that person already has.  For example, someone with A+ blood can only donate to someone else with A+ blood but someone with A- blood could donate to someone with A+ or A- blood. There are two special exceptions.  Someone who has O- blood is called a universal donor.  Since there are no antigens present on their red blood cells, they can donate to anyone without causing an immune response in the recipient.  Someone who has AB+ blood type is a universal recipient.  Since their immune system has already been exposed to all possible antigens, they can receive blood from any individual without a massive immune response because they will not produce antibodies.

Sex – linked Traits

Some traits are inherited by genes found on one of the sex chromosomes.  A trait that was coded for by a gene on the Y-chromosome would only be inherited in men and passed to the male offspring by the father since the mother has only x-chromosomes.  Any trait carried on the X-chromosome could be passed to daughters by either parent but the sons could only receive this trait from their mother, since she donates the X to them and the father donates the Y.

In reference to sex-linked traits, males are hemizygous, because they can only receive ONE possible allele (they only have one X or Y).  Females receive 2 X chromosomes and can be homozygous or heterozygous for a trait.  If a female is heterozygous for the trait, she does not show the trait herself but has the ability to pass it to her son.  She is therefore called a carrier.

Examples of sex-linked traits will be discussed in class.

Mutations:

Changes in the genetic material of a cell are referred to as mutations.  Any substance that can cause a mutation is called a mutagenic agent.

Mutations can be good – cause the formation of an allele that makes an organism better fit to survive than the original allele.

Mutations can be bad – cause the formation of an allele that may diminish the survival of the organism.

Mutations can be indifferent – cause the formation of an allele that is no different from the original and therefore does not change the fitness of the organism or its survival.

Mutagenic agents can be anything from toxins in the earth, air, or water, viruses, or radiation received from the sun or an x-ray.

In this lab you will use UV light as a mutagenic agent against bacteria and then look for lethal (death) or non-lethal (pigment change) mutations.