GENERAL BIOLOGY LAB 1 BSC1010L Lab 8 Mendelian Genetics OBJECTIVES Understand Mendel s laws of segregation and independent assortment Differentiate between an organism s genotype and phenotype Recognize different patterns of inheritance Perform monohybrid and dihybrid crosses Use pedigree analysis to identify inheritance patterns INTRODUCTION Through his studies of the inheritance patterns of the garden pea Pisum sativum Gregor Mendel changed our understanding of heredity Mendel studied characters traits that differed between plants and designed cross fertilization experiments to understand how these characters transmit to the next generation The results of Mendel s work refuted the prevailing hypothesis of blending inheritance and provided a new framework for understanding genetics Ultimately Mendel postulated two laws to explain heredity 1 the law of segregation and 2 the law of independent assortment Monohybrid crosses and the law of segregation The law of segregation also termed the first law states that during gamete formation the alternate forms of a gene i e alleles on a pair of chromosomes segregate randomly so that each allele in the pair is received by a different gamete For example if you were to examine the gene responsible for petal color you may discover that the gene can be expressed as either yellow or white flowers In this scenario the gene is petal color while the alleles are yellow and white Depending on which allele is expressed petal color will vary Examine Figure 1 below making sure that you can follow the path of each allele from parent to offspring Figure 1 Schematic of Mendel s law of segregation 1 In diploid organisms all alleles exist in pairs identical alleles within a pair are homozygous while different alleles are heterozygous Allele forms are represented by a single letter that explains whether a particular trait is dominant or recessive Dominant alleles are assigned an uppercase letter E while recessive alleles are lowercase e In general a dominant trait is expressed when at least one of the alleles present in the resulting allelic pair is dominant EE or Ee In contrast for a recessive trait to be expressed both alleles within the pair must be recessive ee For example when considering ear lobe shape two forms attached and unattached are apparent Fig 2 This trait is regulated by a single gene where unattached ear lobes are dominant E while attached ear lobes e are recessive Figure 2 a Unattached EE or Ee vs b attached earlobes ee An organism s genotype EE Ee ee is the combination of alleles present whereas the phenotype is the physical expression of the genotype In the earlobe shape example above an individual can have a genotype of EE Ee or ee People with EE or Ee genotypes have the unattached earlobe phenotype Fig 2a while those with an ee genotype express the attached earlobe form Fig 2b Note that dominant traits can be either homozygous EE or heterozygous Ee while recessive traits are always homozygous ee Question Given that the allele for brown eyes B is dominant and the allele for blue eyes b is recessive which of the following genotypes would result in individuals with brown eyes Which genotype s is are homozygous and which is are heterozygous BB Bb bb 2 In today s lab you will use the concepts of Mendelian Genetics to solve problems regarding inheritance TASK 1 Patterns of Inheritance I Simple Dominance Simple dominance is the term used to describe a common outcome of allelic combinations where one allele if present will dominate over the other and will be expressed Information about alleles present in a parental population can be used to determine the probability of different genotypic and phenotypic ratios for a variety of traits in the offspring In instances when only 1 or 2 traits are being considered the Punnett square Fig 3 approach is used to predict the possible outcomes of the parental cross When only one trait is being considered the cross is monohybrid while a dihybrid cross involves 2 traits General instructions on how to perform a cross using the Punnett square approach 1 2 3 4 Write down the genotypes of the parents Note the gametes that each parent can contribute Draw a Punnett Square Across the top write the gametes that one parent contributes and along the side write the gametes contributed by the other parent 5 Perform the cross 6 Determine the genotypic and phenotypic ratios Figure 3 Example of a Monohybrid cross In the example above Fig 3 the genotypic ratio is 1 2 1 1 CC 2 Cc 1 cc while the phenotypic ratio is 3 1 Since C curly hair and c straight hair of the possible offspring will have curly hair while only will have straight hair 3 Procedure 1 You will now simulate a cross between two heterozygous individuals Tt and Tt Each group should obtain two coins from your TA You will flip the coins simultaneously to represent the potential outcomes of a cross between two Tt individuals A head represents the dominant tall allele T while a tail symbolizes the recessive dwarf allele t Before you begin flipping the coins perform the Tt x Tt cross in the Punnett square below to estimate the expected genotypic and phenotypic ratios Parent 2 Parent 1 Based on this cross what do you anticipate the genotypic and phenotypic ratios to be Write your hypotheses Ho and Ha in Table 1 Table 1 Expected Genotypic Ratio Expected Phenotypic Ratio 2 Begin flipping the two coins simultaneously for a total of 64 times Record your results in Table 2 4 Table 2 Response Number TT Tt tt Questions a What ratio of allele combinations did you observe b What genotypes and phenotypes result from these crosses c What are the genotypic and phenotypic ratios d How did your results compare to your expectations Do your results support or reject your null hypothesis 5 e Do you think your results would have been closer if you flipped the coins 6400 times instead of just 64 Why or why not 3 Albinism a recessively inherited trait results in organisms that lack pigment in the skin hair or eyes A female with normal pigmentation but who had an albino mother mates with an albino male They have one child Using the information you have learned so far complete Table 3 Table 3 Genotype of child s mother Genotype of child s father Possible gametes of mother Possible gametes of father Possible genotype and phenotype of the offspring Genotypic ratio of children Phenotypic ratio of children TASK 2 Patterns of inheritance II Incomplete vs Complete Dominance Codominance