Lecture Notes-20 Biology 1004

North Arkansas College

Topic: Meiosis

1. Several processes that involves sex cells (gametes).
2. Sex cells are haploid (1N = 23 chromosomes).
3. Two gametes from each of parents combine to form zygote (2N).
4. Two rounds of division occur to produce 4 haploid cells (1N) cells.
5. Actual process of meiosis is similar to mitosis with some exceptions:

1. Chromosome replication and centromere division occurs during the first Interphase stage.
2. Crossing over between chromosomes from the different gametes also occurs during first division. This allows for the mixing/pooling of genetic information or traits.
3. At the end of the first division, the chromosomes are paired (1N) - The pair of chromosomes are joined at the centromeres and are counted as 1 chromosome (a single pair). This division results in 2 cells which have paired chromosomes.
4. In the second meiotic division, the chromosome pairs separate and produce 4 cells which are haploid.

Click below for diagrams of illustrations.

http://taggart.glg.msu.edu/bs110/meiosis.htm

Comparison of Meiosis and Mitosis

1. We all know that poppies give rise to other poppies; zebras produce zebras, and so on. This is due to inherited characteristics or traits.
2. For a long time, it was assumed that the offspring of 2 parents must contain equal amounts of traits from each parent - blending theory of inheritance.
3. There was no way to explain why some crosses always produced offspring with traits of 1 parent and not the other.
4. Gregor Mendel did crosses with several traits in garden peas. He demonstrated dominance and recessiveness among the species.
5. He found that if he crossed true breeding tall plants with true breeding short species, the F₁ generation produced all tall plants.
6. Always produced tall plants in the F₁ generation (first offspring generation) not intermediate height plants as expected from blending theory of inheritance.

1. Monohybrid Cross - 1 set of characteristics. Choose those traits that always breed true. First offspring (F₁) resembled one parent (not blending theory of inheritance).
2. To explain this Mendel suggested:

1. There are duplicate sites for each characteristic - allele (on site present on each chromosome).
2. Some sites are dominant and others are recessive.
3. Now know that these are genes that represent a trait on the allele.
4. The alleles may be of the same type and are called homozygous (AA), or they may be 2 different sites (alleles) of different types and are called heterozygous (Aa).
5. If you cross a homozygous dominant with homozygous recessive parental type, you produce a heterozygous offspring in which the dominant allele expresses itself. If the dominant allele is present, it is expressed, and offspring exhibit the dominant trait (e.g. tall). If true breeding F₁ generation is crossed, ¾ will be like dominant parent, and ¼ will be like the recessive parent.

In a Punnett Square, you place gametes of one parent vertically, and gametes of the other parent horizontally, and then cross the gametes.

Mendel also did dihybrid crosses: More than one set of characteristics - 2 genes (2 characteristics) such as tall green plants that are crossed with short yellow plants.

TTGG are homozygous dominant (true breeding).

ttgg are homozygous recessive.

Earlobes: Attached and Unattached - Unattached earlobes are dominant - EE. Attached earlobes are recessive - ee.

Black coat - dominant; Brown coat - recessive (bb). How can we tell if black is homozygous? If it were homozygous (BB), colts would always be black (Bb). If the coat is due to heterozygous (Bb), then ¼ of offspring will be brown.

1. Mendel was somewhat lucky in that the characteristics that he selected all bred true.
2. In reality, many characteristics do not follow simple Mendelian genetics.

Examples:

1. Incomplete Dominance - Neither allele is dominant, and crosses exhibit characteristics of both parents. Flowers called Four O'Clock's are an example. If you cross red with white flowers, you will get pink (Rr x Rr @ 1:2 ratio). Seems similar to blending theory of inheritance, but this is not the case because if you cross the F₁ hybrid you get: 1 red, 2 pinks & 1 white (1:2:1 ratio). The original parent types are regenerated.
2. Co-Dominance - Both alleles are dominant when present: Blood groups: A, B, O, AB. In AB, both alleles (A & B) are present and express themselves.

Blood group A - AA or AO.

Blood group B - BB or BO.

Blood group O - OO.

Blood group AB - AB.

1. Gene that affects more than 1 characteristic.
2. Marfan Syndrome - A single gene can cause individuals to be tall with long arms, below normal intelligence, their aorta walls are too thin (aortic aneurisms), etc.
3. Sickle Cell Trait - The gene is associated with hemoglobin. RBC's have a tendency to be sickle in shape. They do not transport O₂ properly. Individuals with this disorder may also have problems with the brain due to O₂ deficiency, and they may have heart and kidney problems.

1. A pair of recessive alleles (when present) prevents the expression of dominant alleles.
2. This is called epistasis.
3. Albinism - If aa for white or albinism is present, they will be expressed.

1. The expression of a single characteristic varies depending on the environment (phenotype varies even though genotype is constant).
2. Water Buttercups - Color of flowers is different above and below water.
3. Thalidomide - Was used to prevent morning sickness. If taken during second month of pregnancy, high probability existed that embryo would develop with missing limbs (arms, legs, etc.). Banned in the U.S. years ago. The genotype of the individual doesn't change - expression is due to presence of thalidomide changes.

1. Seen in hair color, especially skin color, seed colors, etc.
2. Seen in coat color in rabbits. There are 4 alleles involved:

1. Agouti coat - brown.
2. Chinchilla coat - gray.
3. Himalayan coat - white; black near points.

C = wild type, Agouti; C^ch = Chinchilla; C^h = Himalayan; C = White.

1. Several genes that affect the same characteristic.
2. They are additive.
3. Humans - skin color. A, B, C, D - all controlled by amount of melanin (skin pigmentation). AA, BB, CC, DD - highest level of melanin and darkest skin. Various levels of pigmentation depend on the amount of dominant genes present. Skin color - 4 or 5 alleles.

1. X chromosome is associated with female.
2. Y chromosome is associated with male.
3. It is the male that controls the sex of offspring.
4. In 1991, Robin Badge-Lovell and Henry Goodfellow isolated the SRY region that controls sex of individual.
5. Embryo's that do not have the SRY region develop ovaries. If SRY region is present, they develop testes.

There are only @ 20 genes on Y chromosome.

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