Understanding Mendelian Genetics: Basics of Inheritance and Genetic Variation

1. Ch 14 Mendelian Genetics

2. Pre-Mendel • belief in blending, child is a mix of parents • problem = traits skipping generations • Mendel – monk, mid 1800’s, bred pea plants Terms • Character = detectable, inherited feature, ex. color • Trait = variant of an inheritable character, ex. green or red color • True-Breeding = always produce plants with same traits as parents, self fertilization • Cross-Breeding = cross parents with different traits to create hybrids

3. Generations are named • P = parental • F1= results of PxP • F2= results of F1 x F1

4. Mendel’s experiment • Mendel looked at 7 characteristics, each had 1 alternate form • hypothesis – if a cross of purple & white gives all purple, then a cross between F1’s would produce purple again • experiment – let F1’s self pollinate • results – 3:1 ratio of purple to white flowers, hypothesis wrong • Mendel crossed true-bred peas, and never saw blending • conclusion – inheritable factor of white must be masked: purple is dominant, white is recessive

6. So… • there are alternate forms of the same gene = alleles, p265 • we inherit one allele from each parent • if alleles are different, one is dominant (noted by capital letter), one is recessive (lowercase letter) • alleles segregate during meiosis

7. More Terms • homozygous – 2 identical alleles for a trait, ex. DD, dd • heterozygous – 2 different alleles for a trait, carrier, ex. Dd • phenotype – organism’s expressed traits, ex. color, height • genotype – organism’s genetic makeup, letters, ex. PP, Pp

8. Testcross – a cross between a recessive and an unknown • tells if it is homo or heterozygous • monohybrid cross – dealing with 1 trait • dihybrid cross – 2 traits • Trihybrid – 3 traits, ouch

9. Law of Segregation • = allele pairs separate randomly during meiosis, p. 266 • There are 2 alleles for flower color, if 1 purple and 1 white: there is a 50% chance of getting either allele • Punnett square predict the results

10. Law of independent assortment p.268-269 • when dealing with 2 or more traits, each allele of the different genes segregates independently of each other • If cross 2 dihybrid heterozygotes, get 9:3:3:1 ratio

11. Probability • = mathematical chance of an event happening • Rule of multiplication- probability of 2 events occurring at the same time = product of their individual probabilities • Ex. 2 coins both coming up heads = ½ x ½ = ¼ • Ex. DdRr x DdRr ? probability of getting DDRR • chance of DD = ¼, chance of RR = ¼ so ¼ x ¼ = 1/16

12. Rule of addition – harder to define, p.270, probability that any one of two or more mutually exclusive events will occur is calculated by adding the individual probabilities • Ex. cross of 2 heterozygotes, ? chance of result being hetero? • Chance of recessive egg + dominant sperm = ½ x ½ = ¼ • Chance of dominant egg + recessive sperm = ½ x ½ = ¼ • chance of hetero child is ¼ + ¼ = ½ • Use → trihybrid AaBbCc x AaBbCc ? chance of AabbCC

13. Extensions: • found that Mendel’s laws were not perfect, in fact, he was lucky that he choose peas which have simple inheritance (except pod shape) • Incomplete dominance = 1 allele is not completely dominant over the other thus, there is a 3rd phenotype, intermediate, ex.Carnations/snapdragonsp. 271

14. Codominance • = both alleles are expressed • Level of expression varies at different levels • Tay-sachs • - molecular level – looks codominant • - biochemical level – looks like incomplete→ an intermediate level of lipid-metabolizing activity • - organismal level – hetero’s =symptom free, homo rec. have

15. Multiple Alleles • = genes that have more than 2 alleles • Ex. blood groups A, B, AB, O (surface carbohydrates) • blood type is the antigen present on the RBC, p. 273 • also contains Rh factor, + or -mendelian

17. Pleiotropy = a single gene has multiple affects

18. Epistasis = one gene effects the expression of another gene, Ex. pigments in mice

19. Polygenic inheritance = many genes affect the same trait • Ex. skin color, very dark to very light, p. 274

20. Environment plays an important part in gene expression, how much is not exactly known, nature vs. nurture argument • Norm of Reaction = The phenotypic range for a genotype, p.275

