Patterns of Inheritance/Mendelian Genetics Chapter 9, 12 Student Learning Goals & Achievement Scale Biology Mendel s Laws, Genetics, and Patterns of Inheritance SC.912.L.16.1 Goals: Use Mendel s Laws of Segregation and Independent Assortment to analyze patterns of inheritance. 4 - Explore Mendel s Laws of Segregation and Independent Assortment to analyze patterns of inheritance. 3 - Use Mendel s Laws of Segregation and Independent Assortment to analyze patterns of inheritance. 2 - Summarize Mendel s Laws of Segregation and Independent Assortment to analyze patterns of inheritance. 1 Define Mendel s Laws of Segregation and Independent Assortment to analyze patterns of inheritance. 2 Learning Objectives 1. Describe how Mendel was able to control how his pea plants were pollinated. 2. Describe the steps in Mendel s experiments on true-breeding garden peas. 3. Distinguish between dominant and recessive traits. 4. State two laws of heredity that were developed from Mendel s work 5. Describe how Mendel s results can be explained by scientific knowledge of genes and chromosomes 3 Learning Objectives 6. Differentiate between the genotype and phenotype of an organism. 7. Explain how probability is used to predict the results of genetic crosses. 8. Use a Punnett square to predict the results of a monohybrid and dihybrid genetic crosses. 9. Explain how a testcross is used to show the genotype of an individual whose phenotype expresses the dominant trait. 10. Differentiate a monohybrid cross from a dihybrid cross. 11. Distinguish between sex chromosomes and autosomes. 12. Explain the role of sex chromosomes in sex determination. 13. Describe how an X or Y-linked gene affects the inheritance of traits. 14. Explain the effect of crossing-over on the inheritance of genes in linkage groups. 15. Distinguish between chromosome mutations and gene mutations. 4 Learning Objectives 16. Analyze pedigrees to determine how genetic traits and genetic disorders are inherited. 17.Explain the inheritance of the ABO blood type 18.Explain how geneticists can detect and treat genetic disorders. Schedule and Announcements Quiz Thursday December 3 Exam 3- Tuesday December 8 over 9, 12 Semester Exam Tuesday December 15 @ 2:15 (cumulative) 5 6 1
Early Ideas of Heredity Before the 20 th century, 2 concepts were the basis for ideas about heredity: 1.) heredity occurs within species 2.) traits are transmitted directly from parent to offspring (The homunculus myth) This led to the belief that inheritance is a matter of blending traits from the parents. 7 Gregor Mendel Mendel song (http://www.youtube.com/watch?v= 2xpTz7SUbnc) Born in 1822 Education: University of Vienna Failed exit examinations Returned to monastery Mendel published his work in 1865. That work was lost until ca. 1900. With the rediscovery of Mendel s conceptual work the hunt was on for the physical nature of the gene. What was it and how did it function? These questions were largely answered from 1940 s through the 1960 s and lead to the biotech 8 revolution beginning of the 1970 s. Pisum sativum Easy to grow Produces many varieties Male and female organs in the same flower Self-fertilization Cross-fertilization What if Mendel choose to work with sheep instead? The Garden Pea Early Ideas of Heredity Mendel s experimental method: 1. produce true-breeding strains for each trait he was studying 2. cross-fertilize true-breeding strains having alternate forms of a trait -perform reciprocal crosses as well 3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait 9 10 Monohybrid Crosses Monohybrid cross: a cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits each trait had 2 alternate forms (variations) Mendel cross-fertilized the 2 true-breeding strains for each trait 11 12 2
Monohybrid Crosses F 1 generation (1 st filial generation): offspring produced by crossing 2 truebreeding strains For every trait Mendel studied, all F 1 plants resembled only 1 parent no plants with characteristics intermediate between the 2 parents were produced 13 14 Monohybrid Crosses F 1 generation: offspring resulting from a cross of true-breeding parents F 2 generation: offspring resulting from the self-fertilization of F 1 plants dominant: the form of each trait expressed in the F 1 plants recessive: the form of the trait not seen in the F 1 plants 15 Monohybrid Crosses F 2 plants exhibited both forms of the trait in a very specific pattern: ¾ plants with the dominant form ¼ plant with the recessive form The dominant to recessive ratio was 3 : 1. Mendel discovered the ratio is actually: 1 true-breeding dominant plant 2 not-true-breeding dominant plants 1 true-breeding recessive plant 16 Monohybrid Crosses gene: information for a trait passed from parent to offspring alleles: alternate forms of a gene homozygous: having 2 of the same allele heterozygous: having 2 different alleles Monohybrid Crosses genotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual 17 18 3
