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GENETICS FULYA KAHRAMAN Who is Gregor Mendel? Known as the father of modern genetics Conducted experiments with pea plants to discover how traits were passed on from generation to generation Identified the concept of dominant and recessive traits and several laws of heredity Mendelian Genetics Vocab Trait A heritable feature such as flower color Allele Alternate versions of a gene that produce different phenotypes Dominant Allele An allele that is fully expressed in the phenotype of a heterozygote Recessive Allele An allele whose phenotype is not observed in a heterozygote Homozygous Having two identical alleles for a given gene Heterozygous Having two different alleles for a given gene Carrier An individual who is heterozygous with one normal allele and one potentially harmful recessive allele. The individual is phenotypically normal but can pass on the harmful allele Mendelian Genetics Vocab Genotype The genetic makeup or a set of alleles of an organism (ie. Aa, AA, aa) Phenotype The physical traits which are determined by genotype (many phenotypes are microscopic in the phenotype of a heterozygote) True Breeding Plants that-when self-pollinated- always produce the same phenotypic traits (homozygous dominant or recessive) Hyrbridization The mating or crossing of two true-breeding varieties (true breeding parents- P generation) Produces all heterozygous F1 generation offspring Test Cross Breeding an organism of unknown genotype (heterozygous or homozygous dominant) with a homozygous recessive individual to determine the unknown genotype. The Ratio of phenotypes in offspring determines unknown genotype Monohybrid Cross The cross between two heterozygotes (F1 generation) for a single trait (ie. Bb x Bb) Dihybrid Cross The cross between two heterozygotes (F1 generation) for two traits (ie. YyRr x YyRr) Mendelian Genetics Vocab cont. Complete Dominance The situation when the phenotypes of the heterozygote and the dominant homozygote are indistinguishable Incomplete Dominance The situation in which the phenotype of heterozygotes is in between the phenotypes of homozygous individuals for either allele Codominance The situation where the phenotypes of both alleles are exhibited in the heterozygote Multiple Alleles When there are three or more alleles for a single gene (as in ABO blood groups) Polygenic Inheritance An additive effect of two or more genes on a single phenotypic trait Pedigree Analysis Predicting the genotypes of individuals in a pedigree chart based on the phenotypes of offspring Brown coat (C); black eyes (E) White coat (c); pink eyes (e) Brown coat (C); black eyes (E) White coat (c); pink eyes (e) Coat-color genes Eye-color genes Brown Black C E C E C E c e c e Meiosis c White e Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE Copyright © 2009 Pearson Education, Inc. 8.19 A karyotype is a photographic inventory of an individual’s chromosomes – A karyotype shows stained and magnified versions of chromosomes – Karyotypes are produced from dividing white blood cells, stopped at metaphase – Karyotypes allow observation of – Homologous chromosome pairs – Chromosome number – Chromosome structure Copyright © 2009 Pearson Education, Inc. Packed red and white blood cells Centrifuge Blood culture 1 Fluid Hypotonic solution Packed red and white blood cells Centrifuge Blood culture 2 1 Fluid Hypotonic solution Packed red and white blood cells Fixative Stain Centrifuge Blood culture 2 White blood cells 3 1 Fluid 4 Centromere Sister chromatids Pair of homologous chromosomes 5 8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome – Trisomy 21 involves the inheritance of three copies of chromosome 21 – Trisomy 21 is the most common human chromosome abnormality – An imbalance in chromosome number causes Down syndrome, which is characterized by – Characteristic facial features – Susceptibility to disease – Shortened life span – Mental retardation – Variation in characteristics – The incidence increases with the age of the mother Copyright © 2009 Pearson Education, Inc. Infants with Down syndrome (per 1,000 births) 90 80 70 60 50 40 30 20 10 0 20 25 40 30 35 Age of mother 45 50 Nondisjunction in