Genetics- That branch of biology that deals with genes and their transmission from one generation to the next, and their effects on external traits, behavior, and other characteristics.
Chromosomes - One of several threadlike structures found in the nucleus of the cell; they are made of protein and DNA, and carry the genes.
The number of chromosomes varies from specie to specie. Within a specie, however, the number is constant. Below is the number of chromosomes in the body cells for the following common animals:
Pig 38
Cat 38
Rabbit 44
Human 46
Sheep 54
Cattle 60
Horse 64
Dog 78
Chicken 78
The size of chromosomes also varies. They can be short with only a few genes attached, or can be long with a large number of genes.
Each chromosome possesses a centromere which serves as the point of attachment for the spindle fiber during cell division. The location of the centromere will vary from chromosome to chromosome as well.
In addition to the fact that numbers of chromosomes remains constant in the cell, chromosomes exist in pairs. The members of a paired set have the same shape, length, and have their centromeres in the exact same location or position.
EX:
Cattle- 30 pairs, Swine- 19 pairs, Dogs- 39 pairs, Humans- 23 pairs

The two members of a paired set of chromosomes are called homologs of each other. Homologous chromosomes are described as two chromosomes that are identical in size, shape, and position of their centromeres. Cells that contain chromosomes in pairs are called DIPLOID. This means they have the total number of chromosomes of that particular specie.
CHEMICAL NATURE OF GENES AND CHROMOSOMES
Chromosomes are made up of a complex combination of protein and DNA. Genes are made up of these DNA strands and each gene is represented by 1DNA molecule.
Ex: If a chromosome has 200 genes attached to it, there would be
200 molecules of DNA.
Genes carry out their function by controlling the synthesis of substances called enzymes. Activities inside every cell are controlled by hundreds of chemical and metabolic reactions.
These activities of cells all working in unison in tissues, organs, anatomical systems, ultimately cause an animal to have a certain appearance or to behave in a certain manner. The physical expression of genes and the physical appearance of an animal is called the animal’s phenotype.
Alleles and Loci
The exact position or location that a gene is found on a particular chromosome is called a locus . Every gene has a locus on that chromosome that never changes. For each gene on a specific chromosome, there exists an identical gene and locus at the exact same position on the homolog of the original chromosome.
The different genes that can occupy the same locus on a paired set of chromosomes are called alleles . REMEMBER, the genes must

be on the same locus of the two homologs, they must control the same trait, but they must control that trait in a different manner.
Ex. One gene is for black hair color, and the other is for red hair color
Cow- Black and red genes
Phenotype- either black or red hair
Genotype- BB or Bb
Example: Given a locus, the possibility of two alleles exists:
Using the letter A- “A” would represent the dominant allele and
“a” would represent the recessive allele and three genotypes are possible: AA. Aa, or aa
Homozygous- Both genes are the same for a given trait AA or aa
Heterozygous- Genes are different for the same trait Aa
The phenotype will depend on the expression of the dominant gene. The only way that the phenotype can be of the recessive gene is when the animal posses TWO of the recessive genes.
We must now learn how genes are passed from one generation to the next in cell division, or how genes are passed from one original parent cell, duplicated and transmitted to daughter cells. This is done by cell division of two types.
One type of division involves a single division of a diploid cell to produce two identical diploid daughter cells. This cell division is called Mitosis. Secondly, a cell division that involves a series of two divisions whereby on parent cell results in the rise of four haploid daughter cells is called Meiosis. These four meiotic products contain exactly one-half the number of chromosomes found in the parent cell. They are not genetic duplicates of the parent cell either. Since they contain half the chromosome number of the parent, these cells are said to be haploid .

MITOSIS
Mitosis is the simpler of the cell divisions. Mitosis occurs in all body cells with the exception of sex cells or gametes. Mitosis occurs in these cells to increase the number of cells in growing animals, or to maintain the number of cells in tissues as old cells wear out or die.
It is not completely known what causes a cell to begin dividing.
The static or non-dividing cell is described as being in interphase.
This is not a phase or stage of Mitosis, it is simply the phase f the cells when Mitosis is not taking place. The four actual stages of
Mitosis are:
Prophase, Metaphase, Anaphase, and Telophase.
Segregation and Recombination of Genes
Genes separate and recombine as a result of meiosis in spermatogenesis and ovigenisis. The two chromosomes of a paired set are distributed to a separate gamete. This is random and the segregation is completely by chance.
Individuals of the genotype AA can only produce on type of gamete. A Likewise , the aa genotype can only produce gametes with a genes. Therefore, homozygous individuals will only produce 1 type of gamete while heterozygous individuals will always produce 2 types of gametes.
During the mating process, as gametes from the male and female unite, the diploid number of chromosomes is again restored. This new cell is called a zygote. Genes again are recombined to pairs.
The varieties of genetic possibilities are even more diverse with genes from the two parent individuals that influence the newly formed embryo. The new genotype is a result of the recombination of the two parent’s genes and DNA.

