Introduction to Genetics and Heredity Although these dogs have similar characteristics they are each unique! I. Early Ideas About Heredity A. The Theory of Blending Inheritance Each parent contributes factors that blend in their offspring - ex. A short plant crossed with a tall plant would produce a medium sized plant. B. Gregor Mendel Born in 1822 in Czech Republic Worked as a teacher and performed research on the heredity of pea plants C. Genetics The scientific study of heredity
D. Mendel s Experiment 1. Procedure: a. Cross-pollinate purebred pea plants b. Use pea plants with different characteristics for the same trait - Traits include: seed shape and color, flower position, plant height etc.
D. Mendel s Experiment 2. Results: a. The F 1 (offspring) generation had the traits of only one of the parents b. The P 1 (parental) generation s traits did not blend
C. Mendel s Experiment 3. Conclusions: a. Individual factors, which do not blend, control each trait of a living thing. These factors are called genes. b. The different forms of a gene are called alleles. For example, the gene for plant height occurs in tall and short form. c. Some alleles are dominant, while others are recessive. The effects of a dominant allele are seen even if a recessive allele is present. The effects of a recessive allele are seen only if a dominant allele is not present.
II. Using Genetic Vocabulary A. Defining Terms: 1. DNA is organized into chromosomes. There are 23 pairs of chromosomes (46 total) in all human cells (except sex cells). 2. Genes are small segments of DNA present on chromosomes that code for a particular protein. Multiple genes are found on one chromosome. - The estimated # of genes in the human genome is between 30,000 and 35,000 3. Genes code for proteins, ultimately resulting in the expression of specific traits (characteristics).
II. Using Genetic Vocabulary Cont. 4. Genes come in different forms called alleles. Alleles are either dominant (A) or recessive (a). 5. Organisms that have two identical alleles for a particular trait are said to be homozygous (AA) or (aa). 6. Organisms that have two different alleles for the same trait are heterozygous (Aa). 7. The genotype (genetic make-up) for a particular trait determines the phenotype (physical characteristic).
III. A Simple Example The organisms in our fictional example have one pair of chromosomes per body cell. A gene on the chromosome codes for either dark (D) or light (d) flower color. P 1 (parental) Generation Male: Female: Genotype: dd DD Phenotype: Homozygous recessive Light flower color Homozygous dominant Dark flower color A cross between two parents with different forms of a gene is called a hybrid.
To prepare for mating, special cells called gametes must form. The process used to make gametes is called meiosis. Gamete cells are haploid (containing only one homologous chromosome per pair). P 1 Generation Male: Female: Pollen (sperm) d d D D Ovule (egg) d d D D Note: In animals, 3 of the egg cells (polar bodies) will degenerate!
When organisms mate, 1 sperm cell (or pollen grain) from dad will randomly combine with an egg cell (or ovule) from mom. These combined gametes will produce a diploid (2N) offspring with 2 complete sets of chromosomes. In this example, all of the pollen cells contain the recessive allele (d) for flower color and the ovule cell contains the dominant allele (D) for flower color Male gamete: pollen or sperm d Female gamete: ovule or egg Fertilization D F1 (offspring) generation Dd = heterozygous Dark flower color = All offspring will be Dd with Dark flowers Genotype Phenotype
IV. Punnett Squares D d D = dark flowers D DD Dd d = light flowers d Dd dd A Punnett square is used to predict the probability Of producing offspring with certain characteristics. The probability of two heterozygous dark flowered plants Producing a dark flowered plant is 3/4 while the probability Of producing a light colored plant is 1/4.
V. Special Cases of Dominant and Recessive Incomplete Dominance: a genetic cross where one allele is Not completely dominant over another. Example - red and White flowers combine to produce pink flowers Codominance: a genetic cross where both alleles show up In the phenotype for the organism. Example - red and White flowers combine to produce speckled red and white Flowers Multiple Alleles: Characteristics that have more than one Possible allele. Example - the gene for human eye color Comes in many different forms. Polygenic Traits: Traits that are controlled by two or more Genes. Example - about four different genes control human Skin color