Genetics PDF - BIOL S103F Essential Biology

BIOL S103F ESSENTIAL BIOLOGY GENETICS Week 9 Mendel’s Law 2    Genetics is the scientific study of heredity  In 1866, Gregor Mendel  Parents pass on to their offspring discrete “heritable factors”  Heritable factors (genes) retain their individuality generation after generation Character  A h...

BIOL S103F ESSENTIAL BIOLOGY GENETICS Week 9 Mendel’s Law 2    Genetics is the scientific study of heredity  In 1866, Gregor Mendel  Parents pass on to their offspring discrete “heritable factors”  Heritable factors (genes) retain their individuality generation after generation Character  A heritable feature that varies among individuals Trait  Each variant for a character Character Traits Recessive Dominant Flower color Purple White Axial Terminal Yellow Green Round Wrinkled Inflated Constricted Green Yellow Flower position Seed color Seed shape Pod shape Pod color Stem length Tall Dwarf @2015 Pearson Education Ltd Mendel’s Law 3    Mendel deduced the principles of genetics by breeding garden peas Technique 1 Mendel chose to track only those characters that occurred in 2 distinct alternative forms He also used varieties that were truebreeding (plants that produce offspring of the same variety when they self-pollinate) 2 Stamens Parental generation (P) Carpel 3 4 Results 5 First filial generation offspring (F1) @2015 Pearson Education Ltd Mendel’s Law of Segregation Describes the Inheritance of a Single Character     Monohybrid cross  A cross between 2 individuals differing in a single character P generation  True-breeding parental plants The Experiment P generation (true-breeding parents) Purple flowers F1 generation All plants have purple flowers F1 generation  Hybrid offspring F2 generation  A cross of F1 plants White flowers Fertilization among F1 plants (F1 × F1) F2 generation 𝟑 of plants (n=705) 𝟒 have purple flowers 𝟏 of plants (n=224) have white flowers 𝟒 @2015 Pearson Education Ltd Mendel’s Law of Segregation 5      When Mendel crossed contrasting, true-breeding white- & purple-flowered pea plants, all F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of ~3:1 (purple : white flowers) in the F2 generation Mendel observed the same pattern of inheritance in 6 other pea plant characters, each represented by 2 traits What Mendel called a “heritable factor” is what we now call a gene @2015 Pearson Education Ltd Mendel’s Hypotheses 6    The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis Mendel needed to explain why white color  Seemed to disappear in the F1 generation  Reappeared in ¼ F2 offspring Mendel observed the same patterns of inheritance for six other pea plant characters @2015 Pearson Education Ltd Mendel’s Hypotheses 7  4 hypotheses (described using modern terminology) 1. Alleles are alternative versions of a gene that account for variations in inherited characters 2. For each character, an organism inherits 2 alleles, one from each parent  The alleles can be the same or different  A homozygous genotype has identical alleles  A heterozygous genotype has 2 different alleles Alleles are Alternative Versions of a Gene 8   Locus (plural, loci)  Specific location of a gene along a chromosome For a pair of homologous chromosomes (homologs), alleles of a gene reside at the same locus  Homozygous individuals have the same allele on both homologs  Heterozygous individuals have a different allele on each homolog @2015 Pearson Education Ltd Mendel’s Hypotheses 9  4 hypotheses (described using modern terminology) 3. If the alleles of an inherited pair differ, then one determines the organism’s appearance (dominant allele B); the other has no noticeable effect on the organism’s appearance (recessive allele b)  The phenotype is the appearance / expression of a trait  The genotype is the genetic makeup of a trait  The same phenotype may be determined by > 1 genotype 4. Law of segregation  A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes  The fusion of gametes at fertilization creates allele pairs once again The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Mendel’s Hypotheses 10  Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation  F1 hybrids all have a Pp genotype  A Punnett square shows the 4 possible combinations of alleles that could occur when these gametes combine Gametes All P All p F1 generation (hybrids -- Offspring of 2 different varieties) Cross-fertilization / genetic cross  Hybridization F2 generation Results: All Pp Alleles segregate Gametes 1 2 1 2 P p Fertilization p Sperm from F1 plant P Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Eggs from F1 plant P PP Pp p Pp pp Results @2015 Pearson Education Ltd The Law of Independent Assortment is Revealed by Tracking 2 Characters at Once 11   Dihybrid cross  Mating of parental varieties that differ in 2 characters  F2 had new nonparental combinations of traits (9:3:3:1 phenotypic ratio) Law of independent assortment  Inheritance of one character has no effect on the inheritance of another  Dihybrid cross is the equivalent to 2 monohybrid crosses @2015 Pearson Education Ltd Check your concept.. 12      In pea plants, green pod color dominates yellow pod color. If a pea plant with yellow pods is crossed with a pea plant heterozygous for pod color, ____ of the offspring will have yellow pods. A) 25% B) 50% C) 75% D) 100% Testcross to Determine Unknown Genotypes 13   Testcross  Mating between an individual of unknown genotype & a homozygous recessive individual  An individual with the dominant phenotype could be either homozygous dominant or heterozygous What is the genotype of the black dog?  