Chapter 14 - Mendelian Inheritance Preview (PART 1) PDF

Announcements Drop deadline is tomorrow  Chapter 14 Mendelian Inheritance Gregor Mendel father of modern genetics Austrian monk who conducted experiments that paved the way for our understanding of genetic inheritance. His experiments used pea plants as they have many varieties with various di...

Announcements Drop deadline is tomorrow  Chapter 14 Mendelian Inheritance Gregor Mendel father of modern genetics Austrian monk who conducted experiments that paved the way for our understanding of genetic inheritance. His experiments used pea plants as they have many varieties with various different heritable traits that he could observe. And, because he could study many generations in a relatively short period of time  Mendel’s Experiments Mendel chose to track only those characters that occurred in two distinct alternative forms A heritable feature that varies among individuals is called a character Each variant for a character is called a trait STOP & THINK What is the character? What are the traits? © 2017 Pearson Education, Inc. Mendel studied many different characters that each had 2 traits  Mendel’s Experiments Mendel started with varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) If a plant has purple flowers, and if when that plant is self-pollinated all its offspring also has purple flowers, then that plant is said to be true- breeding for the purple flower trait. © 2017 Pearson Education, Inc. Experiment P Generation × true-breeding parents Purple White flowers flowers Experiment P Generation × true-breeding parents Purple White flowers flowers F1 Generation Filial Generation (hybrids) All plants had purple flowers Self- or cross-pollination Experiment P Generation × true-breeding parents Purple White flowers flowers F1 Generation Filial Generation (hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 3/4 705 purple-flowered 224 white-flowered plants plants Mendel found the same ratio across the different characters he tested: One trait was found in 75% of the F2 generation, and the other trait was found in only 25%.  Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation Dominant vs Recessive Whichever trait is observed in F1 is called dominant. Whichever trait is hidden in F1 is called recessive. We didn’t see a light purple flower, only ever one of the original (heritable) traits. What Mendel called a “heritable factor” is what we now call a gene © 2017 Pearson Education, Inc.  Mendel’s Model Based on his results, Mendel made 4 conclusions: 1. alternative versions of genes account for variations in inherited characters 2. for each character, an organism inherits two alleles, one from each parent 3. if the two alleles at a locus differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect 4. The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes © 2017 Pearson Education, Inc.  1. alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are called alleles Each gene resides at a specific locus on a specific chromosome © 2017 Pearson Education, Inc.  1. alternative versions of genes account for variations in inherited characters A gene resides at a specific locus on a specific chromosome Alternative versions of a gene are called alleles © 2017 Pearson Education, Inc.  STOP & THINK STOP & THINK How many alleles are there for Characters are determined by each character Mendel studied? ______________ and traits are determined by ______________ © 2017 Pearson Education, Inc.  So far we have talked about how we inherit chromosomes from our parents. We said each homologous chromosomes have the same genes. (ex. Chromosome 1 carries the same genes regardless if they it came from mom or dad) Each homologous chromosome carries the same genes, but they may be different versions of that gene  1. alternative versions of genes account for variations in inherited characters Alleles are represented by a letter The dominant allele is indicated with a capital letter The recessive alleles is indicated by a lower case letter Usually, the letter of the allele corresponds to the dominant trait EXAMPLE: In the case of flower color, the dominant trait was purple, so the allele is represented by the letter “p” (for purple). P = dominant allele = purple flower color p = recessive allele = white flower color © 2017 Pearson Education, Inc.  2. for each character, an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about chromosomes An organism inherits 2 alleles for a gene, one from each parent. Those 2 alleles could be the same, or they could be different. An organism with two identical alleles for a character is called a homozygote An organism with two different alleles for a gene is a heterozygote STOP & THINK Do you think the P generation in the experiment were homozygotes or heterozygotes? Do you think the F1 plants in the experiment were homozygotes or heterozygotes? © 2017 Pearson Education, Inc.  STOP & THINK Do you think the P generation in the experiment were homozygotes or heterozygotes? How would you write the alleles for each parent? PP pp Are the F1 plants in the experiment homozygotes or heterozygotes? © 2017 Pearson Education, Inc.  2. for each character, an organism inherits two alleles, one from each parent When we write down the two alleles for an organism, that is called their genotype. When we write down the observable appearance or trait for that genotype, it is called a phenotype Genotype Phenotype True-breeding purple flower True-breeding white flower F1 generation (hybrids) © 2017 Pearson Education, Inc. Figure 14.6 Homozygous dominant Homozygous recessive  3. if the two alleles at a locus differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant © 2017 Pearson Education, Inc. Practice Write the genotypes and phenotypes for each of these characters: Seed color Seed shape Stem length You will need to decide the best letter to use for your alleles.  4. The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles that are present in the organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis pp Pp Gametes have only 1 allele for each gene! © 2017 Pearson Education, Inc. There’s a 50% chance each allele will be selected Pp pp We can use a Punnet Square to predict all potential outcomes.  Punnet Square Example: Pp x pp Results:  Punnet Square Example: PP x pp Results: Experiment P Generation STOP & THINK × true-breeding What happens if a Pp parents Purple White crosses with another Pp? flowers flowers F1 Generation Filial Generation (hybrids) All plants had purple flowers  Experiment P Generation × true-breeding parents Purple White STOP & THINK flowers flowers What is the phenotype ratio? F1 Generation (ratio of purple to white flowers) Filial Generation (hybrids) All plants had purple flowers What is the genotype Self- or cross-pollination ratio? F2 Generation 3/4 705 purple-flowered 224 white-flowered plants plants  STOP & THINK What is the phenotype ratio? (ratio of purple to white flowers) What is the genotype P p ratio? P p  An organism’s traits do not always reveal its genetic composition We distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes © 2017 Pearson Education, Inc. Can you always tell the phenotype if you are given the genotype? Can you always tell the genotype if you are given the phenotype? How can you determine the genotype of a purple flowered plant? The Testcross unknown genotype  The Testcross An individual with the dominant phenotype could be either homozygous dominant or heterozygous To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous © 2017 Pearson Education, Inc. Practice Draw the Punnet square for a hybrid cross for: Seed color Seed shape Stem length Show how you would determine the genotype of a plant with yellow seeds green seeds Hint: you may need to do a test cross  Mendel’s Model Based on his results, Mendel made 4 conclusions: 1. alternative versions of genes account for variations in inherited characters 2. for each character, an organism inherits Questions? two alleles, one from each parent 3. if the two alleles at a locus differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect 4. The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes © 2017 Pearson Education, Inc.  The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids, heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Punnett squares are useful for determine the genotypes of offspring © 2017 Pearson Education, Inc.  Dihybrid cross Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently Mendel developed the law of independent assortment- It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation © 2017 Pearson Education, Inc. Figure 14.8 Practice Draw the Punnet square for a dihybrid cross for: Flower color AND stem length Seed color AND seed shape  Remember from Chapter 13 1. Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis © 2017 Pearson Education, Inc. Some traits have inheritance patterns that are more complex than simple Mendelian genetics Degrees of dominance Multiple alleles Pleiotropy Epistasis Polygenic inheritance  Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Ex. Pea plant flower color: Purple (dominant), White (recessive) In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Ex. Snapdragon flower color: Red (dominant), White (recessive), Pink (hybrid) In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Ex. Cows: Redish brown (dominant), White (dominant), Roan (hybrid) © 2017 Pearson Education, Inc. P Generation Red White CRCR × CWCW Gametes CR CW P Generation Red White CRCR × CWCW Gametes CR CW F1 Generation Pink CRCW Gametes 1/ 2 CR 1/ 2 CW P Generation Red White CRCR × CWCW Gametes CR CW F1 Generation Pink CRCW Gametes 1/ 2 CR 1/ 2 CW F2 Generation Sperm 1/ 1/ 2 CR 2 CW 1/ 2 CR CRCR CRCW Eggs 1/ CW 2 CRCW CWCW  codominance https://biology4me.weebly.com/sc912l161-genetics.html  Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles One baby out of 400 in the United States is born with extra fingers or toes This condition, polydactyly, is caused by a dominant allele, found much less frequently in the population than the recessive allele © 2017 Pearson Education, Inc. Some traits have inheritance patterns that are more complex than simple Mendelian genetics Degrees of dominance ‒Complete dominance Dominant allele has capital letter ‒Incomplete dominance Dominant allele has capital letter ‒Codominance with a superscript (ex. CR) Multiple alleles Pleiotropy Epistasis Questions? Polygenic inheritance  Multiple Alleles 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 for the enzyme that attaches A or B carbohydrates to red blood cells: IA, IB, and i STOP & THINK What are all the possible phenotypes if both parents are both Type A? What are all the possible phenotypes if both parents are both Type O? What are all the possible phenotypes if both parents are both Type AB? © 2017 Pearson Education, Inc.  Multiple Alleles 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 for the enzyme that attaches A or B carbohydrates to red blood cells: IA, IB, and i STOP & THINK If Mom has Type A, and Dad has Type B, is it possible for them to have children with type A? What about children with type AB? What about children with type O? © 2017 Pearson Education, Inc.  Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy (1 gene  many effects) For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease © 2017 Pearson Education, Inc.  Genotype Phenotype Black Epistasis Brown Yellow In epistasis, expression of 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 1. One gene determines the pigment color (with alleles B for black and b for brown) 2. The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair Since we are looking at 2 genes, we must do a dihybrid cross to determine F2 outcomes Practice: make a Punnet square for a BbEe x BbEe cross © 2017 Pearson Education, Inc.  BbEe × BbEe Sperm Genotype Phenotype 1 4 BE 1 4 bE 1 4 Be 1 4 be Eggs Black 1 BE 4 BBEE BbEE BBEe BbEe Brown Yellow 1 bE 4 BbEE bbEE BbEe bbEe 1 Be 4 BBEe BbEe BBee Bbee 1 be 4 BbEe bbEe Bbee bbee 9 : 3 : 4  Polygenic Inheritance Multiple genes affect a single phenotype Height is a good example of polygenic inheritance: Over 180 genes affect height Skin color in humans is also controlled by many separately inherited genes Quantitative characters are those that vary in the population along a continuum © 2017 Pearson Education, Inc. Some traits have inheritance patterns that are more complex than simple Mendelian genetics Degrees of dominance Multiple alleles Pleiotropy Epistasis Polygenic inheritance Questions?  Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The phenotypic range is broadest for polygenic characters Traits that depend on multiple genes combined with environmental influences are called multifactorial © 2017 Pearson Education, Inc. End of Thursday lecture The rest of chapter 14 will be posted on BrightSpace on Friday