Mendelian Genetics

By Kelly Diep
March 6, 2015
Period 1

Mendel's Peas

Mendel picked common garden pea plants to test for his research because they can be grown easily in large numbers and their reproduction can be manipulated. Pea plants have both male and female reproductive organs, meaning they can either self-pollinate themselves or cross-pollinate with another plant.  In his experiments, Mendel was able to selectively cross-pollinate purebred  plants with particular traits and observe the outcome over many generations.  Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.  For instance, the pea flowers are either purple or white by crossing white and purple peas. He allowed all 7 F1 generation plants to self-pollinate and then crossed those offspring with each other to get F2 Plants. He found that in the F1 plants, during fertilization the alleles for tallness or shortness separated but paired up again in the F2 Generation.

Crossing Over

Thomas Hunt Morgan, in the early 1900’s, used a common species of fruit fly, Drosophila melanogaster, to expand the understanding of genetics. Morgan was the first to show through experiments that genes were located on chromosomes. He also found that certain fruit fly traits (e.g., white vs. red eye color) are found on the same chromosomes that also determine their sex. 

Mitosis

Mitosis is the first stage of cell division, and it is the division of the nuclues, Mitosis is split in four phases: prophase, metaphase, anaphase, and telophase. Prophase is the longest phase in mitosis, and by the time prophase is over, the chromosomes coil more tightly, the nucleolus disappears, and the nuclear envelope breaks down. In metaphase, the chromosomes line up across the center of the cell. In anaphase, the chromosomes split into 2 groups, and finally in telophase, the chromosomes become dense tangles and are surrounded by a nuclear envelope. Mitosis results in 2 daughter cells genetically identical to the parent cell. Genotype is determined by the makeup of something called "alleles", the form of a gene that produces different effects. The phenotype is the physical expression or characteristics of that trait.
Meiosis

Meiosis is a specialized type of cell division which reduces the chromosome number by half. This process occurs in all sexually reproducing eukaryotes including animals, plants, and fungi. Meiosis has two successive divisions of a diploid cell of a sexually reproducing organism that results in four haploid progeny cells. As homologous chromosomes form tetrads, they exchange portions of their chromatids in a process called crossing over. The homologous chromosomes separate and new cells are formed. Since all pairs of homologous chromosomes were separated, neither of the daughter cells has 2 complete sets of chromosomes that it would have in a diploid cell. The 2 cells produced by meiosis I have different alleles and chromosomes from the diploid cell that had first entered meiosis I. In meiosis II, the 2 cells produced by meiosis I now enter a second meiotic division.

Natural Selection

Natural selection is the gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms

interacting with their environment.It is a key mechanism of evolution and it was one of Darwin's most famous theories stating that evolutionary change comes through the production of variation in each generation and differential survival of  individuals with different combinations of these variable characters. Individuals with characteristics which increase their probability of survival will have more opportunities to reproduce and their offspring will also benefit from the heritable, advantageous character. Over time, these variants will spread through the population.

Incomplete Dominance & Codominance

Incomplete dominance describes a case in which one allele is not fully dominant over the other. For example: Red roses (RR) and White roses (WW) combine to form pink roses (RW). Many genes have more than 2 alleles. Codominance is a situation in which both alleles in a gene contribute to a phenotype. Co-dominance is a relationship between two versions of a gene. Individuals receive an an allele of a gene from

 each parent. If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked.

Sex Determination & Linkage

2 of a human's 46 chromosomes are called sex chromosomes because they define an individual’s sex. Female's have two X chromosomes, whereas males have one X and one Y chromosome. Sex-linked genes are genes located on the sex chromosomes. 3 human genes associated with color vision are found on the X chromosome, and in males, a defective version of any one of those genes produces color blindness. Males have just 1 X chromosome, so all X-linked alleles are expressed in males.The recessive phenotype of a sex linked genetic disorder is much more common among males than females.

Genetic Counseling

Genetic counseling is the process in which the patients or relatives at risk of an inherited disorder are advised of the consequences and nature of the disorder, the probability of developing or transmitting it, and the options open to them in management and family planning. Genetic tests are done by analyzing small samples of blood or body tissues. They determine whether you, your partner, or your baby carry genes for certain inherited disorders.

Punnett Squares

A punnett square is a diagram drawn to determine the gene combinations from a genetic cross. An organism is homozygous for a certain trait if it has two identical alleles for that trait. An organism is heterozygous for a trait if it has two different alleles for that trait. Homozygous organisms are true-breeding for a certain trait, and heterozygous organisms are hybrid for that trait. Punnett squares show all possible combinations of a maternal allele and paternal allele combination.