Six Basic Crosses
When dealing with any 1 trait controlled by two alleles, three different genotypes are possible:
AA, Aa, and aa
It is possible to combine these three genotypes into six basic crosses.
Genotypes of parents
1.
AA x AA
2.
AA x Aa
3.
AA x aa
4.
Aa x Aa
5.
Aa x aa
6.
aa x aa
Genotypes of progeny all AA
½ AA, ½ Aa all Aa
¼ AA, ½ Aa, ¼ aa
½ Aa, ½ aa all aa
Mitosis
Prophase
Prophase includes most of the activities that prepare the cell for division. Each chromosome manufactures a completely new identical chromosome strand. The two strands are called chromatids and are united as one structure by a common centromere. The DNA and genes in each chromatid are identical to one another.
As division begins the chromosomes become shorter and thicker than the chromosome in the interphase cell. The centriole located just outside the nucleus divides and migrates to opposite ends of the cell. As they move, the nuclear membrane disappears. By the time they have completed their migration to what is referred to as the poles of the cell, the spindle fibers have also formed. These extend from one centriole to the other and each also attaches to the centromere of the doubled chromosomes.

Metaphase
Metaphase is also a predivision phase of mitosis. In this phase the doubled chromosomes become aligned and oriented along the equilateral plane of the cell. With the centrioles representing the poles of the cell, an imaginary line running through the poles of the cell from pole to pole would represent the axis. The equilateral plane of the cell would be a plane perpendicular to the axis of the cell. At this point the cell is ready to divide.
Anaphase
Anaphase is given to the stage of mitosis where the centromeres of the chromosome pairs divide longitudinally between the chromatids of their respective chromosomes and the separated chromatids move to either pole of the cell.
The two chromatids from each chromosome appear to be drawn to opposite poles of the cell by the spindle fiber that connects its centromere to the centriole.
Each chromatid can now be called a chromosome.
Meiosis
The second type of cell division occurs with certain specialized cells located in the testes and the ovaries. The process begins with diploid cells called germ cells. Ovigonia in the ovaries and spermatogonia in the testes.
Altough this cell division is called Meiosis, more specifically it is referred to as spermatogenesis in the male, and ovigenesis in the female.
The overall process involves two divisions resulting in four products from one original germ cell. These meiotic products are haploid meaning they possess one-half the normal number of chromosomes present in the normal body cells of the animal.

First Meiotic Division
This stage of cells division begins in much the same way as
Mitosis. The activities we discussed in prophase of mitosis also occur in Prophase I of Meiosis in the first meiotic division. The most important activity of this stage, as in mitosis, is the duplication of the chromosomes. In addition, the now doubled chromosomes seek each other out in the cell and lay side by side.
This is called synapsis. The doubled chromosomes of each homologous pair give the appearance of a four stranded structure called a tetrad. As an example, there would be 19 tetrads in the swine cell in Prophase I since the cells of swine contain 19 pairs of chromosomes, or 38 total chromosomes. Next the tetrad become aligned along the equilateral plane as in prophase of mitosis. In meiosis, this is called Metaphase I.
During the period prior to the first meiotic division which the homologous chromosomes are in synapsis, a phenomenon called crossing over occurs. A chromatid from one homolog will lie across one of the chromatids of the other homolog, forming an X shaped appearance. This occurs at the same locus on each chromatid. When the homologous chromosomes separate during anaphase I, they often exchange parts or “ends” of the chromatids.
Genes that were once on one chromosome are now found on the homologous partner chromosome. This is not an uncommon occurance in meiosis. It is a natural process and is important in the
“mixing” of genes on the homolog in the randam districbution of genes in the resulting gamete cells.
The events of Anaphase I are different than those of mitotic anaphase. In mitosis, there was no tetrad. During anaphase, the centromeres of each chromosome divide and separate resulting in the formation of two new chromosomes from the two chromatids of each doubled chromosome. In Anaphase I of meiosis, this does not occur. Instead, the homologous chromosomes become

“unsynapsed” meaning that they separate and move toward the poles of the meiotic cell. Each chromosome still possesses two chromatids. As the cell continues to divide, a new nuclear membrane forms around each cluster of chromosomes and the cell constricts between the two new cells. This phase of of the division is called telophase I. The completion of Telophase I produces two haploid daughter cells. Now synapsis should become clear. No two chromosomes from the same pair of homologs will end up in the same daughter cell. Because the chromosome number is reduced to haploid in the first division, this division is sometimes referred to as reduction division.
Second Meiotic Division
The function of the second meiotic division is basically two single stranded chromosomes from each of the doubled chromosomes. In this sense, the second division of meiosis is very similar to the single mitotic division. It does, however, differ in several ways.
One of which, the products are not genetically identical to each other or the parent cell.
As in the first division, the second is divided into stages. There is a prophase II and metaphase II that include activities to prepare the cell for division. In anaphase II the actual division of the paired chromosomes takes place, resulting in the formation of two sets of chromosomes that migrate to the opposite poles of the cell.
Telophase II completes the division process. In this stage, secondary haploid cells produce two new haploid daughter cell.
Thus, one diploid parent cell produces four haploid daughter cells, none of which are identical to each other or the parent cell.

There are two basic reasons that the four cells are not identical:
1.
They contain only one of the two homologous chromosomes, and most homologous chromosomes are not identical in their genetic make-up
2.
The crossing over phenomenon that occurs in prophase I further mixes the genes between the homologs.
Mitosis Questions
What are the two purposes that Mitosis occurs in the cell.
1.
2.
List the 4 stages of Mitosis.
1.
2.
3.
4.
What is the ultimate result of the mitotic process?
Name the most important event that takes place in each stage of mitosis.
1.
2.
3.
4.
Define the following terms:
Centromere
Centriole
Pole
Spindle fiber
Equilateral Plane
Chromatid

2.
3.
4.
5.
6.
Axis
Differences between Mitosis and Meiosis:
1.