4. GENETICS

4.1 Chromosomes, genes, alleles, and

mutations

Inventory of your traitsTraits are observable characteristics, find out which traits your share with the rest of the class.

First, fill out the inventory for yourself

Second, tally up the results of your group and then add them to the entire class.

Third, draw a bar graph with the results

Homework: make a graph using excel just like we did in class

MeiosisMeiosis: A type of cell division that produces four cells, each with half the number of chromosomes as the parent cell.

Homologous chromosomes: As a consequence of fertilization, humans have pairs of chromosomes with one chromosome in a pair from each parent. The two chromosomes of each matching pair are called homologous.

Diploid: A nucleus with two chromosomes of each type. (2n)

Haploid: A nucleus with only one chromosome of each type. (n)

The process of meiosis

1) Prophase I: Homologous chromosomes pair up and stick together along their length and exchange genetic material in the process called crossing over.

Meiosis I: Four stages. Previously, during interphase, the chromosomes replicate, so that each chromosome has an identical chromatid (2n).

2) Metaphase I: Homologous pairs (tetrads) move to the equator of the cell. Orientation of maternal and paternal on either side is random and independent.

3) Anaphase I: Homologous pairs are separated and migrate to opposite poles of the spindle.

The process of meiosis4) Telophase I and Cytokinesis: Each pole now has a haploid daughter nucleus (eventhough each chromosome consists of two chromatids). No replication occurs. Cytokinesis splits the cell. Nuclear membrane appears and chromosomes uncoil.

Meiosis II: Four stages as well.

1) Prophase II: Chromosomes condense, nuclear envelope disintegrates, and a spindle forms.

2) Metaphase II: The chromosomes line up in the middle of the cell.

The process of meiosis3) Anaphase II: Centromeres separate and sister chromatids are moved to opposite poles.

4) Telophase II and cytokinesis: The chromatids reach the poles, nuclear envelope forms, cytokinesis splits the cell again, producing four haploid daughter cells as a final result.

www.biology.com: Online activity 10. 4

Non-disjunctionIt occurs when homologous chromosomes fail to separate at anaphase.

The result will be a gamete (sex cell) that either has an extra chromosome or is deficient in a chromosome.

An abnormal number of chromosomes will often lead to a person possessing a syndrome, a collection of physical signs or symptoms

An example is Down syndrome, or trisomy 21, which is due to a non-disjunction that leaves the individual with 3 chromatids of the chromosome 21, instead of 2

Other examples of non-disjunction are Turner’s syndrome, Klinefelter’s syndrome and Patau syndrome.

KaryotypingKaryotype: It is the number and type of chromosomes that the nucleus of a cell contains.

To build a karyotype, the chromosomes are stained, a micrograph (microscopic photograph) is taken and then the chromosomes are arranged according to their size and structure, starting with the largest and ending with the smallest.

As most cells are diploid, the

chromosomes in a karyotype usually occur in pairs.

KaryotypingTwo procedures for obtaining the fetal chromosomes to produce the karyotype.

1)Amniocentesis: involves passing a needle through the mother’s abdominal wall, to withdraw a sample of amniotic fluid from the amniotic sac of a developing fetus.

2)Chorionic villus sampling: This procedure samples cells from the placenta, specifically from the villi of the chorion, rather than the amniotic fluid.

Complete Human Karyotype

Class activityMake a Karyotype:

Part I: Cut and paste the chromosomes in order. Remember to look for similar sizes, band patterns, and location of centromeres.

Part II: Cut and paste the chromosomes in order. Analyze the karyotype and write down whether the patient is male, female and if they have a genetic disorder.

This graphic should help you locate bands and

centromeres.

Mutations2 kinds of mutations:

a) Gene mutations: Produce changes in a single gene

a1) Point mutations: Involve changes in one or a few nucleotides, they occur at a single point in the DNA sequence = substitutions.

a2) Frameshift mutations: They shift the reading frame (every codon) of the genetic message, that changes every amino acid that follows the point of mutation = insertion or deletion.

Mutationsb) Chromosomal mutations: Involve changes in the number or structure of chromosomes. They may change the locations of genes on chromosomes and even the number of copies of some genes = deletions, duplications, inversions and translocations.

A. DeletionB. Inversion

C. TranslocationD. Duplication