Monohybrid Crosses Principle of Segregation Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization. 19 20 Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross for example: RR YY x rryy The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait. Dihybrid Crosses The F 2 generation is produced by crossing members of the F 1 generation with each other or allowing self-fertilization of the F 1. for example RrYy x RrYy The F 2 generation shows all four possible phenotypes in a set ratio: 9 : 3 : 3 : 1 21 22 Dihybrid Crosses Principle of Independent Assortment In a dihybrid cross, the alleles of each gene assort independently. 23 24 4
Probability Predicting Results Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities. When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining pp (1/4) ¼ + ¼ = ½ Probability Predicting Results Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities. When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ = 1/16 25 26 Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent 27 28 Extensions to Mendel Mendel s model of inheritance assumes that: each trait is controlled by a single gene each gene has only 2 alleles there is a clear dominant-recessive relationship between the alleles Most genes do not meet these criteria. Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 29 5
Extensions to Mendel Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes. Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes. 31 32 Multiple Alleles Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene: I A, I B, and i. The enzyme (I) adds specific carbohydrates to the surface of blood cells The enzyme encoded by I A adds the A carbohydrate, and the enzyme encoded by I B adds the B carbohydrate; the enzyme encoded by the i allele adds neither Figure 11.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele Carbohydrate (b) Blood group genotypes and phenotypes Genotype Red blood cell appearance A I A I A I A or I A i B I B I B or I B i I B I A I B i none ii Extensions to Mendel Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait. The phenotype is an accumulation of contributions by multiple genes. These traits show continuous variation and are referred to as quantitative traits. For example human height Phenotype (blood group) A B AB O 36 6
Epistasis Figure 11.12 BbEe BbEe In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in Labrador retrievers and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Eggs ¼ BE BBEE ¼ be ¼ Be Sperm ¼ BE ¼ be BbEE ¼ Be BBEe ¼ be BbEe BbEE bbee BbEe bbee BBEe BbEe BBee Bbee ¼ be BbEe bbee Bbee 9 : 3 : 4 bbee Polygenic Traits Extensions to Mendel Pleiotropy refers to an allele which has more than one effect on the phenotype. This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. In these diseases, multiple symptoms can be traced back to one defective allele. 39 40 Extensions to Mendel Extensions to Mendel The expression of some genes can be influenced by the environment. for example: coat color in Himalayan rabbits and Siamese cats an allele produces an enzyme that allows pigment production only at temperatures below 30 o C 41 42 7
Figure 11.14 Key Male 1st generation (grandparents) 3rd generation (two sisters) Female Ww ww Widow s peak Affected male WW or Ww ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww Affected female Ww ww No widow s peak (a) Is a widow s peak a dominant or recessive trait? Mating FF or ff Attached earlobe ff ff Offspring, in birth order (first-born on left) ff FF or ff Free earlobe (b) Is an attached earlobe a dominant or recessive trait? The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal Most people who have recessive disorders are born to parents who are carriers of the disorder Figure 11.15 Eggs A a Normal Aa Sperm A AA Normal Aa Normal (carrier) Parents Normal Aa a Aa Normal (carrier) aa Albino Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals, all hemoglobin is abnormal (sickle-cell) Symptoms include physical weakness, pain, organ damage, and even paralysis Figure 11.UN05 Relationship among alleles of a single gene Description Example Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms About one out of ten African-Americans has sickle-cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous Complete dominance of one allele Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Heterozygous phenotype same as that of homozygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters PP Pp C R C R C R C W C W C W I A I B ABO blood group alleles I A, I B, i Sickle-cell disease 8
