Unit 5

2. Unit 5 Chapter 13: Patterns in Inherited Traits

3. Gregor Mendel • Austrian monk, Father of Genetics • Genetics: the study of heredity • Heredity: the passing of traits from parent to offspring (INHERITANCE) • Mendel used pea plants to study heredity • Pea plants self-fertilize, so Mendel cross-pollinated peas by hand to observe traits of their offspring • Started with pea plants that “bred true” for a trait (stays the same every generation), then cross-fertilized pea plants with different traits • Offspring appeared in predictable patterns, he concluded that hereditary information is passed in discrete units

4. Gregor Mendel • Parent Generation (P): 1st line of crosses • First Generation(F1): offspring of the parent generation • F2 Generation: second cross, using the F1 offspring

5. Inheritance in Modern Terms • Individuals share certain traits because their chromosomes carry the same genes • The DNA sequence of each gene occurs at a specific location on a particular chromosome

6. Sexual Reproduction • Alleles: Different forms of the same gene • For example: height • Tall or short • We inherit one allele for a gene from each parent • Offspring of sexual reproducers inherit new combinations of parental alleles which produces new traits • Genetic variation!!!!

7. Inheritance in Modern Terms • Alleles are either dominant or recessive • Dominant alleles mask recessive alleles, the recessive allele is there, just hidden • A dominant allele is represented by italic capital letters (A) • A recessive allele is represented by italic lowercase letters (a)

8. Inheritance in Modern Times • Individuals only need to inherit one dominant allele to see that trait – DOMINANCE • TWO recessive alleles must be inherited to see that trait • Homozygous: An individual carrying identical alleles for a gene [true or pure] – AA or aa • Heterozygous: An individual carrying two different alleles of a gene [hybrids] – Aa • Parents of these offspring breed true for a trait • We see the dominant trait, the organism carries the recessive trait

9. Inheritance in Modern Terms • Genotype: the particular set of alleles that an individual carries [heterozygous or homozygous] • Phenotype: the observable traits, such as flower color • Genotype gives rise to phenotype

10. Inheritance in Modern Terms • EXAMPLE – • If detached earlobes are dominant, attached earlobes are recessive • TT – homozygous dominant • Tt – heterozygous, since the dominant allele is present, it will show • tt– homozygous recessive T t = Tall GENOTYPE (HETEROZYGOUS) PHENOTYPE ALLELE ALLELE

11. Mendel’s Law of Segregation • Two alleles for a trait separate during meiosis and are reunited during fertilization • Each gamete will have a different allele

12. Mendel’s Law of Independent Assortment • When homologous chromosomes separate during meiosis, they are randomly assorted in to any nucleus • Gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes • Every person with brown hair doesn’t have brown eyes • Some genes are inherited together (LINKED) because the genes are very close to each other on the chromosome. • People with red hair are also fair-skinned. • Punnett squares can be used to predict inheritance

14. Punnett Square • Punnett Square: grid used to predict the genotypic and phenotypic outcome of a cross • Monohybrid: crossing one trait at a time • Dihybrid: crossing two traits • Testcross: Breeding experiments used to determine genotype

15. Punnett Square • An individual shows a dominant trait (RR or Rr) and is crossed with one that is homozygous recessive (rr) • What is the unknown genotype? • If all offspring have the dominant trait = homozygous dominant • If any offspring have the recessive trait = heterozygous

16. Dominant Trait (T) is tongue roller • Recessive Trait (t) is non-tongue roller • 2 heterozygous (Tt) parents T t Genotypic Homo Dominant = 1 Heterozygous = 2 Homo Recessive = 1 1:2:1 TT Tt T Tongue roller Tongue roller tt Tt t Phenotypic Tongue Roller = 3 Non-tongue Roller = 1 3:1 Non-tongue roller Tongue roller Probability of having a child who could roll their tongue? [3 of 4 = 75%] A child who could not? [1 of 4 = 25%]

17. Dihybrid Cross • Individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed

18. Complex Inheritance • Simple Dominance: dominant allele fully masks the expression of a recessive one • Other patterns of inheritance are not so simple, and cannot be explained by Mendel’s law of inheritance • Incomplete dominance • Codominance • Epistasis • Pleiotropy

19. Complex Inheritance • Incomplete Dominance: One allele is not fully dominant over another • heterozygous phenotype is a blend of homozygous phenotypes • Codominance: heterozygous genotype expresses BOTH alleles

20. Complex Inheritance • Multiple Alleles: gene for which three or more alleles exist in a population • Example: an ABO gene for blood type • This doesn’t mean an organism has more than 2 alleles for atrait, just that more than 2 exist in the population

21. Complex Inheritance • The A and the B allele are codominant when paired • Genotype AB = type AB • The O allele is recessive when paired with either A or B • Genotype AA or AO= type A • Genotype BB or BO= type B • Genotype OO = type O

22. Complex Inheritance • Epistasis: a trait is influenced by the products of multiple genes • Fur color in dogs – A dominant allele (B) specifies black fur, recessive (b) specifies brown fur, but a dominant allele of a different gene (E) causes color to be deposited in fur and the recessive (e) reduces color • E and B allele =black fur, E and bb = brown, ee = yellow fur regardless of Bor b alleles

23. Complex Inheritance • Pleiotropy: A gene whose product influences multiple traits, also called polygenic traits • Mutations in pleiotropic genes are associated with complex genetic disorders • Sickle-cell anaemia, Cystic fibrosis, Marfan syndrome

24. Epigenetics • Potentially inheritable mechanisms that alter genes without changing the DNA sequence • These can cause environmental adaptation faster than evolution, and the changes can be reversed • Chemicals and diet can cause changes, but traumatic experiences in our past, or even our recent ancestors’ pasts, can leave molecular scars on our DNA

25. Environmental Factors • Epigenetic research is revealing that environment can influence phenotype • Toxic agents, Diet and exercise, Sunlight and water, Temperature, Medications • Conditions can cause a gene to shut down or turn on • Twins have identical genes, scientists conclude that twins with different phenotypes are influenced by the environment • “Nature versus nurture”