Genetics: CH9 Patterns of Inheritance

Transcription

1 Genetics: CH9 Patterns of Inheritance o o Lecture note Directions Highlight Key information (10-30% of most slides) My Thoughts: Questions, comments, additional information, connections to prior knowledge, diagrams, rephrasing any MT related to the subject. Name: Starting Date: Period: Pre lecture stamp 10pnts (If not printed, min. of first 2 pgs handwritten.) o Not prepared for pre lecture check bring in for 5/10 late check! Lecture turn in on test day o Wow (20/20); Standard complete notes (17/20); o minor issues (14/20); significant issues but at least 50% complete (10/20) o something, but less than 50% complete (5/20) /30 Total Score Purebreds and Mutts A Difference of Heredity Genetics is the science of heredity These black Labrador puppies are purebred their parents and grandparents were black Labs with very similar genetic makeups The parents of these puppies were a mixture of different breeds Mendel s Principles: The science of genetics has ancient roots The science of heredity dates back to ancient attempts at selective breeding Until the 20th century, however, many biologists erroneously believed that characteristics acquired during lifetime could be passed on characteristics of both parents blended irreversibly in their offspring Experimental genetics began in an abbey garden Modern genetics began with Gregor Mendel s quantitative experiments with pea plants Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation Mendel studied seven pea characteristics Mendel s principle of segregation describes the inheritance of a single characteristic From his experimental data, Mendel deduced that an organism has two genes for each inherited characteristic One characteristic comes from each parent A sperm or egg carries only one gene of each pair Principle of Segregation Homologous pairs of genes segregate (separate) during gamete formation (meiosis). The joining of gametes at fertilization pair the genes once again. Homologous chromosomes bear the two alleles for each characteristic Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes Genetic Vocabulary Gene a segment of DNA that contains the instructions that code for a particular trait Locus specific location of a gene on a chromosome Allele alternate versions of a gene at a single locus

2 Homozygous when the alleles of a gene are the same on the homologous chromosomes Heterozygous when the alleles of a gene are different on the homologous chromosomes Dominant the allele that is expressed when the alleles are heterozygous. Represented by an upper case letter Recessive the allele that is not expressed when the alleles are heterozygous. Represented by a lower case letter. To be expressed the cell must have 2 copies of the recessive allele Phenotype the physical appearance of a trait in an organism Genotype the genetic make up of an organism with respect to a trait. The genotype of a trait can be homozygous dominant, heterozygous or homozygous recessive Mendel s principles reflect the rules of probability Inheritance follows the rules of probability Predicting the Outcome of a Monohybrid Cross Predict the results of the following cross (using R to denote tongue-rolling ability): P generation: RR x RR What genotype(s) will be found in the F1 generation? What phenotype(s) will be found in the F1 generation? Explain why you made these predictions. Predicting the Outcomes of a Monohybrid Cross Predict the results of the following cross: P generation: RR x rr What genotype(s) will be found in the F1 generation? What phenotype(s) will be found in the F1 generation? Explain why you made these predictions. Predicting the Outcome of a Monohybrid Cross Predict the results of the following cross: P generation = Rr x Rr Draw the Punnett square. What are the possible genotypes in the F1 generation? What is the genotypic ratio of this cross? What are the possible phenotypes in the F1 generation? What is the phenotypic ratio for this cross? Geneticists use the testcross to determine unknown genotypes The offspring of a testcross often reveal the genotype of an individual when it is unknown Test cross A testcross is the mating between an individual of unknown genotype with a homozygous recessive genotype. Usually performed when the phenotype of the unknown individual is dominant. Understanding Test Cross Brown coat color (B) in rabbits is dominant and white coat color is recessive. Suppose you have a group of rabbits some brown and some white. a. For which phenotype(s) do you know the genotype(s)? b. For which phenotype(s) are you unsure of the genotype(s)?

3 Understanding Test Cross Using B and b to symbolize the brown and white alleles What are the possible genotypes of a white rabbit in your group? What are the possible genotypes of a brown rabbit? Suppose you wanted to find out the genotype of a brown rabbit. What color rabbit would you mate it with? Understanding Test Cross A brown buck (male) is mated with a white doe (female). In their litter of 11 young, 6 are white and 5 are brown. Using a Punnett square to check your answer, what is the genotype of the buck? Use a Punnett square to determine the ratio of brown and white offspring that would have been produced by the above mating if the brown buck had been homozygous. Mendel dehydrate experiment The principle of independent assortment is revealed by tracking two characteristics at once By looking at two characteristics at once, Mendel found that the genes of a pair segregate independently of other gene pairs during gamete formation This is known as the principle of independent assortment Independent assortment of two genes in the Labrador retriever

