What You’ll Learn

· You will identify the basic concepts of genetics.
· You will examine the process of meiosis.

Mendel’s Laws of Heredity

Gregor Mendel was an Austrian Monk that lived during the 1800’s. He studied what humans had noticed for centuries- the passing of traits from parents to offspring, or heredity. Mendel was the first to predict why this happened, becoming the first real ‘geneticist,’ or person to study heredity.

Mendel was a great gardener, so his chosen subject was the garden pea. He chose to use garden peas because they reproduced sexually- meaning that they needed both male and female sex cells (gametes) to produce offspring. Fertilization occurs when the male and female reproductive cells combine and the resulting fertilized cell (which eventually forms into a baby) is called a zygote. In plants, the process of fertilization is also called pollination, or when the transfer of pollen from the male plant to the female plant occurs. There are two different kinds of pollination. There is self-pollination, which occurs when a plant fertilizes itself, and there is cross-pollination, where the pollen is transferred from one plant to another one to trigger fertilization.
Cross Pollination Animation
Mendel was sure to use purebred plants (ones that were homozygous dominant or recessive for a trait-meaning that they had genes for only one type of allele) so that his results would be correct. Mendel was experimenting with hybrids- offspring of parents with different types of a trait. He started off with monohybrid crosses- breeding only to observe the frequency of one trait- height. First he crossed a tall plant with a short plant and the offspring were all tall. Then, the allowed the second generation to self pollinate and found that their offspring (the third generation) were ¾ tall plants and ¼ short plants.
Mendel's Peas Animatio​n Activity

From that experiment, Mendel concluded that two things controlled each trait organisms have. The things are genes located on chromosomes and are called alleles. Basically, Mendel concluded that an organism would have alleles from both parents (the law of segregation). With these traits Mendel determined that some were dominant and appeared in offspring, and that some were recessive and disappeared in offspring. Dominant traits are represented with capital letters (RR) and recessive ones are represented with lowercase letters (rr).
Phenotype is the way an organism looks and behaves, genotype is its allele combinations. A genotype cannot always be seen by what an organisms phenotype is because of recessive traits. Organisms that are homozygous for a trait (have two of the same alleles for a trait, i.e. tall tall, short short) show their genotypes in their phenotypes. Organisms that are heterozygous (have two different alleles for a trait, i.e. tall short) do not have phenotypes that show their genotypes.
Mendel’s second rounds of crosses were dihybrid. He took round yellow seeds and crossed them with wrinkled green seeds. The first generation of offspring from those offspring showed the dominant trait- they were all round and yellow. However, when he let that second generation self pollinate they showed all of the possible combination of traits with this ratio- 9 round yellow seeds, 3 round green seeds, 3 wrinkled yellow seeds and one wrinkled green seed.
This second experiment caused Mendel to create the law of independent assortment which says that traits are inherited separate from each other (i.e. height and weight). This causes the variation because instead of clumping together traits can combine in many different ways.
Punnett Squares
Reginald Punnett (an English biologist) created an easy way to find the expected outcomes of possible offspring a specific cross could have.
Punnett Square Animation

Above is an example of a monohybrid cross. This type of cross shows the results of simple crosses where a trait is either one option or the other. Above is an example of a cross where a homozygous recessive pea plant is crossed with a heterozygous plant. The result is two heterozygous plants and two homozygous recessive plant. Simply, you take the alleles that a plant has (say, two recessive ones meaning the plant is green) and cross them with the alleles of another plant (say, a dominant one and a recessive one meaning the plant is yellow). The results of possible types of offspring are shown.
Monohybrid Cross animation

More complicated than that are Dihybrid crosses. Show above with kitties and the traits of tail length (long or short) and color (brown or white). Dihybrid crosses show the law of independent assortment by showing all the possible combinations of two separately inherited traits.
Dihybrid Cross Animation
Using Punnett squares one can find the probability of a trait. In the monohybrid cross above the ratio is 2:2 because there are two different outcomes that are shown to occur 2 times each. The larger the probability of a trait the more often it will occur in offspring.
Mitosis- Process of nuclear division where two new cells each receive a full set of chromosomes
Mitosis animation.
Diploid- A cell with two of each kind of chromosome or 2n number of chromosomes.
Haploid- A cell with one of each kind of chromosome or n number of chromosomes.[[@http://Homologous Chromosome|Homologous Chromosome]]
[[http://Homologous Chromosome|Homologous Chromosome]]- Chromosomes with genes for the same traits in the same order.
Meiosis- Cell division where one cell produces four gametes each with half the chromosomes.
Sperm- Haploid male sex cells produced by meiosis.
Egg- Haploid female sex cell produced by meiosis.
Sexual Reproduction- Reproduction that involves the production and fusion of haploid sex cells.
Crossing Over- Exchange of genetic material between chromatids during prophase I of meiosis
making new allele combinations.
Genetic recombinationGenetic recombination- Genetic variation during meiosis by reassortment or crossing over.
Genetic Recombination Animation
Nondisjunction- Failure of homologous chromosomes to separate during meiosis properly,
making gametes with too many or too few chromosomes.