21. Humans • Pedigree – family tree that shows inheritance over many generations, shows patterns •  = male, O = female, ●= diseased, ○= non-diseased

22. Recessive human disorders • - usually caused by a defective protein • - heterozygotes are carriers • Cystic Fibrosis– most common, membrane protein that controls Cl traffic, causes increase mucus in lungs • Tay-Sachs – higher in “Jews”, can’t break down a type of lipid • Sickle cell – substitution in one hemoglobin, causes RBC to sickle and clog, carriers are immune to malaria, p. 278 • Consanguinity – mating with relatives

23. Dominant inherited disorders • – rarer than recessive, not masked • Achondroplasia – type of dwarfism • Huntington's – late acting degeneration of nervous system, chromosme #4 • Multifactoral disorder- many different factors effect, ex. Heart disease, diabetes, cancer

24. Genetic testing and counseling • 1) carrier recognition - help make decisions about children • 2) fetal tests • amniocentesis – take amniotic fluid from around fetus, do karyotype • chorionic villus sampling (CVS) – take villi, do karyoptype, fast, earlier, more risk, p. 280 • ultrasound – imagery using sound waves, look for physical problems • fetoscopy – fiber optics • 3) Newborn screening – ex. PKU

26. Ch 15 Chromosomes and Inheritance

27. Chromosome theory of inheritance: genes are located on chromosomes, they segregate and independently assort

28. T.H.Morgan • rediscovered Mendel’s work 1900’s, specific genes on specific chromosomes? • work on fruit fly, why? • fast repro., easy to handle, 4 pairs of chromosomes (1 pair are sex chromosomes) • gene symbol is based on the mutant or recessive ex. curly is recessive = Cy, if normal then Cy+ • wild type is the type seen in nature = +

29. Experiment- p 289 • white eyed male (♂)→ crossed with a red eyed female (♀)→ in F2 only males had white eyes ?→ eye color and sex are linked • Linked = when genes are on the same chromosome, so they are inherited together (? crossing over), no independent assortment • Sex linked = located on a sex chromosome, p. 290, ex. Hemophilia

30. Recombination • = offspring with different combinations of traits than the parents, caused by crossing over or mutations • Parental types – same phenotype as a parent • Recombinants – differ from parents, *p. 293-294

32. Sturtevant • made chromosome maps • find relative distance between farthest genes, find distance of an end and a middle, fill in other genes • double crossovers can occur too, throw # off a little • 50% frequency of recombinants = 2 genes on different c’somes • use recombination frequency to determine distance of genes • 1% = 1 map unit = 1 cm (centimorgan), ex

34. Heterogametic- produces 2 kinds of gametes • Homogametic- produces only 1 kind of gamete • Humans - ♂ is XY, ♀ is XX, other animals differ • few genes on the Y, thus most sex-linked diseases are seen in males b/c on the X (not masked), females often carriers, p. 290 • X-inactivation = females inactivate one of their X’s, why? inactive X becomes a Barr body = Lyon hypothesis, p.291

35. Nondisjunction –division error, chromosomes don’t separate, mitotic and meiotic, p. 297 • Aneuploidy = having an abnormal # of chromosomes • Trisomy – 3 copies of 1 chromosome • Monosomy – 1 copy of the chromosome • Polyploidy = more than normal chromosome set • Triploidy – 3 chromosome sets (3N)

36. Mutations • Deletion – chromosome loses a piece, p. 298 • Duplication – double of gene • Inversion – chromosome is in reverse • Translocation – gene moves to another chromosome • →caused by UV light, chemicals or random • →effects can be silent, lethal or in between

37. Down Syndrome – trisomy 21, female age makes more frequent? • Klinefelters – XXY, XXXY male, sterile, some female features • XYY – male, usually normal, XXX- female, usually normal • Turner syndrome– X, female, sterile, few sexual features • Some effects of chromosomal abnormalities depend on what parent inherited by (genomic imprinting, p.300) • - prader–willi– deletion of part of #15 from dad, retardation • - angelman – deletion of same part of # 15 from mom, motor issues