Gen e e n t e i t c c V a V ri r abi b li l ty Biolo l gy g Lec e tur u e e 9 : 9 Gen e et e ic I n I her e itan a ce

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1 Genetic Variability Biology 102 Lecture 9: Genetic Inheritance Asexual reproduction = daughter cells genetically identical to parent (clones) Sexual reproduction = offspring are genetic hybrids Tendency to inherit best traits of both parents Survival advantage against environmental change, competition, disease, etc. Siblings will often look similar, but not identical Each inherits 50% from each parent, but not the same 50% Crossing over Genetic Variability Ultimate sources of variability Mutations Genetic Variability Crossing over (recombination) Independent assortment Genetic Variability Problem with inbreeding Limited number of genes Increased chances that deleterious mutations will show up Remember how mutations affect genes Protein product altered in 1 of 4 ways 1) No effect Silent mutation Mutations 2) Protein is altered, but it doesn t matter Neutral change HAT vs CAP 3) Protein loses some or all of its function Deleterious change - HAT vs CAT 4) Protein functions better Example: HIV resistance 1

2 Genetics All somatic cells contain 23 pairs of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes Genes contained in each pair of chromosomes are identical Gene: Portion of genetic material that codes for a specific protein Allele: Any form of a given gene in the population Humans are diploid Genetics For any given gene, we carry 2 alleles Homozygous: Both alleles are the same for a given gene Heterozygous: 2 different alleles for a given gene Heterozygosity 2 alleles for a given gene Each codes for a slightly different protein Which will be made? Both? Dominant One allele is usually chosen over the others Consistently chosen across the species Called the dominant allele Need only be present in one copy to be expressed Recessive Consistently ignored alleles are recessive Only expressed if present in 2 copies Can be passed on to offspring, even if not expressed Recessive does NOT mean rare, or even less common! (Lab 9) Describes both alleles present for a given gene Capital letter = dominant Lower case letter = recessive Homozygous dominant = AA Heterozygous = