Ciera Ventura L11 Patterns of Inheritance I Guided Reading Qs Chapter 9 selected modules Reading Objectives Define and use the terms that geneticists use to communicate about inheritance Construct Punnett squares to examine the offspring that arise from independent segregation within parents for autosomal and sex linked traits Determine the types of gametes that form through independent assortment in a dihybrid and link this to metaphase I of meiosis Explain why a tester is a useful tool in genetic crosses Module 9 2 What made the pea plant a good model for studying genetics as opposed to say humans It had shorter generation times produced large numbers of offspring from each mating many readily distinguishable varieties and mating between them can be strictly controlled One important aspect for Mendel was that he had pea plants that could breed true what does this mean For example if a plant was a purebreeding purple flower plant what does this indicate about the plant s two alleles that code for flower color True breeding varieties over many generations of self pollination produced only the same variety as the parent plant The organisms are homozygous for the characters under consideration Module 9 3 Take time to be sure you know the bold faced terms You need a good understanding of these terms to speak like a geneticist List here any that you are not already familiar with and define them These terms should become more obvious as you start applying them in the next section too Allele an alternative version of a gene Homozygous having two identical alleles for a given gene Heterozygous having two different alleles for a given gene Dominant allele the allele that determines the phenotype of a gene when the individual is heterozygous for that gene Recessive allele an allele that has no noticeable effect on the phenotype of a gene when the individual is heterozygous for that gene Law of segregation a general rule in inheritance originally formulated by Gregor Mendel that individuals have two alleles for each gene and that when gametes form meiosis the two alleles separate each resulting gamete ending up with only one allele of each gene also known as Mendel s first law of inheritance Ciera Ventura Punnett square a diagram used in the study of inheritance to show the results of random fertilization Phenotype the expressed traits of an organism Genotype the genetic makeup of an organism The law of segregation is useful to predict how alleles for one character will segregate into gametes What type of gametes can a parent that is homozygous dominant PP make through meiosis Dominant gametes P gametes What type of gametes can a parent that is homozygous recessive pp make through meiosis Recessive gametes p gametes What type of offspring will they make if the gametes of these two parents fuse through fertilization Pp heterozygous Practice constructing a Punnett square when a purple heterozygote Pp is crossed to another purple heterozygote Pp What will be the phenotypes of the offspring and in what ratio will they be to each other p p P p P Pp Pp P PP Pp P Pp Pp p Pp pp 3 purple 1 white PP purple Pp purple pp white Ciera Ventura Module 9 4 What is similar about homologous chromosomes What might be different Does a drawing help you here Carry alleles of the some genes at the same locations Gene Loci Alleles might be different heterozygous Module 9 5 Mendel used single gene or monohybrid experiments to describe his law of segregation He used 2 characters or dihybrid experiments to come up with what law Law of independent assortment When an organism that is heterozygous for two genes RrYy is crossed to another organism that is the same genotype RrYy the phenotypic ratio will always be what Dihybrid x dihybrid 9 3 3 1 What fraction will have the dominant phenotype for both traits 9 16 What fraction will have the dominant phenotype for trait A and be recessive for B 3 16 What fraction will have the dominant phenotype for trait B and be recessive for a 3 16 What fraction will have the recessive phenotype for both traits 1 16 Using the traits in Labradors discussed in the book predict the phenotype of offspring obtained by mating a black Lab homozygous for coat color and normal vision with a chocolate Lab that is blind from PRA bn bn BN BN BNbn BNbn BNbn BNbn Ciera Ventura Phenotype black with normal vision Module 9 6 What is a testcross useful for To determine the unknown genotype of an individual with the dominant phenotype What is the genotype of one of the individuals that is always crossed in a testcross We call this individual a tester Homozygous recessive Module 9 7 Use the rule of multiplication what is the chance that a woman who is pregnant four different times will give rise to a family with four sons 50 x 50 x 50 x 50 6 25 Apply the rule of addition to a single gene Punnett square when Bb x Bb What fraction of offspring at heterozygotes See Fig 9 7 Module 9 8 In any pedigree what does a circle indicate female A square male What is the difference between these shapes when they are filled in vs not filled in Filled in homozygous recessive for given trait Please note that the pedigree is a tool originally designed to follow royal bloodlines and selective breeding of animals While still a useful tool in genetics it is important to clarify a few points for modern use A pedigree chart is not a family tree It only represents the passing down of genetic traits from ancestors to descendants Squares and circles denote male and female sex respectively not gender As most human and nonhuman individuals never receive a karyotype test the symbols usually represent external genitalia rather than chromosomal sex A pedigree chart shows phenotype and not genotype unless otherwise indicated We typically construct a pedigree chart out of macroscopic observations only then we apply our knowledge of genetics when analyzing the chart When two individuals are connected by a horizontal line this represents a union of their gametes It does not necessarily represent a marriage relationship or even sexual activity because reproductive technologies may be used Vertical lines represent genetic parent offspring relationships with 50 of the parent s genetic material being transferred These lines do not necessarily represent parenting caregiving or any social relationship Ciera Ventura What is the probability that two individuals who are carriers for the cystic fibrosis will have a child with cystic fibrosis To
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