4 Principle of Independent Assortment Dihybrid cross An experimental mating of individuals in which the inheritance of 2 traits is tracked. When the inheritance of 2 traits is tracked in an individual, the dominant/recessive traits doe not always appear together. The individual may be dominant in one of the traits and recessive in the other. Principle of Independent Assortment Genes for different characteristics are not connected and each pair of genes for a characteristic separate independently during meiosis. Solving Dihybrid Problems 1. List the genotypes of each parent. 2. Make all possible combinations of the gametes 3. Construct a 16 square Punnett square. 4. List the possible genotypes of the offspring and determine the genotypic ratio. 5. List the possible phenotypes of the offspring and determine the phenotypic ratio. Solving Dihybrid Problems Example: In humans freckles (F) is dominant and no freckles (f) is recessive. Normal arches (A) are dominant and flat feet (a) is recessive. A man who has freckles and flat feet (FFaa) marries a woman without freckles and normal arches (ffaa). What are the possible genotypes and phenotypes of their children? 1. List the genotypes of each parent. 2. Make all possible combinations of the gametes 3. Construct a 16 square Punnett square. Geneotypes of Possible offspring produced by a dihybrid cross 4. List the possible genotypes of the offspring and determine the genotypic ratio. 5. List the possible phenotypes of the offspring and determine the phenotypic ratio. Connection: Genetic traits in humans can be tracked through family pedigrees The inheritance of many human traits follows Mendel s principles and the rules of probability Family pedigrees are used to determine patterns of inheritance and individual genotypes Connection: Many inherited disorders in humans are controlled by a single gene Most such disorders are caused by autosomal recessive alleles Examples: cystic fibrosis, sickle-cell disease Inherited Single Gene Disorders Recessive disorders Most single gene disorders Relatively harmless disorders to deadly diseases Most born to normal parents who are carriers Carrier an individual who is heterozygous for a recessive disorder and does not show symptoms of the disorder Carriers have a 1 in 4 chance of having a child with a recessive disorder

5 Dominant disorders A few are caused by dominant alleles Genetic Disorder Examples Connection: Fetal testing can spot many inherited disorders early in pregnancy Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions Fetal cells can be obtained through amniocentesis Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping Examination of the fetus with ultrasound is another helpful technique VARIATIONS ON MENDEL S PRINCIPLES The relationship of genotype to phenotype is rarely simple Mendel s principles are valid for all sexually reproducing species However, often the genotype does not dictate the phenotype in the simple way his principles describe Incomplete dominance results in intermediate phenotypes When an offspring s phenotype such as flower color is in between the phenotypes of its parents, it exhibits incomplete dominance Incomplete dominance in human hypercholesterolemia Incomplete Dominance In a cross between a homozygous dominant parent and a homozygous recessive parent the phenotype of the offspring is in between the phenotypes of the parents. Example: When red snapdragons are crossed with white snapdragons all the offspring have pink flowers Codominance The alleles for A and B blood types are codominant, and both are expressed in the phenotype Many genes have more than two alleles in the population Multiple allele traits 3 or more alleles of the same gene code for a single trait Example: the three alleles (I A, I B, i) for ABO blood type in humans A single gene may affect many phenotypic characteristics Pleoitropy A single gene may affect phenotype in many ways Example: the allele for sickle-cell disease

6 A single characteristic may be influenced by many genes Skin color Example: Dominant alleles ABC code for DARK skin tone Recessive alleles abc code for light skin tone Polygenic traits Trait that is controlled by 2 or more genes. This situation creates a continuum of phenotypes When the range of traits is graphed a bell shaped curve is seen Example: skin color, eye color Match the description with its pattern of inheritance 1. There are 3 different alleles for a blood group, I A, I B, and i, but an individual has only two at a time. 2. The sickle cell allele, s, is responsible for a variety of phenotypic effects, from pain and fever to damage to the heart, lungs, joints, brain or kidneys. 3. If a red shorthorn cow is mated with a white bull, all their offspring are roan, a phenotype that has a mixture of red and white hairs. 4. Independent genes at 4 different loci are responsible for determining a person s HLA tissue type, important in organ transplants and certain diseases. 5. When graphed, the number of individuals of various heights forms a bell shaped curve. 6. Chickens homozygous for the black allele are black, and chickens homozygous for the white allele are white. Heterozygous chickens are gray. The chromosomal basis of Mendel s principles Genes are located on chromosomes

7 Their behavior during meiosis accounts for inheritance patterns Genes on the same chromosome tend to be inherited together Linked genes Genes that are located close together on the same chromosome tend to be inherited together These genes usually do not follow Mendel s principle of independent assortment SEX CHROMOSOMES AND SEX-LINKED GENES Many animals including humans have a pair of sex chromosomes A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring Other systems of sex determination exist in other animals and plants The genetic basis of sex determination isn t fully understood: Gene SRY on the Y chromosome plays a crucial role SRY triggers testis development Absence of SRY results in overy development Sex-linked genes exhibit a unique pattern of inheritance All genes on the sex chromosomes are said to be sex-linked In many organisms, the X chromosome carries many genes unrelated to sex Fruit fly eye color is a sex-linked characteristic Their inheritance pattern reflects the fact that males have one X chromosome and females have two Connection: Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Sex-Linked Disorders Other sex-linked disorders are Duchenne muscular dystrophy weakening and loss of muscle tissue Fragile X syndrome abnormal X chromosome, most common cause of mental retardation in boys Solving Sex-Linked Problems Example One: Eye color is a sex-linked trait in fruit flies and is carried on the X chromosome. Red eye color (R) is dominant over white eye color (r). What is the sex and eye color of the offsrping of a homozygous red eyed female and a white eyed male? Example Two: What is the sex and eye color of the offspring of a heterozygous red eyed female fruit fly and a red eyed male fruit fly?