Mendelian Genetics

Mendel’s Principles of Heredity Genetics studies ways in which hereditary information is passed from parents to offspring. Mendel studied the genetics of peas plants. Some traits come in two forms (tall or short, yellow or green seeds). Some plants ‘bred true’: after several generations of breeding the trait is remaining the same. What happens if you cross plants with contrasting traits?

First Generation: one trait seem to disappear. F1 offspring, from pure parents are called hybrids

Second generation Is the yellow trait lost forever?

Law of dominance The trait expressed in the F1 generation is the dominant trait. The trait hidden in F1 is called recessive. When an organism is hybrid for a pair of contrasting traits, only the dominant trait can be seen in the hybrid.

Law of segregation Each trait is controlled by a pair of “factors” (Mendel did not know yet about DNA and chromosomes) In a cross between two parents the offspring received both kinds of factors, one from the father and one from the mother. Factors that occur in pairs are separated from each other during gamete formation and recombined at fertilization.

Gene and chromosome theory. The factors are carried by homologous chromosomes. The separation of chromosomes pairs during meiosis and their recombination during fertilization account for the separation and recombination of the Mendelian factors.

Fundamentals of Genetics Alleles: each body of an organism has two copies of the gene for each trait. The two copies of the gene for a certain trait can be alike or different. Different copies for forms of a gene controlling a certain trait are called alleles. Homozygous: the alleles for a certain trait in an organism are the same. (pure) Heterozygous: the alleles for a certain trait are different. (hybrid)

Genotypes and Phenotypes A capital letter is used for the dominant allele. And a lower case letter is used for the recessive allele. The genetic make-up of an organism is called genotype. The physical trait that an organism develops because of the alleles is the phenotype. You can determine the ratio of each phenotype using probabilities.

Punnett square A Punnett square is helpful in determining the result of any cross. A test cross is a Punnett square used to determine the unknown genotype of an organism.

Law of independent assortment. A monohybrid cross is when only one pair of contrasting traits is being studied. If you follow two pairs of contrasting traits at the same time you can study independent assortment. It is a dihybrid cross. Genes for different traits are separated and distributed to gametes independently of one another.