meiosis I Normal meiosis II Gametes n+1 n+1 n–1 Number of chromosomes n–1 Normal meiosis I Nondisjunction in meiosis II Gametes n+1 n–1 n Number of chromosomes n 8.21 Accidents during meiosis can alter chromosome number – Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis – During Meiosis I – Both members of a homologous pair go to one pole – During Meiosis II – Both sister chromatids go to one pole – Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes Copyright © 2009 Pearson Education, Inc. Example of Complete Dominance If you cross Tall (TT) and Short (tt), what do you expect? All talls? Some tall, some short? All shorts? Medium height? Since T is completely dominant, offspring are all tall Example of Incomplete Dominance If you cross CRCR and CWCW, what do you expect? All red (CRCR) All white? (CWCW) Some red and some white? All pink? Some red, some white, some pink? All pink, because the phenotype is a mixture of both traits What do you expect for the F2 offspring? Example of Codominance If you cross STST and SRSR, what do you expect? All triangle spots? All round spots? Some triangle, some round? No spots? Both spots? Both spots, because both traits are expressed without mixing STST SRSR STSR What is the law of segregation? Mendel’s first law- stating that Each allele in a pair (diploid set) separates into a different gamete (haploid) during gamete formation *Offspring receive only one allele for a trait What is the law of independent assortment? Mendel’s second law that Each pair of alleles separates independently during gamete formation (Means which allele of Trait A a gamete receives is not at all related to which allele of Trait B the gamete receives) This law only applies when genes for two traits are located on different chromosomes. The influence of the law of segregation and the law of independence on gamete formation. Both laws mix up the genes so that each gamete receives = genetic diversity Punnet Square Parent/Organism One Parent/Organism Two Used to examine how one trait will be passed down to offspring Use a Punnet Square with 4 cells Trait A Allele 1 Trait A Allele 1 Trait A Allele 2 Trait A Allele 2 Monohybrid Cross Cross between two heterozygotes for a single trait What would the genotypes of the parents be in a monohybrid cross? Aa x Aa What would the genotypes of the offspring be? AA, Aa, and aa What would the ratio of phenotypes in the offspring be? 1:2:1 1 Homozygous Dominant: 2 Heterozygous: 1 Homozygous Recessive Test Cross Used to identify the unknown genotype of one individual Cross an unknown genotype with a Homozygous recessive phenotype Why homozygous recessive? Because you know the genotype: Two recessive alleles Example of Test Cross Captured Blue Mouse B B b Bb Bb b Bb Bb Hom. Rec. White Mouse Hom. Rec. White Mouse In Bodine mice, blue fur is dominant and white fur is a recessive trait. A captured mouse has blue fur. How would you determine its genotype? Mate the blue mouse with a white mouse Possible genotypes and offspring: Captured Blue Mouse B b b Bb bb b Bb bb If the offspring were all blue (Bb), what is the genotype of the original blue mouse? Homozygous dominant (BB) Two-Trait Punnet Squares Used to examine how two nonlinked traits are passed down in relation to each other Use a Punnet Square with 16 cells Dihyrbid crosses use 16-cells; cross between two heterozygotes Trait A, allele 1 Trait B, allele 1 Trait A, allele 1 Trait B, allele 1 Trait A, allele 2 Trait B, allele 1 Trait A, allele 1 Trait B, allele 2 Trait A, allele 2 Trait B, allele 2 Trait A, allele 2 Trait B, allele 1 Trait A, allele 1 Trait B, allele 2 Trait A, allele 2 Trait B, allele 2 How Dihybrid Crosses Work Remember dihybrid is a cross between two F1 individuals (AaBb) Combine the traits like a regular 4-celled Punnet Square Work from left to right Arrange the traits in pairs, one B / b per cell one E / e per cell Org. 2 Org. 1 How would you set up a dihybrid cross between BbEe and BbEe? How would the allele arrangement look for organism 1? Organism 2? Now solve the Punnet Square How many different types of offspring do we have? Dominant B, Dominant E BBEE, BbEE, BBEe, BbEe Dominant B, Recessive e Bbee, Bbee Recessive b, Dominant E