Intro to Genetics

Genetics Before DNA and chromosomes were discovered, heredity was one of the greatest mysteries of science. Heredity is the passing of characteristics from parents to offspring. Genetics is the study of heredity. The term character refers to inherited characteristics. The term trait refers to a single form of a character (having purple flowers is a trait).

Gregor Mendel Gregor Johann Mendel was an Austrian monk who carried out experiments in which he bred different varieties of the garden pea. Through these tests, Mendel became the first to accurately predict patterns of heredity.

Monohybrid Crosses Mendel began his experiments by doing monohybrid crosses. A monohybrid cross is a cross that involves one pair of contrasting traits.

Mendelian Theory of Heredity: Based on the results of his experiments, Mendel made four hypotheses which now make up the Mendelian theory of heredity: For each inherited character, an individual has two copies of the gene—one from each parent. There are alternative versions of genes. Today, these different versions of a gene are called alleles.

When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. The expressed form is called dominant. The form not expressed is called recessive. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited character. When gametes unite during fertilization, each gamete contributes one allele.

Alleles: Letters are used to represent the different alleles. Dominant alleles are represented by capital letters. Recessive alleles are represented by lowercase letters. If the two alleles for a particular gene are the same, they are called homozygous. If the two alleles for a gene are different, they are called heterozygous.

Alleles: The set of alleles an individual has for a character is its genotype. The physical appearance of a character is its phenotype.

Punnett Squares A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross. Genotypic ratio: 2 BB : 2 Bb : 0 bb Phenotypic ratio: 4 brown : 0 blue

Punnett Square Practice: P = purple p = white Cross PP x pp What are the genotypic and phenotypic ratios of the offspring?

Punnett Square Practice: Cross the offspring from the last Punnett square. What are the genotypic and phenotypic ratios?

Punnett Square Practice: