Patterns of Inheritance

1. Patterns of Inheritance Chapter 9

2. An Old Genetic Experiment • Genetics is the study of heredity/inheritance • Dogs bred for specific traits • Genome completed in 2003 • Wolves and domestic dogs share a common ancestor

3. History of Inheritance • Blending hypothesis • Information (cells) from each parent produce mixed offspring • Tall and short adults had medium height children • Didn’t explain disappearance/reappearance of traits b/w generations • Gregor Mendel 1860 • Father of genetics • Parents pass on specific heritable factors to offspring • ‘Genes’ don’t blend, but remain the same over generations

4. UsefulGenetics Terminology

5. Alleles • Alternate versions of the same gene, located at the same loci on a specific chromosome • Dominant alleles mask others when both are present (UPPERCASE LETTERS) • Dominance implies it determines phenotype, not superiority or increased prevalence • Recessive alleles are easily masked by others (lowercase letters) • Recessive traits often more common • Individuals inherit 2 alleles  1 maternal and 1 paternal • Actual combination determines genotype • Resulting physical expression determines phenotype

6. Phenotype and Genotype • Homozygous dominant (PP) • 2 dominant alleles • Express dominant trait • Heterozygous (Pp) • 1 dominant and 1 recessive allele • Express dominant trait • Homozygous recessive (pp) • 2 recessive alleles • Express recessive trait • Phenotypic vs. genotypic ratios

7. Why a Pea? Traits Characters • Peas (Pisumsativum) have several characters that vary among individuals • Have distinct traits, or variants, of each character • Can control types of mating/crosses that occurred • Self-fertilize  natural, involves 1 plant • Cross-fertilize  artificial, involves 2+ different plants • Can create true-breeding lines • All individuals genetically identical • Only contain 1 character variant

8. Mendel’s Initial Work • Started with monohybrid crosses • Differ by only 1 trait • Can use Punnett squares to represent hypothetical crosses • All crosses produced same results • Crossing true-breeding tall and short (P) = only tall (F1) • Cross any resulting tall hybrids (F1) = 3:1 ratio (type of ratio?) of tall to short (F2) • Short phenotype disappears but reappears in next generation • Held true for all 7 tested characters

9. Constructing Punnett Squares • Allows determination of all possible genotypes and phenotypes • Remember: • All individuals have 2 alleles for every gene • 1 from mom and 1 from dad • Meiosis produces haploid gametes from diploid cells • Aa mother = A or a eggs • Steps • Place gametes (haploid) of one parent along top, other along the left side • Combine all possible female gametes with all possible male gametes = fertilization • Boxes with 2 alleles = possible offspring (diploids)

10. Practice • Background: • Tall (T) and short (t) • Fill in the boxes to show genotypes • For each box, identify the phenotype • For each punnett square list the genotypic and phenotypic ratios 1._________ 2 ._________ 3._________ 4 ._________ 1._________ 2 ._________ 3._________ 4 ._________ 1._________ 2 ._________ 3._________ 4 ._________ 1._________ 2 ._________ 3._________ 4 ._________ Adopted from: http://www.exploringnature.org/db/detail.php?dbID=22&detID=2290

11. Identifying Individuals in Crosses • P (parental) generation • F1(first filial) generation • F2(second filial) generation • And so on …

12. Mendel’s Work (cont.) • Mendel wanted to be able to identify genotypes of all individuals • Designed a testcross • Cross recessive phenotype with a dominant phenotype • Why is recessive phenotype required? • Determine genotype of a dominant trait • Still used in current research

13. Mendel’s Work (cont.) • Continued with dihybrid crosses • Peas differed by 2 characters • All crosses produced same results • Crossing true-breeding round yellow and wrinkled green (P) = only round yellow (F1) • Cross any resulting round yellow hybrids (F1) = 9:3:3:1 ratio (type of ratio?) of round yellow to round green to wrinkled yellow to wrinkled green(F2) • Wrinkled green phenotype disappears but reappears in next generation along with 2 new phenotypes

14. Mendel’s Law of Segregation • Two alleles of a trait separate during gamete formation • Remember • Homologous chromosomes each carry 1 allele • Meiosis separates these chromosomes  forms haploid gametes

15. Mendel’s Law of Independent Assortment • Genes located on different chromosomes are inherited independently • E.g. hair color doesn’t determine eye color

16. Autosomal Recessive Disorders • Only affects homozygous recessive individuals • Heterozygous is “carrier” • Prevents complete removal of allele from a population • Albinism • Lack of normal amounts of melanin (pigment) in body • Cystic fibrosis • Thick mucus in lungs & digestive tract • Cl- channel abnormality • Most common lethal genetic disorder among Caucasians