bbEE, bbEe Recessive b, Recessive e bbee Different Genotypes in Dihybrid Cross How many different genotypes? Try to find them. Nine: Homozygous dominant B and E Homozygous dominant B, heterozygous E Heterozygous B, homozygous dominant E Heterozygous B, heterozygous E Homozygous recessive B, homozygous dominant E Homozygous recessive B, heterozygous E Homozygous dominant B, homozygous recessive E Heterozygous B, homozygous recessive E Homozygous recessive B, homozygous recessive E (BBEE) (BBEe) (BbEE) (BbEe) (bbEE) (bbEe) (BBee) (Bbee) (bbee) (1) (2) (2) (4) (1) (2) (1) (2) (1) Different Phenotypes in Dihybrid Cross How many different phenotypes? Four 1. Dominant B and Dominant E 2. Dominant B and Recessive e 3. Recessive b and Dominant E 4. Recessive b and recessive e How many of each? 1. nine 2. three 3. three 4. one Recombinants Recombinants are the offspring that have a different phenotype than their parents If parents are XxHh and xxHh, Recombinants are: Xxhh xxhh Sex Linkage / Sex-Linked Traits When the gene for a trait is on the X or Y chromosome (sex chromosomes) Sex traits express themselves more in one sex than the other- often more in males than females. In humans the term usually refers to X-linked characters: genes located only on X chromosomes Fathers can pass X-linked alleles to their daughters, but not sons Mothers can pass sex-linked alleles to both sons and daughters Dominant Sex-Linked Traits Dominant gene on X chromosome Affected males pass to all daughters and none of their sons Genotype= XAY If the mother has an X-linked dominant trait and is homozygous (XAXA) all children will be affected If mother is heterozygous (XAXa), there is a 50% chance of each child being affected Recessive Sex-Linked Traits Gene located on the X chromosome A female will express the phenotype only if she is homozygous recessive XrXr If a male receives the recessive allele from his mother he will express the phenotype XrY Females can only inherit if the father is affected and mother is a carrier (hetero) or affected (homo) An affected female will pass the trait to all her sons Daughters will be carriers if father is not affected Males cannot be carriers (only have one X so either affected or not) More males than females affected (males inherit affected X from mother) Can skip generations Examples: Colour blindness Haemophilia Dominant Sex-Linked Trait Pedigree Look for: More males being affected Affected males passing onto all daughters (dominant) and none of his sons Every affected person must have an affected parent Recessive Sex-Linked Trait Pedigree Look for: More males being affected Affected female will pass onto all her sons Affected male will pass to daughters who will be a carrier (unless mother also affected) Unaffected father and carrier mother can produce affected son Pedigree Analysis for Sex-Linked Trait Dominant or recessive? Recessive, III-4 does not have trait even though father gave her affected X chromosome Pedigree Analysis for Multiple Alleles Who was the father of 2nd-1? Why? 1st-3, because 1st-1 has no B allele to give If 2nd-3 marries a man with AB blood, what could their offspring have? If she is AA, offspring can be AA, AB If she is AO, offspring can be AA, AB, AO, BO Gene Linkage Genes are on the same chromosome Are usually inherited together for example, if blue eyes and freckles are linked, if your offspring have blue eyes, they will almost always have freckles Do these genes usually undergo independent assortment? No! Under what circumstance would the genes be inherited separately? If there is crossing over that mixes up the genes IB Exam Question 1. Define the terms gene and allele and explain how they differ. (4 marks) gene is a heritable factor / unit of inheritance; gene is composed of DNA; gene controls a specific characteristic / codes for a polypeptide/protein; allele is a form of a gene; alleles of a gene occupy the same gene locus/same position on chromosome; alleles differ (from each other) by one / a small number of base(s) / base pair(s); IB Exam Question 2. What is a difference between autosomes and sex chromosomes? (1 mark) A. Autosomes are not found in gametes but sex chromosomes are. B. Sex chromosomes are found