Meiosis Video
Walk Through Meiosis animation
How Meiosis Works with explaination

Analyze- how meiosis maintains a constant number of chromosomes within a species.

Infer- how meiosis leads to variation in a species.
Relate- Mendel’s laws of heredity to the events of meiosis.

Organisms have thousands of genes that determine traits. Genes are lined along chromosomes within the nucleus. A chromosome can contain more than a thousand genes along its length. In most cells, chromosomes are in pairs. One chromosome is from the male parent and one is from the female parent. A cell with two of each kind of chromosome is called a diploid cell. This supports Mendel’s conclusion that organisms have two factors (alleles) for each trait.
Organisms produce gametes that contain one of each kind of chromosome. These are called haploid cells and are used in sexual reproduction. Haploid cells support Mendel’s claims that parent organisms give one factor for each trait to their offspring.
Homologous chromosomes are the pair of chromosomes in a diploid cell that have genes for the same traits. On homologous chromosomes, these genes are arranged in the same order but because there are two possible alleles for the same gene, the two chromosomes are not always identical.
Meiosis is used so that through combination of an egg and sperm, the offspring has half of each of its parents’ chromosomes. This provides the ability for genetic variation and evolution. Meiosis has two stages, meiosis I and meiosis II. When both stages are completed, there are four haploid cells. Haploid cells are called sex cells or gametes. Male gametes are called sperm and female gametes are called eggs. When a sperm fertilizes an egg, a zygote is formed with a diploid number of chromosomes.
The beginning of meiosis is interphase. During interphase, a cell replicates its chromosomes. Next is prophase I. Here the DNA coils and homologous chromosomes line up across from each other gene by gene. These pairings are called tetrads. The chromosomes in a tetrad are bound so tightly together, that they can actually exchange genetic material. This process of exchange is called crossing over. Crossing over results in new combinations of alleles on a chromosome.

The next phase is metaphase I. During metaphase I, the centromere from each chromosome is attached to a spindle fiber which pulls the tetrads to the middle. In the next phase, anaphase I, chromatids separate and move to opposite ends of the cell. This separatioin is essential in ensuring that each new cell will receive only one chromosome from each homologous pair. The last phase in the first stage of meiosis is Telophase I. Here the spindle is destroyed, chromosomes uncoil, and the cytoplasm divides into two new cells. After this first stage of meiosis, the process occurs again known as mitosis II. By the end of meiosis, there are four haploid sex cells usable for reproduction.

Crossing over during meiosis provides ways to rearrange alleles. Also, genetic recombination through fertilization can provide many different alleles. This potential means that genetic variation is much more likely to occur through meiosis and sexual reproduction. Mendel’s results are explained by modern science’s understanding and discoveries. Anaphase I of meiosis explains Mendel’s observation that each parent gives one allele to the offspring. The segregation of chromosomes during anaphase I also explains how genes for different traits are inherited independently from one another like Mendel theorized.
Sometimes chromosomes do not separate properly during meiosis. When this occurs it is called nondisjunction. One kind of non disjunction creates two kinds of gametes. A gamete with an extra chromosome, and a gamete missing a chromosome. Another kind of nondisjunction is when homologous chromosomes fail to separate at all. This creates children with three sets of chromosomes. Organisms with more than the normal number of chromosomes are called polyploids. Polyploids are more common in plants because in animals it almost always causes the death of the zygote. Plants that are polyploidy are often bigger and better which has led some plant breeders to use chemicals that cause nondisjunction on plants.
Genes sometimes appear to be inherited together. This is because if they are close on the same chromosome, they are more likely to be inherited together. Chromosomes rather than individual genes follow Mendel’s law of independent assortment.

Crossing Over-
1. Roll out four long strands of clay at least 10 cm long to represent two chromosomes, each with two chromatids.
2. Tie these chromatids (or strands of clay) together about 3 cm from their tops with twist ties.
3. Poke the letter C into the tops (above the twist ties) of all four chromatids with a toothepick, making the two on the first chromosome capital C’s the second two lower case. Poke the letter B lower down on all four chromatids, making all the letters capitals. Then, near the bottom of all the chromatids poke the letter A, having the ones on the chromosome with the capital C’s be capital and the ones on the chromosome with the lower case A’s be lower case.
4. Assume that the chromosomes started out as a single pair before they were replicated. The chromosomes you have will resemble prophase I in meiosis. With this first set of chromosomes assume that crossing over does not take place. Draw how these chromosomes will appear in the end of meiosis. In this drawing record where the chromatids end up and what genes are located on them (i.e. the letters).
5. Repeat steps 1-3. This time assume that crossing over between genes B and C on the chromosomes occur. Remove one of the longer ends of each chromosome and switch them. Now draw and record the results as detailed in step 4.

By Kassie Amann and Jeff Todd- Period 2
2010- By Angie Schwendiman and Jane Raty pd. 5