Chapter 12

1. Chapter 12 Mendel and Heredity

2. Standard 3 • Students know and understand the characteristics and structure of living things, the processes of life, and how living things interact with each other and their environment. • Benchmark 3.13: some traits can be inherited while others are due to the interaction of genes and the environment. • Assessment objective 3.13a: Classify well-known conditions as being purely genetic or the result of the interaction of genes and the environment.

3. Vocabulary • Character Homozygous • Trait Heterozygous • Hybrid Punnett square • Generation Probability • Allele Pedigree • Dominant Genetic disorder • Recessive Polygenic character • Genotype Codominance • Phenotype Linked

4. Origins of Hereditary Science • Modern genetics is based on Mendel’s explanations for the patterns of heredity in garden pea plants. • Seven characters Mendel studied in his plants included: • Flower color • Seed color • Seed shape • Pod color • Pod shape • Flower position • Plant height Gregor Mendel Introduction (1:30)

5. Contrasting Traits • In the study of heredity, physical features that are inherited are called characters. • A trait is one of several possible forms of a character. • For example, the color of a flower is inherited and therefore a character. • A variation of the color is a possible form of a character and therefore a trait.

6. In garden pea plants there are both male and female reproductive parts. • Plants can self-pollinate (fertilize itself) or cross-pollinate with other pea plants. • Plants with different traits that cross-pollinate produce offspring called hybrids. Gregor Mendel Research and Principles (4:30)

7. A generation is a group of offspring from a given group of parents. • Mendel first ensured that his plants were true-breeding for a certain trait by letting the plants self-pollinate for several generations. • The first group of parents that are crossed are called the parental generation or P generation.

8. Mendel then crossed two P generation plants that had contrasting traits like white and purple flowers. • He called the offspring of the P generation the first filial generation or F1 generation.

9. Mendel then let the F1 generation plants self-pollinate and produce new plants. He called this new generation of offspring the second filial generation or F2 generation. • Notice how many of each trait was produced—what’s the deal?

10. Mendel’s Theories(2:51) • Mendelian theory explains simple patterns of inheritance. In these patterns, tow of several versions of a gene combine and result in one of several possible traits. • An organism’s traits come from different versions of genes. Each version is called an allele. • Traits can come from either parent—because of meiosis. • For every pair of traits, only one expresses itself. • The allele that expresses itself is said to be dominant. • The other allele is a recessive allele and its trait is not expressed.

11. Random Segregation of Alleles(4:41) • Even though a gamete receives an allele from each parent, only chance (or fate) determines which one it will receive. • The law of segregation holds that when an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles. • Dominant alleles are indicated with a capital letter and recessive alleles have a lower case letter. • Given “a” and “A”, which is dominant and which is recessive?

12. Mendel’s Findings in Modern Terms • The set of alleles that an individual has for a character is called the genotype. • The trait that results from a set of alleles is the phenotype. • Genotype determines phenotype.

13. Homozygous & Heterozygous • If an individual has two of the same alleles of a certain gene, the individual is homozygous for the related character. • If an individual has two different alleles of a certain gene, then they are heterozygous. • Between pp and Pp, which is homozygous and which is heterozygous? Significance of test cross as genetic tool (1:30)

14. Mendel’s Second Experiment • Mendel used dihybrid crosses which involve two characters like color and shape to further test how patterns appear. • The Law of Independent Assortment holds that during gamete formations, the alleles of each gene segregate independently. • Alleles can mix and match. Introduction dihybrid crosses (3:15)

15. Mendel’s Second Law • Genes are linked to each other as parts of chromosomes. • Genes that are located close together on the same chromosome will rarely separate independently, thus they are said to be linked because they’re so close together. Simulating a dihybrid cross (2:57)

16. Punnet Squares • Punnet squares show all the genotypes that could result from a given cross. • As shown, the simplest box is 4 squares with the parent’s traits on the top and side of the square. • Watch the video • Do the Test Cross QuickLab on page 277 Punnet Square (4:49)

17. Using Probability(7:04) • Probability is the likelihood that a specific event could occur. • Punnet squares predict probability • Probability can be calculated using the formula: Number of one kind of possible outcome • Probability = Total number of all possible outcomes • Probability formulas can be used to predict the probabilities that specific alleles will be passed on to offspring.

18. Using a Pedigree • A pedigree is a family history that shows how a trait is inherited over many generations. • Genetic disorders or diseases can be inherited. • Pedigrees can answer questions associated with sex linkage, dominance, and heterozygosity. Sex linked inheritance (29:06)

19. Dominant/RecessiveHeterozygous/Homozygous • If a person has a trait that is autosomal and dominant and has even one dominant allele, they will show the trait. • If a person has a recessive trait and only one recessive allele, they will not show the trait but could pass it on. • If a person is either heterozygous or homozygous dominant for an autosomal gene, their phentoype will show the dominant trait. • Homozygous recessive shows the recessive trait. • If a child shows the recessive trait, that means that both parents are heterozygous carriers of the recessive allele.

20. When several genes affect a character it is called a polygenic character. • Examples are eye color, skin color and height. • Most characters are polygenic. • Incomplete dominance occurs when an offspring has a phenotype that is in-between the traits of its two parents. • An example is when a red and white snapdragon flower is crossed and a pink offspring results. It shows that neither the red or the white is completely dominant. • Incomplete dominance in snapdragons (2:07) • Incomplete dominance in humans and plants (2:01)

21. Multiple alleles and Codominance • Multiple alleles are genes that have three or more possible alleles. • In humans, blood types are determined by 3 alleles: IA, IB, and i. These can produce blood types of A, B, AB and O • Codominance is a condition in which both alleles for the same gene are fully expressed. • Human blood groups are examples of codominance. Multiple Alleles and Codominance in Blood (11:09)

22. Genes affected by environmentand genes linked within chromosomes • Phenotype can be affected by conditions in the environment such as nutrients and temperature. • Many arctic animals have genes that cause their fur to be dark in the summer and light in the winter in order to increase chances of survival through camoflauge. • Environmental Factors Which Influence the Expression of Traits (02:02) • During meiosis, genes that are close together on the same chromosome are less likely to be separated than genes that are far apart. • Genes that are close together as well as the traits they determine are said to be linked. • QuickLab pg 281