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What happens to all those genes? Genetic inheritance Copyright © 2009 Pearson Education, Inc. Let’s look at an interesting example… the hybrid Hybrid: the offspring of two different varieties Cross a female tigress (Panthera tigress) with a male lion (Panthera leo) and get a LIGER Males are sterile, females “can” reproduce VERY large; have lived to 24 years but often die shortly after birth Copyright © 2009 Pearson Education, Inc. http://www.bing.com/images/search?q=liger&view=detail&id=73F46F59A3487 133F45EC7D338EC493CF0A7EA0D&first=1&FORM=IDFRIR&qpvt=liger What determines what the liger will look like, if it will live, if it can reproduce…? Copyright © 2009 Pearson Education, Inc. What determines what the liger will look like, if it will live, if it can reproduce…? Genes Copyright © 2009 Pearson Education, Inc. Gregor Mendel discovered genetics using the garden pea Genes: Heritable factors passed from parent to offpsring Advantages of using pea plants Controlled matings Self-fertilization or cross-fertilization Observable characteristics with two distinct traits Copyright © 2009 Pearson Education, Inc. Petal Stamen Carpel White 1 Removed stamens from purple flower Stamens Carpel Parents (P) 2 Purple 3 Transferred pollen from stamens of white flower to carpel of purple flower Pollinated carpel matured into pod 4 Offspring (F1) Planted seeds from pod Character Flower color Traits Purple White Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Tall Dwarf Flower position Stem length P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F1 plants (F1 ´ F1) F2 generation 3 – 4 Copyright © 2009 Pearson Education, Inc. of plants have purple flowers 1 – 4 of plants have white flowers A monohybrid cross Track ONE character Parental (P) generation: purple flowers white flowers F1 generation: all plants with purple flowers F2 generation: ¾ of plants with purple flowers ¼ of plants with white flowers Questions? Why did one trait seemed to disappear in the F1 generation? Why that trait reappeared in one quarter of the F2 offspring? Copyright © 2009 Pearson Education, Inc. Here is why (4 hypotheses): 1. Genes are found in alternative versions called alleles (all alleles/genes found in an organism are called the genotype) 2. For each characteristic, an organism inherits two alleles; the alleles can be the same or different A homozygous genotype has identical alleles A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc. Here is why (4 hypotheses): 3. If alleles are different: - The dominant allele determines the organism’s appearance (phenotype) - The recessive allele has no noticeable effect 4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene How do we use these to explain Mendel’s experiment?? Copyright © 2009 Pearson Education, Inc. Genetic makeup (alleles) pp PP P plants Gametes All p All P F1 plants (hybrids) All Pp Gametes 1 – 2 1 – 2 P p Sperm P F2 plants Phenotypic ratio 3 purple : 1 white p P PP Pp p Pp pp Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp Punnett square On homologous chromosomes, alleles of a gene reside at the same locus Homozygous individuals have the same allele on both homologues Heterozygous individuals have a different allele on each homologue Gene loci P a B P a b PP Homozygous for the allele Copyright © 2009 Pearson Education,dominant Inc. Genotype: Dominant allele aa Homozygous for the recessive allele Recessive allele Bb Heterozygous Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F1 generation rryy RRYY Gametes RY RrYy RrYy Sperm 1 – 2 F2 generation 1 – 2 RY 1 – 2 Eggs 1 – 2 1 – 4 ry RY ry 1 – 4 RY 1 – 4 rY Eggs 1 – 4 Hypothesized (not actually seen) ry 1 – 4 RY 1 – 4 Sperm 1 – Ry rY 4 1 – 4 RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy 9 –– 16 Ry RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry Actual results (support hypothesis) Copyright © 2009 Pearson Education, Inc. ry 3 –– 16 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled A dihybrid cross Track TWO characters Parental generation: round yellow seeds x wrinkled green seeds F1 generation: all plants with round yellow seeds F2 generation: 9/16 of plants with round yellow seeds 3/16 of plants with round green seeds 3/16 of plants with wrinkled yellow seeds 1/16 of plants with wrinkled green seeds Questions? Why nonparental combinations were observed Why a 9:3:3:1 ratio was observed among the F2 offspring Copyright © 2009 Pearson Education, Inc. Law of independent assortment Each pair of alleles segregates independently of the other pairs of alleles during gamete formation For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry Copyright © 2009 Pearson Education, Inc. Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F1 generation rryy RRYY ry Gametes RY RrYy RrYy Sperm Sperm 1 – 2 F2 generation 1 – 2 RY 1 – 2 1 – 4 ry 1 – 4 RY Eggs 1 – 2 RY 1 – 4 ry Hypothesized (not actually seen) 1 – 4 rY 1 – 4 Ry 1 – 4 ry RY RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy rY Eggs 1 – 4 1 – 4 9 –– 16 Ry RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry Actual results (support hypothesis) 3 –– 16 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled Let’s try a dihybrid cross…. Characters: 1. Earlobes- Free is dominant (F) Attached is recessive (f) 2. Tongue rolling- Rollers are dominant (R) Nonrollers are recessive (r) Cross a true breeding unattached roller with an attached nonroller to the F2 generation Copyright © 2009 Pearson Education, Inc. What if we did NOT know the genotype of our unattached tongue roller?? Testcross: Mating between an individual of unknown genotype and a homozygous recessive individual Will show whether the unknown genotype includes a recessive allele Used by Mendel to confirm true-breeding genotypes Copyright © 2009 Pearson Education, Inc. Remember: Mendel’s laws reflect probability and statistics What is the probability that the Olsen sisters have such similar genes but are NOT identical twins? Copyright © 2009 Pearson Education, Inc. The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. 