Figure 11.UN06 Relationship among two or more genes Epistasis Description The phenotypic expression of one gene affects the expression of another gene Example BbEe BbEe BE be Be be BE Chromosomes, Mapping, and the Meiosis-Inheritance Connection be Be be 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc Chromosome Theory Chromosomal theory of inheritance developed in 1902 by Walter Sutton proposed that genes are present on chromosomes based on observations that homologous chromosomes pair with each other during meiosis supporting evidence was provided by work with fruit flies Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila red-eyed female X white-eyed male gave a F 1 generation of all red eyes Morgan concluded that red eyes are dominant 51 52 Chromosome Theory Morgan crossed F 1 females X F 1 males F 2 generation contained red and whiteeyed flies but all white-eyed flies were male testcross of a F 1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene is linked to the X chromosome 53 Chromosomal basis of sex linkage White-eyed male flies X red-eyed females F1 flies all have red eyes F2 flies, all of the white-eyed flies are males because the Y chromosome lacks the white gene 54 9
Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes 2 X chromosomes = female 1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome 2 X chromosomes = female having a Y chromosome (XY) = male 55 Sex Chromosomes In many organisms, the Y chromosome is greatly reduced or inactive. genes on the X chromosome are present in only 1 copy in males sex-linked traits: controlled by genes present on the X chromosome Human X-linked disorders Color blindness, Muscular dystrophy, Hemophilia, Fragile X syndrome Sex-linked traits show inheritance patterns different than those of genes on autosomes. 56 Royal Hemophilia Pedigree Sex Chromosomes Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1. In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body. Females heterozygous for genes on the X chromosome are genetic mosaics. 57 58 Genetic basis behind a calico cat Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes. traits controlled by these genes do not follow the chromosomal theory of inheritance genes from mitochondria and chloroplasts are often passed to the offspring by only one parent 59 60 10
Chromosome Theory Exceptions Maternal inheritance: uniparental (oneparent) inheritance from the mother the mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization in plants, the chloroplasts are often inherited from the mother, although this is species dependent Human X Chromosome Gene Map 61 62 Human Genetic Disorders Sickle-Cell Anemia Some human genetic disorders are caused by altered proteins. the altered protein is encoded by a mutated DNA sequence the altered protein does not function correctly, causing a change to the phenotype the protein can be altered at only a single amino acid (e.g. sickle cell anemia) 63 64 Human Genetic Disorders Down Syndrome Some genetic disorders are caused by a change in the number of chromosomes. nondisjunction during meiosis can create gametes having one too many or one too few chromosomes fertilization of these gametes creates trisomic or monosomic individuals Down syndrome is trisomy of chromosome 21 65 66 11
Human Genetic Disorders Nondisjunction of sex chromosomes can result in: Syndrome Sex Disorder Chromosome Spontaneous # abortions Live births Abnormalities in the # of sex chromosomes Turner F XO 45 1/18 1/ 2,500 Klinefelter M XXY OR XXXY Poly-X F XXX OR XXXX 47 or 48 1/300 1/800 47 or 48 0 1/ 1,500 Jacobs M XYY 47? 1/1,000 Down M or F Trisomy 21 47 1/40 1/800 67 68 Human Genetic Disorders Amniocentesis Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring. Some genetic disorders can be diagnosed during pregnancy. amniocentesis collects fetal cells from the amniotic fluid for examination chorionic villi sampling collects cells from the placenta for examination 69 70 Chorionic villi sampling 71 12