Notes: Genetics

1. Notes: Genetics Mendel & Human Genetics (Honors/Gifted)

2. Genetics & Heredity • What is genetics? • the science of heredity • What is heredity? • passing of traits from parents to offspring

3. Gregor Mendel • Austrian Monk interested in inheritance • Studied offspring from different “matings,” or crosses, of pea plants. • He began his research with pea plants that were considered true-breeding. • Plants that have the ability to produce offspring identical to themselves, through the use of their own gametes = “Self-Pollination” = true breeding

4. Gregor Mendel • Most plants reproduce via cross-pollination, the combining of gametes from two separate plants. • Mendel DID NOT want to have this extra variable in his research, so he prevented his plants from “self-pollinating” by crossing the pea plants himself. • He began his research looking at 7 different pea plant TRAITS • a characteristic that varies from one organism to another

5. Mendel’s Research • All 7 traits Mendel studied occurred in one of 2 forms. • He cross-pollinated plants with different forms of the same trait (i.e. purple x white) to examine offspring.

6. After crossing these plants Mendel discovered that… • Inheritance is determined by genes. • Segments of DNA that determine a specific trait • Some alleles (different forms of a gene) are considered Dominant and some are Recessive: The Principle of Dominance. • Dominant alleles that are expressed (usually capital letters) • Recessive  alleles that are hidden (usually lower-case)

7. His Experiments • Mendel conducted Monohybrid Crosses (mating involving only one pair of traits). • He cross-pollinated 2 pea plants with opposing traits • The original mating pair he usedwas called the Parental or P – Generation. • Ex  Purple flowering plant & white flowering plant

8. Mendel - Monohybrid Crosses • The offspring of this Parental cross is called the F1Generation, meaning “first filial.” • Results: ALL PLANTS’ FLOWERS WERE PURPLE!

9. Mendel – Monohybrid Crosses • Mendel’s 2nd Experiment… • Mendel allowed the F1 Generation (all purple) to self-pollinate. • Results: The F2 Generation (“second filial”) produced flowers in a ratio of 3 purple: 1 white • This 3:1 ratio occurred for each of the 7 traits in the F2 Generations!

10. Alas…Mendel’s Conclusions • Every inherited trait has 2 copies of the gene – one from each parent. • There are alternative versions of genes (alleles). • When 2 different alleles occur together, one can be completely expressed (dominant) while the other can be hidden (recessive). • Gametes (sperm and eggs) each carry one allele for a given trait; during fertilization, the offspring receives 1 allele from each parent.

11. Now let’s review some basic Terms… • Gene: Segment of DNA that determines a specific trait • Trait: a characteristic that varies from one organism to another • Allele: Different forms of a gene • Dominant Allele: an allele that hides a recessive trait; usually characterized by a capital letter. • Recessive Allele: an allele that can be “masked” or hidden by a dominant allele; usually characterized by a lower-case letter

12. Homozygous vs. Heterozygous • Homozygous: Offspring have two identical alleles (PP = homozygous dominant; pp = homozygous recessive); also known as “pure-bred” • Heterozygous: Offspring has one of each allele (Pp = heterozygous); also known as “hybrid”

13. Genotype v. Phenotypes • Genotype: the set/ combination of alleles an organism has for a certain trait…the “letters” (ex: PP, pp, or Pp) • Phenotype: the physical appearance of a trait in an organism…. the “looks” (ex: blonde hair) Genotype Phenotype

14. The Law of Segregation • 2 alleles for a trait segregate (separate) when gametes are formed; each offspring receives one trait from their parents

15. The Law of Independent Assortment • Alleles for different genes separate independently of each other during gamete formation. • One trait, like flower color, does not influence the inheritance of another trait, like plant height.

16. Probability & Genetics • Probability = the likelihood that a particular event will occur. • Principles of probability can be used to predict the outcomes of genetic crosses. • The more trials conducted, the closer the result will come to the EXPECTED ratio. • The Punnett square can predict the “probability” of outcomes resulting from a genetic cross.

17. Predicting Results:Punnett Squares • Punnett Squaresare diagrams that use the Laws of Segregation and Independent Assortment to predict offspring • Possible gametes for 1 parent are placed along the top of the square; possible gametes for the other parent are written on the left of the square. • The genotypes are predicted by combining alleles from each parent. R r R r

18. For example… • We can use ratios to express genotypes and phenotypes. • Genotypic Ratio = ___ YY: ____ Yy :___yy • Phenotypic Ratio = ____ yellow : _____ green

19. Predicting Results:Punnett Squares • Genotypic Ratio =0 YY : 4 Yy : 0yy4 Yy or 100% Yy • Phenotypic Ratio = 4 yellow : 0 green 100% yellow

20. Predicting Results: Punnett Squares • Dihybrid crosses(crosses involving 2 traits) are a little more complex. • Possible combinations for the different types of alleles are placed at the top and sides of the square. • Example: (Round, Yellow)  (wrinkled, green) RrYyrryy

21. Part II: Beyond Dominant & Recessive Alleles

22. Beyond Dominant & Recessive Alleles… • Some traits are Polygenic, meaning that more than one gene determine the phenotype. • Examples human hair color, eye color, weight, skin color

23. Incomplete Dominance • One form of a trait is NOT dominant or recessive to the other. Results in an “in-between” phenotype - threedifferentphenotypes are possible. • ExampleFour O’Clocks flowerRed (RR) X White (WW)  Pink (RW)

24. Co-dominance • When two dominant alleles are expressed at the same time in an offspring • Example: A homozygous red horse mates with a homozygous white horse to produce a horse with BOTH red and white hair (called a Roan coat).

25. Part III: Blood Types

26. Blood Types: A Result of Multiple Alleles Multiple Alleles– Genes with 3 or more possible alleles determining a trait • Each individual receives only 2 alleles, but there are more than 2 in the population • Example = Blood Type • Blood Phenotypes = A, B, AB, and O • Blood Alleles = IA, IB, and i • IA and IB are both dominant (codominant when together), & i is recessive

28. Blood Typing Chart

29. Blood Type Frequency in Population

30. Part IV: Human Genetics

31. Human Genetics • Sex ChromosomesThe two chromosomes that determine an individuals sex are XX (mom) or XY (dad). • Autosomal Chromosomes The other 44 chromosomes, not sex chromosomes • Karyotype = a picture of chromosomes arranged in 23 matching pairs; Sex chromosomes are ALWAYS on pair #23.

32. 2 Types of Disorders • Sex-Linked Disorders • Chromosomal Disorders ***But first, let’s talk a little about Sex-Linked Traits…***

33. Sex-Linked Traits • All eggs carry an X chromosome • Females are XX; Males carry XY • In females, if a defective gene rides on one of the X chromosomes, the other X is likely to have a good copy of the gene that can take over for the “bad” gene • Males do not carry the backup copy of the X chromosome, so the gene is expressed

34. Sex-Linked Disorders • The information/ traits on the sex chromosomes are called sex-linked genes. Because these chromosomes determine sex, disorders caused by these genes are sex-linked disorders. Typically, these genes are found on the X chromosome. • Examples • Colorblindness • Hemophilia • Duchene Muscular Dystrophy

35. Chromosomal Disorders • Nondisjunction (failure of homologous chromosomes to separate) occurs during meiosis. The resulting individual has an abnormal number of chromosomes and that results in a disorder! • Examples: • Down Syndrome (Trisomy 21) • Turner’s Syndrome (XO) sterile • Klinefelter’s Syndrome (XXY)