Genetics Genetics is the scientific study of heredity. Chapter 11 Sections 1-3

The Work of Mendel 1822 – Gregor Mendel used purebred pea plants to study the inheritance of traits. Fertilization – the process in sexual reproduction in which male & female reproductive cells join to form a new cell.

Mendel ’ s Experiment with Pea Plants Mendel used the pea flower because they have both male & female parts. Self-pollinating – sperm cell in pollen fertilize the egg cells in the same flower. The offspring inherit characteristics from a single parent. Ex. Pea Plant flowers True-breeding - the peas plant flowers were self-pollinating and could produce offspring identical to themselves.

Experiment I: Pea Plant Mendel had to prevent self-pollination so he made a controlled cross between plants. He cut away the male parts & dusted pollen from another plant on the flower. Cross-pollination – seeds produced are the offspring of 2 different parents. Mendel studied a few traits, or specific characteristics of the pea plant such as color & height.

Mendel Studies 7 Traits / Characters pg. 264 Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height

Genes and Dominance Mendel crossed 2 plants with different characters, or forms, for the same trait. Ex. 1 tall & 1 short The plants that grew were hybrid. Hybrid – are the offspring of crosses between parents with different traits. Parent plant – P generation First set of offspring – F 1 generation

Experiment I Concluded: Traits are inherited through the passing of factors from parents to offspring. The hybrid plants looked like only 1 parent and the character of the other parent seemed to disappear. Each trait is controlled by 1 gene. Alleles – controls the different forms of a gene. Genes – chemical factors that determine traits.

Principle of Dominance States that some alleles are dominant & others are recessive. Whenever a living thing inherits a dominant allele, that trait is visible. The effects of a recessive allele are not seen if the dominant allele is present.

Experiment II: Segregation Mendel crossed a tall plant (dominant) with a short plant (recessive), the F 1 plant inherited an allele for tallness from the tall parent & an allele for shortness from the short parent. Pg. 265

What happened to the recessive allele? Mendel allowed his hybrid plants to self-pollinate. Some showed recessive traits, the recessive traits did not disappear. Earlier, the dominant masked the recessive, so it was not visible.

Experiment II Conclusion Alleles for the same trait can be separated. Segregation – When sex cells, or gametes, are formed. Each gamete carries only 1 copy of each gene. Therefore, each F 1 plant produces 2 types of gametes (some with an allele for tallness & some with an allele for shortness). Ex. T, t, T, t = TT, Tt, Tt, tt

Probability & Punnett Squares Probability – the likelihood that a particular event will occur. Ex. Flipping a coin. The probability that it will land on tails is ½.

The Principles of Probability It is used to predict the traits of the offspring produced by genetic crosses. Mendel used a Punnett Square to show the gene combinations that might result from a genetic cross.

Punnett Squares: The F 1 parent is shown along the top & left sides. The possible F 2 offspring are in the 4 boxes that make up the square. Letters represent alleles. T is dominant allele for tallness. T is recessive allele for shortness. Pg. 268

The Punnett Square show: All the possible results of a genetic cross. The genotypes of the offspring. The alleles in the gametes of each parent.

Punnett Squares Homozygous – organisms that have 2 identical alleles for a particular trait; they are true-breeding. Ex. TT or tt Heterozygous – organisms that have 2 different alleles for the same trait; they are hybrid. Pg. 269 Genotype – genetic make up. Phenotype – Physical characteristics; don ’ t have the same genotype.

Probability of Segregation Pg ¼ (25%) of F 2 plants have 2 alleles for tallness (TT). ½ (50%) of F 2 plants have 1 allele for tallness & 1 allele for shortness (Tt). ¾ (75%) of F 2 plants are tall because the allele for tallness is dominant over allele for shortness. Overall, there are 3 tall for every 1 short plant in F 2 generation; Ratio of 3:1

Independent Assortment states that during gamete formation, genes for different traits separate with out influencing each other ’ s inheritance. Helps account for genetic variations. Ex. Pea shape & pea color.

Experiment III The Two-Factor Cross: F 1 Mendel wanted to see if genes that determine 1 trait have anything to do with genes that determine another. He followed 2 different genes as they passed from one generation to the next. Mendel crossed true-breeding plants - round yellow peas (RRYY) with wrinkled green peas (rryy). The F 1 offspring were all round & yellow showing that both were dominant alleles. The genotype is RrYy. Pg. 270

Experiment III The Two-Factor Cross: F 2 The F 1 plants were all heterozygous with a genotype of RrYy. The F 1 plants were segregated to each other to produce an F 2 generation. Mendel compared the variations in the seeds.

Experiment III Conclusion Mendel found some offspring that were: Round / yellow (P phenotype) Wrinkled / green (P phenotype) Many with combinations of alleles (not found in either parent) This showed that the alleles for seed shape segregated independently of those for seed color (independent assortment). Pg. 271

Summary of Mendel ’ s Principles Genes are passed from parents to their offspring. Some forms of the gene may be dominant and others may be recessive. Each adult has 2 copies of each gene – one from each parent. The genes are segregated from each other when gametes are formed. Alleles for different genes usually segregate independently of one another.

Different Patterns of Dominant & Recessive Incomplete Dominance – 1 allele is not completely dominant over another. Ex. Red flower (RR) & white flower (WW), F 1 is a pink flower (RW). Pg. 272 Codominance – both alleles contribute to the phenotype. Ex. A cross of a black chicken (BB) with a white chicken (WW) will produce all speckled offspring (BBWW); colors appear separately.

Different Patterns of Dominant & Recessive Multiple Alleles – have more than 2 alleles. Ex. Coat color in rabbits. Polygenic Traits – traits controlled by 2 or more genes; “ having many genes ”. Ex. Variation in human skin color.

Thomas Hunt Morgan Geneticist who looked for a model organism to use in studying genetics He chose the fruit fly because it produces large numbers of offspring.

Genes & the Environment Mendel ’ s principles apply to all organisms not just plants. Characteristics of any organism are not determined solely by genes it inherits. Characteristics are determined by interaction between genes and the environment. Ex. Genes affect the height and color of a sunflower but the same characteristics are influenced by climate, soil, conditions, and availability of water. Genes provide a plan for development, but how the plan unfolds depends on the environment.