Genetics & Heredity EDFN 645 Reading Notes October 1 st 2008

Definitions  Genotype- constant, inherited, genetic endowment  Phenotype- observable, measurable features, characteristics & behaviors

Genetic Influence Gene- nucleotide (chemical building blocks like adenine, thymine, etc.) within a chromosome that codes enzyme production influencing trait transmission. Allele- alternate form of a gene influencing individual difference Gametes- sex cells (egg & sperm carry only ½ of a pair of genes Chromosome- DNA in nucleus of cell forming a collection of genes Human genome- gene pairs making up all recessive and dominant trait inheritance in an individual Summary:

Alleles The genes on both chromosomes have the exact same function: to determine this person's eye color. However, there are two different alleles present. The allele on the chromosome on the left codes for brown eyes. The allele on the chromosome on the right codes for blue eyes. There is only “one gene” (the gene for eye color) present, but there are two different alleles. With every gene pair, there will be two alleles. Those alleles may be identical (e.g. both for brown eyes), or they may be different, as in this example.

Dominant & Recessive Genes Dominant and Recessive Genes In the example above, there are two alleles for eye color. One allele tells the body that the eyes should be brown, and the other allele tells the body that the eyes should be blue. Which allele does the body pay attention to? The answer to that question depends on which allele is dominant. A dominant allele is one which is expressed even if only one copy is present; it “dominates” its partner allele. A recessive allele, on the other hand, requires two copies to be present in order to be expressed. A recessive allele is not expressed if it is partnered with a dominant allele. In our example, the allele for brown eyes is dominant, and the allele for blue eyes is recessive. The dominant allele, the one for brown eyes, is the one that will be expressed; the individual will have brown eyes. A person would also have brown eyes if the two alleles were identical and both coded for brown eyes. For this person to have blue eyes, both of the alleles would have to code for blue eyes.

Homozygous vs. Heterozygous In the example of eye color above, the two alleles for the gene are different from one another. One is for brown eyes, the other is for blue. When a person has two different alleles for a trait, it is said that they are heterozygous for that trait. Heterozygous simply means having two non-identical alleles at a given locus (i.e. location on a chromosome). If a person has two identical alleles for a particular trait, that person is said to be homozygous. A person can be homozygous dominant or homozygous recessive. A person who is homozygous dominant for a certain trait has two dominant alleles, like two alleles for brown eyes. A person who is homozygous recessive for a certain trait has two recessive alleles, like two alleles for blue eyes.

Chromosome Duplication Mitosis: Cell division taking place in most cells of the human body and resulting in identical material in each of 46 chromosomes per cell Meiosis: cell division forming gametes = 23 chromosomes in each egg and sperm cell

Chromosomal Abnormalities  What are chromosome abnormalities?  A chromosome abnormality reflects an abnormality of chromosome number or structure. There are many types of chromosome abnormalities. However, they can be organized into two basic groups:  Numerical Abnormalities:  When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome is an example of monosomy, where the individual - in this case a female - is born with only one sex chromosome, an X.  Structural Abnormalities:  When the chromosome's structure is altered. This can take several forms:  Deletions: A portion of the chromosome is missing or deleted.  Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.  Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.  Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.  Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.  Most chromosome abnormalities occur as an accident in the egg or sperm. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism, where some cells have the abnormality and some do not.  Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why chromosome studies are often performed on parents when a child is found to have an abnormality.

Chromosomal Abnormalities continued  Structural Abnormalities:  When the chromosome's structure is altered. This can take several forms:  Deletions: A portion of the chromosome is missing or deleted.  Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.  Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.  Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.  Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.  Most chromosome abnormalities occur as an accident in the egg or sperm. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism, where some cells have the abnormality and some do not.  Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why chromosome studies are often performed on parents when a child is found to have an abnormality.