in animal cells and autosomes are found in plant cells. C. Autosomes are diploid and sex chromosomes are haploid. D. Sex chromosomes determine gender and autosomes do not. Correct answer: D IB Exam Question 3. Explain the relationship between Mendel’s law of segregation and meiosis. (3 marks) law of segregation states that one half of the alleles enter one gamete and the other half enter the other gamete; meiosis reduces the chromosome number by half / diploid to haploid; homologues carrying alleles separate (in anaphase I); end result is four cells, half with one allele/homologue and the other half with the other allele; IB Exam Question 4. A parent organism of unknown genotype is mated in a test cross. Half of the offspring have the same phenotype as the parent. What can be concluded from this result? (1 mark) A. B. C. D. The parent is heterozygous for the trait. The trait being inherited is polygenic. The parent is homozygous dominant for the trait. The parent is homozygous recessive for the trait. Correct answer: A IB Exam Question 5. In peas the allele for round seed (R) is dominant over the allele for wrinkled seed (r). The allele for yellow seed (Y) is dominant over the allele for green seed (y). If two pea plants with the genotypes YyRr and Yyrr are crossed together, what ratio of phenotypes is expected in the offspring? (1 mark) A. 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green B. 3 round yellow : 3 round green : 1 wrinkled yellow : 1 wrinkled green C. 3 round yellow : 1 round green : 3 wrinkled yellow : 1 wrinkled green D. 1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green Correct answer: C IB Exam Question 6. In garden peas, the pairs of alleles coding for seed shape and seed colour are unlinked. The allele for smooth seeds (S) is dominant over the allele for wrinkled seeds (s). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y). If a plant of genotype Ssyy is crossed with a plant of genotype ssYy, which offspring are recombinants? A. B. C. D. SsYy and Ssyy SsYy and ssYy SsYy and ssyy Ssyy and ssYy Correct answer: C (1 mark) IB Exam Question 7. A polygenic character is controlled by two genes each with two alleles. How many different possible genotypes are there for this character? (1 mark) A. B. C. D. 2 4 9 16 Correct answer: C I.e. BBEE, BbEE, BBEe, BbEe, BBee, Bbee, bbEE, bbEe, bbee IB Exam Question 8. A woman who is a carrier of hemophilia marries a man who is not affected. What are the possible genotypes of their children? (1 mark) A. XHXh, XHXH, XHY, XhY B. XHXh, XHXH, XHYh, XHYH C. XHXh, XhXh, XHYh, XhYh D. XHXh, XhXh, XHY, XhY Correct answer: A woman can give normal or hemophilic X allele man can only give normal X allele trait is sex-linked, meaning nothing will be on the Y IB Exam Question 9. Two genes A and B are linked together as shown below. If the genes are far enough apart such that crossing over between the alleles occurs occasionally, which statement is true of the gametes? A. All of the gametes will be Ab and aB. B. There will be 25 % Ab, 25 % aB, 25 % ab and 25 % AB. C. There will be approximately equal numbers of Ab and ab gametes. D. The number of Ab gametes will be greater than the number of ab gametes. Correct answer: D Because Ab is the undisturbed gamete, ab is when crossing over occurs, which only happens occasionally IB Exam Question 10. Using an example you have studied, explain a cross between two linked genes, including the way in which recombinants are produced. (9 marks) linked genes occur on the same chromosome / chromatid; genes (tend to be) inherited together / not separated/do not segregate independently; non-Mendelian ratio / not 9:3:3:1 / 1:1:1:1; real example of two linked genes; Award [1] for each of the following examples of a cross between two linked genes. key for alleles involved in the example of a cross; homozygous parental genotypes and phenotypes shown; F1 genotype and phenotype shown / double heterozygote genotype and phenotype; possible F2 genotypes and phenotypes shown; recombinants identified; recombinants due to crossing over; in prophase I of meiosis; diagram / explanation of mutual exchange of parts of chromatids during crossing over; www.bodine.phila.k12.pa.us