17. Autosomal Dominant Disorders • Affects all dominant phenotypes • Lethal types less common • Achondroplasia • Embryonic cartilage in skeleton doesn’t develop properly • “Dwarf”, average 4’ tall • Huntington’s Disease • Nervous system deteriorates • Symptoms often not seen until after 30 • Die in 40s or 50s

18. Pedigrees: an application practice problem Recessive trait: attached earlobe • Diagram family relationships and phenotypes • Allow human heredity to be studied • Can’t control human mating, so look at those naturally occurring • Can indicate type of gene responsible • Sex-linked or autosomal recessive/dominant • Can deduce genotypes of most members from phenotypes • Mendelian genetics and logic female male affected unaffected carrier

19. Not All Genetics Are Simple • Mendel used characters exhibiting complete dominance, offspring look like one of the two parents (simple) • Not applicable to all characters • Genotype and phenotype relationship not so simple • Single genes can have alleles that aren’t completely dominant or recessive • Characters can have 1+ genes (complex) • Basic principles of segregation and independent assortment still apply

20. Variations on Mendelian Genetics • Incomplete dominance • Codominance • Epistasis • Polygenic inheritance • Ignore environmental influences • Nutrition can effect height, sun exposure can alter skin color, exercise can change build, etc. • Nature vs nurture still major debate

21. Incomplete Dominance • Heterozygote offspring has an intermediate of parent’s phenotype • Doesn’t support blending • Each genotype has own phenotype • True breeding red and white cross • Homozygous red and white offspring • Heterozygous pink offspring • 1:2:1 is and genotypic and phenotypic ratio

22. Codominance • Heterozygote offspring expresses two alleles at the same time • Blood type • 3 alleles • 4 phenotypes • 6 genotypes • Universal donor? • Universal acceptor?

23. Epistasis • Gene at one locus alters the phenotypic expression of another gene at a second locus • Coats of mice and Labrador retrievers • B (black) & b (brown) • C (melanin) & c (no melanin) • Possibilities • B_C_ = black • bbC_ = brown • B_cc and bbcc = white or yellow

24. Polygenic Inheritance • An additive effect of 2+ genes on one phenotypic trait • Range of small differences in a trait • Skin color due to different amounts and types of melanin • Height, weight, and iris (eye) color too

25. Chromosomal Inheritance

26. Human Sex Determination • 2 sex chromosomes and 44 autosomes • XX = female and XY = male • Eggs = all X and sperm = X or Y • Sperm cell determines sex • Gene on Y chromosome responsible for ‘maleness’ • SRY gene (TDF production) triggers testes development • Without, ovaries develop • Default sex is female

27. Other Sex Determining Systems • Insects have 1 sex (X) chromosome • Females XX, males X0 • Bees and ants are haploid or diploid • Queen decides • Diploid females, haploid males • Marine fish commonly change • Social hierarchy and balance of sex’s • Alligators and turtles rely on incubation temperature • Plants are complex

28. Sex-linked Genes • Genes that reside on sex chromosomes, but unrelated to genetic sex • X chromosome in humans (generally) • Fathers pass X to all daughters, but no sons • Mothers pass X to all offspring • Can you justify these statements? • X-linked disorders more common and most likely to affect males • X chromosome is larger • Male affected with 1 = hemizygous • Females affected with 2; 1 = carrier • Represented in crosses differently • Need sex chromosome and UPPER or lower case letter to imply affected or not (XnY = affected male, XNXn = carrier female) • Y-linked is rare • Used to track ancestry through male lines

29. Sex-linked Genes • Genes that reside on sex chromosomes but unrelated to genetic sex • X chromsome in humans (generally) • Fathers pass X to all daughters, but no sons • Mothers pass X to all offspring • Can you justify these statements? • X-linked disorders more common and most likely to affect males • X chromosome is larger • Males affected with 1 = hemizygous • Females affected with 2; 1 = carrier • Represented in crosses differently • Need sex chromosome and UPPER or lower case letter to imply affected or not (XnY = affected male, XNXn = carrier female) • Y-linked is rare • Used to track ancestry through male lines

30. Red-Green Color Blindness • Involves several X-linked genes • Some heterozygous females affected (rare) • N represents color-blind gene carried at an X chromosome loci • XN = trait not present, Xn = trait present F: carrier; M: normal F: carrier; M: carrier F: normal; M: affected

31. Hemophilia • X-linked recessive • Allele for clotting factor VIII mutated • Introduced into ruling houses of Russia and Europe