If any offspring display the recessive phenotype, the mystery parent must be Testcross heterozygous Mendel used testcrosses to verify that he had true-breeding varieties of plants Genotypes bb B_? Two possibilities for the black dog: Bb BB or Gametes B b Offspring Bb All black B b b Bb bb 1 black : 1 chocolate @2015 Pearson Education Ltd Many Inherited Traits in Humans are Controlled by a Single Gene 14    Dominant traits are not necessarily  “Normal”  More common than a recessive trait  e.g. 1 in 400 baby in US is born with extra fingers or toes (dominant) Wild-type traits  Those most often seen in nature  Not necessarily specified by dominant alleles Many human traits are controlled by single genes on autosomes  Inherited as dominant / recessive traits controlled by a single gene  Show simple inheritance patterns like the traits Mendel studied in pea plants 15 Many Genetic Disorders are Inherited as Recessive Traits 16    Thousands of human genetic disorders are inherited as recessive traits  Ranging in severity from relatively mild, e.g. albinism to invariably fatal, e.g. cystic fibrosis Most people with recessive disorders are born to normal parents who are  Both heterozygotes  Carriers of the recessive allele for the disorder  Phenotypically normal Normal Aa Parents Offspring If a recessive allele that causes a disease is rare, then the chance of 2 carriers meeting & mating is low A Normal Aa A Sperm a AA Normal Aa Normal (carrier) Aa Normal (carrier) aa Albinism Eggs a @2015 Pearson Education Ltd  Consanguineous matings (i.e. matings between close relatives) increase the chance of mating between 2 carriers of the same rare allele Variations on Mendel’s Laws 17  Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations  When alleles are not completely dominant or recessive  When a gene has more than two alleles  When a gene produces multiple phenotype Degrees of Dominance 18    Complete dominance  Phenotypes of the heterozygote & dominant homozygote are identical  Mendel’s pea crosses always looked like one of the 2 parental varieties Incomplete dominance  Appearance of F1 hybrids falls between the phenotypes of the 2 parental varieties  Expression of one intermediate trait in heterozygous individuals  e.g. Hypercholesterolemia  Dangerously high levels of blood cholesterol  Heterozygotes have intermediately high cholesterol levels Codominance  2 dominant alleles affect the phenotype in separate, distinguishable ways  e.g. ABO blood group Incomplete dominance in human hypercholesterolemia 19 Many Genes have More than 2 Alleles in the Population 20  Human ABO blood group phenotypes  The 4 phenotypes of the ABO blood group in humans are determined by 3 alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i Blood Carbohydrates Present Group Genotypes on Red Blood Cells (Phenotype) A I AI A or I Ai Carbohydrate A Carbohydrate B B I BI B or I Bi AB I AI B O ii Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O AB A B Anti-B Anti-A Carbohydrate A and Carbohydrate B None Anti-A Neither Anti-B No reaction Clumping reaction @2015 Pearson Education Ltd Multifactorial Traits 21    Traits that depend on multiple genes combined with environmental influences  Only genetic influences are inherited Many characters result from a combination of heredity & the environment  Skin color is affected by exposure to sunlight  Many diseases (e.g. heart disease, diabetes, alcoholism, mental illnesses, cancer) have both genetic & environmental components  No matter what our genotype, our lifestyle has a tremendous effect on phenotype Identical twins show that a person’s traits are the results of  Genetics  Environment basic soil acidic soil @2015 Pearson Education Ltd Sex-Linked Genes Exhibit a Unique Pattern of Inheritance 22      Sex-linked genes  Located on either of the sex chromosomes X chromosome carries many genes unrelated to sex, whereas most Y-linked genes are related to sex determination X-linked genes follow specific patterns of inheritance For a recessive X-linked trait to be expressed  A female needs 2 copies of the allele (homozygous)  A male needs only 1 copy of the allele (hemizygous) X-linked recessive disorders are much more common in males than in females Human Sex-Linked Disorders Affect Mostly Males 23   Most sex-linked human disorders  Due to recessive alleles  Seen mostly in males A male receiving a single X-linked recessive allele from his mother will have the disorder  A female must receive the allele from both parents to be affected  X-linked recessive human disorders    Hemophilia (血友病) Red-green colorblindness Duchenne muscular dystrophy (杜興氏肌肉營養不良症) @2015 Pearson Education Ltd Am I colorblind? 24 XNXN XnY Mom Dad Xn Y XN XNXn XNY XN XNXn XNY Sister Myself Sperm Eggs (a) Normal female  colorblind male © 2013 Pearson Education, Inc. Is my nephew or niece colorblind? 25 XNXn Key XNY Brother-in-law Sister Unaffected individual XN Y XN XNXN XNY Xn XNXn XnY Sperm Carrier Colorblind individual Eggs Niece Nephew (b) Carrier female  normal male © 2013 Pearson Education, Inc. Is my nephew or niece colorblind? 26 XNXn Key XnY Sister Unaffected individual Brother-in-law Xn Y XN XNXn XNY Xn XnXn XnY Sperm Carrier Colorblind individual Eggs Niece Nephew (c) Carrier female  colorblind male © 2013 Pearson Education, Inc.