Genetics

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

2. 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.

3. 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.

4. 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.

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

6. 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 – F1 generation

7. 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.

8. 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.

9. Experiment II: Segregation • Mendel crossed a tall plant (dominant) with a short plant (recessive), the F1 plant inherited an allele for tallness from the tall parent & an allele for shortness from the short parent. Pg. 265

10. 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.

11. 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 F1 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

12. 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.

13. 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.

14. Punnett Squares: • The F1 parent is shown along the top & left sides. • The possible F2 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

15. 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.

16. 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.

17. Probability of SegregationPg. 268-269 • ¼ (25%) of F2 plants have 2 alleles for tallness (TT). • ½ (50%) of F2 plants have 1 allele for tallness & 1 allele for shortness (Tt). • ¾ (75%) of F2 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 F2 generation; Ratio of 3:1

18. 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.

19. Experiment III The Two-Factor Cross: F1 • 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 F1 offspring were all round & yellow showing that both were dominant alleles. The genotype is RrYy. Pg. 270

20. Experiment IIIThe Two-Factor Cross: F2 • The F1 plants were all heterozygous with a genotype of RrYy. • The F1 plants were segregated to each other to produce an F2 generation. • Mendel compared the variations in the seeds.

21. 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

22. 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.

23. Different Patterns of Dominant & Recessive • Incomplete Dominance–1 allele is not completely dominant over another. Ex. Red flower (RR) & white flower (WW), F1 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.

24. 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.

25. 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.

26. 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.