1. Rule of multiplication Multiply the probabilities of events that must occur together Example: Cross two AaBbCc parents What is probability that offspring will be aabbcc? 2. Rule of addition Add probabilities of events that can happen in alternate ways Copyright © 2009 Pearson Education, Inc. F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 – 2 1 – 2 1 – 2 B B B b B B 1 – 4 1 – 4 1 – 2 b b B 1 – 4 F2 genotypes b b b 1 – 4 Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Law of independent assortment: Each pair of alleles segregates independently of the other pairs of alleles during gamete formation VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc. Incomplete dominance P generation Red RR White rr r Gametes R Neither allele is dominant over the other F1 generation Pink Rr 1 Heterozygous individual showsan intermediate phenotype Gametes–2 R 1 – 2 F2 generation 1 – 2 1 – 2 r Sperm 1 – r R 2 R RR rR r Rr rr Eggs 1 – 2 Copyright © 2009 Pearson Education, Inc. Multiple alleles A diploid individual can carry any two of these alleles Example: ABO blood group Three alleles: IA, IB, and i Four phenotypes: Type A, Type B, Type AB and Type O blood Copyright © 2009 Pearson Education, Inc. Codominance Neither allele is dominant over the other Expression of both alleles is observed as a distinct phenotype in the heterozygous individual Copyright © 2009 Pearson Education, Inc. Blood Group (Phenotype) Genotypes Red Blood Cells O ii A IAIA or IAi Carbohydrate A B IBIB or IBi Carbohydrate B AB IAIB Pleiotropy One gene influencing many characteristics Example: The gene for sickle cell disease Affects the type of hemoglobin produced Affects the shape of red blood cells Causes anemia Causes organ damage Is related to susceptibility to malaria Copyright © 2009 Pearson Education, Inc. Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Physical weakness Impaired mental function Anemia Heart failure Paralysis Pain and fever Pneumonia and other infections Accumulation of sickled cells in spleen Brain damage Damage to other organs Rheumatism Spleen damage Kidney failure Polygenic inheritance Many genes influence one trait Skin color is affected by at least three genes Copyright © 2009 Pearson Education, Inc. P generation aabbcc (very light) AABBCC (very dark) F1 generation AaBbCc F2 generation 1 – 8 1 – 8 1 – 8 1 – Eggs 1 8 – 8 1 – 8 1 – 8 1 – 8 1 –– 64 1 – 8 6 –– 64 1 – 8 15 –– 64 AaBbCc Sperm 1 – 1 – 1 – 8 8 8 20 –– 64 15 –– 64 1 – 8 1 – 8 6 –– 64 1 – 8 1 –– 64 Summary of variations on Mendel’s laws Incomplete dominance White rr Red RR Pink Rr Pleiotropy Single gene Multiple genes Multiple characters Polygenic inheritance Single characters (such as skin color) INHERITANCE OF GENES Genetic traits in humans can be tracked through family pedigrees Pedigree: Shows the inheritance of a trait in a family through multiple generations Demonstrates dominant or recessive inheritance Can also be used to deduce genotypes of family members Free earlobe Copyright © 2009 Pearson Education, Inc. Attached earlobe First generation (grandparents) Ff Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters) Female Male Affected Unaffected Ff ff ff ff Ff Ff Ff ff ff FF or Ff AUTOSOMAL DISORDERS 1. Autosomal recessive inheritance Two recessive alleles are needed to show disease Heterozygous parents are carriers Probability of inheritance increases with inbreeding Parents Normal Dd Normal Dd ´ Sperm D Offspring d DD Normal Dd Normal (carrier) Dd Normal (carrier) dd Deaf Eggs d Copyright © 2009 Pearson Education, Inc. D 2. Autosomal dominant inheritance One dominant allele is needed to show disease Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc. SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc. Chromosomes determine sex in many species (male) 44 + XY X-Y system in mammals, fruit flies XX = female; XY = male X-O system in grasshoppers and roaches XX = female; XO = male Chromosome number in ants and bees Diploid = female; haploid = male Copyright © 2009 Pearson Education, Inc. 22 + X Parents’ diploid cells (female) 44 + XX 22 + Y Sperm 22 + X 44 + XX 44 + XY Egg Offspring (diploid) 22 + XX 22 + X 32 16 Sex-linked genes exhibit a unique pattern of inheritence Sex-linked genes are located on either of the sex chromosomes X-linked genes (Xg) can be passed from: ____________ to _______ and __________ ____________ to _______ Y-linked genes (Yg) can be passed from: ____________ to _______ Copyright © 2009 Pearson Education, Inc. Example of sex-linked genes: fruit fly eye color Female Male Xr Y XR XR Sperm Eggs XR Xr Y XR Xr XR Y R = red-eye allele r = white-eye allele Female Male XR Xr Xr Y Sperm Xr Y XR XR XR XR Y Xr Xr Xr Xr Y Eggs Sex-linked disorders affect mostly males Males express X-linked recessive disorders because they only have ONE copy Hemophilia Colorblindness Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Copyright © 2009 Pearson Education, Inc. THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2009 Pearson Education, Inc. Mendel’s Laws rely on chromosome separation in meiosis The law of segregation depends on separation of homologous chromosomes in anaphase I The law of independent assortment depends on alternative orientations of chromosomes in metaphase I Copyright © 2009 Pearson Education, Inc. F1 generation All round yellow seeds (RrYy) R r y Y r R y Y R Y y Y y R R Y y Anaphase I of meiosis r Y R r R Y Metaphase I of meiosis (alternative arrangements) r Metaphase II of meiosis r Y y r R Y y y Y Y r r r 1 – ry 4 1 – rY 4 Fertilization among the F1 plants F2 generation R Gametes y 1 – RY 4 r 9 :3 :3 :1 y y R R 1 – 4 Ry