Ch. 11: Mendelian Genetics

1. Ch. 11: Mendelian Genetics 11.1: What Mendel did 11.2: Mendel’s Law of Segregation 11.3: Mendel’s Law of Independent Assortment 11.4: Human Genetic Traits 11.5: Beyond Mendel

2. 11.1: What Mendel Did Right • Peas: easy to grow, fast, true-breeding • Simple traits: no intermediate types (Fig. 11.2) • Mathematical interpretation: Laws of Probability → “Particulate Theory of Inheritance” (NOT Blending Concept)

3. Required Vocabulary • True-breeding / segregating • P, F1, F2 generations • Monohybrid cross • Allele • Locus • Homozygous, heterozygous • Dominant, recessive • Phenotype, genotype

4. True-breeding: Always producing the same type of offspring, generation after generation, when self-pollinated or when crossed to an identical individual Segregating: Producing more than one type of offspring, following one or more cycles of self-pollination or crossing to an identical individual True-breeding vs. Segregating

5. Generations • P = parental (P1, P2) • F1 = filial generation, offspring, the next generation; results of a hybridization • F1 individuals are heterozygous for the trait in question • F2 = the next generation, produced by intermating F1 individuals from the same cross, or by self-pollinating one F1 individual

6. Monohybrid Cross • A cross between two parents that are different from each other in only one trait • E.g., Tall x short; in all other respects the 2 plants are the same

7. Genes & Alleles • Gene: a section of a chromosome responsible for one trait; e.g., gene for flower color, gene for seed shape. • Remember, chromosomes come in pairs, so a diploid organism has two pieces of DNA for each trait. • Those two pieces of DNA are on homologous chromosomes (Fig. 11.4) • Allele: alternate form of a gene; one of two or more possible versions of a gene • E.g., a diploid pea plant could have a “purple” allele for flower color, and a “white” allele.

8. Gene Locus • Each chromosome carries hundreds – thousands of genes. • Genes for different traits are arranged one after another along the length of the chromosome. • The gene locus is the position on the chromosome where a particular allele exists – its “address” • Ordinarily, that “address” doesn’t change, even as chromosomes are passed from parents to offspring.

9. Heterozygous & Homozygous • Heterozygous: having two different alleles at a gene locus (e.g., T and t) • Homozygous: having identical alleles at a gene locus (e.g., T and T, or t and t) • Homozygous dominant: T T • Homozygous recessive: t t

10. Phenotype: the physical appearance of an organism (regarding the trait in question) e.g., “tall”, or “short”, or “purple” or “white”, etc. See Table 11.1 Genotype: the alleles that an organism carries, causing it to have a specific phenotype e.g., “T T” or “T t” or “P p” or “heterozygous” or “homozygous dominant” etc.

11. One-Trait Problems • Usually start with homozygous parents • Each parent has two alleles • Remember: gametes are haploid: they receive only one allele. • F1 individual gets one allele from each parent. • F1 individuals can produce 2 kinds of gametes. • Remember: gametes are haploid: they receive only one allele. • F2 = F1 x F1

12. 11.2: Mendel’s Law of Segregation: • An individual (diploid) has 2 factors for any trait. • Those 2 factors segregate when gametes form. • Each gamete gets only one of the two factors. • With fertilization, a new individual is formed, who again has 2 factors.

13. Laws of Probability • Multiplicative: Probability of two independent events occurring together is the product of their probabilities occurring separately. • Additive: Probability of an event that can occur in two or more different ways is the sum of the individual probabilities.

14. Punnett Square • Female gametes across top • Male gametes on left side • Squares show genotypes of offspring • Numbers of squares predict expected outcomes • Punnett squares use probability laws: • Frequency of different alleles • Frequency of combining alleles • Frequency of independent events

15. The Testcross • A cross between an individual with dominant phenotype and one with recessive phenotype • It’s a way to determine genotype of dominant-phenotype individual • Resulting phenotypic ratio determines whether that individual was homozygous dominant or heterozygous. • E.g., T - x t t ?

16. 11.3: Independent Assortment • Dihybrid cross: a cross in which true-breeding individuals differ in two traits • Law says: • 2 factors for one trait segregate (separate) independent of the 2 factors for the other trait • Gametes have all possible combinations

17. More on Independent Assortment • Addresses which kinds of gametes form in a dihybrid cross: where true-breeding individuals differ in two traits • E.g., LLGG x llgg, or TTgg x ttGG • Each gamete receives only one allele for each trait. • Parents can contribute only those alleles that they possess.

18. Two-Trait Genetics Problems • Dihybrid: parents differ for two traits • F1 individuals are heterozygous: AaBa • F1 individuals produce 4 types of gametes • Probability of any type of gamete: ¼ = 25% • Probability of any one gamete meeting up with any one gamete from other parent = ¼ x ¼ = 1/16 (multip. Law of Prob.) • Probability of a certain genotype being produced in the F2 = sum of all the ways of producing that genotype (add. Law of Prob.)

19. Two-Trait Problems, cont’d: • Fig. 11.8: F2 • Probability of lg gamete from either F1 parent = ¼ • Probability of llgg genotype in F2 generation = ¼ x ¼ = 1/16 • Notice: 1 of the 16 boxes in the Punnett square has the genotype llgg (not a coincidence!) • Phenotypic ratios: count no. of boxes with each possible phenotype (e.g., “long grey”)

20. 11.4: Human Genetic Disorders • Autosomes: Non-sex-chromosomes (1-22) • Pedigree: diagram to track human traits • Circles = females • Squares = males • Shaded = affected • Rows = generations • Connecting lines: • Horizontal: marriage • Vertical: offspring

21. Autosomal Recessive Disorders • Individual must be aa to have disorder • Examples: • Tay-Sachs • Cystic Fibrosis • Phenylketonuria • Sickle-cell Disease • But heterozygotes show some symptoms

22. Autosomal Dominant Disorders • Genotypes AA or Aa have disorder • Examples: • Neurofibromatosis • Huntington Disease • Symptoms don’t show up until mid-life • Achondroplasia (dwarfism) • Genotype AA is lethal

23. 11.4: Beyond Mendel Some traits don’t behave like those Mendel described: • Incomplete Dominance:Aa phenotypes are distinguishable from AA phenotypes • E.g., flower color, sickle-cell disease • Multiple Allelic Traits: More than 2 alleles • E.g., ABO blood types

24. Beyond Mendel, Cont’d • Polygenic Inheritance: More than one gene affects a single trait • Examples: Seed color in wheat; height, skin color in humans • Environmental Effects: Environment affects whether or not an allele is expressed • E.g., fur color in rabbits, Siamese cats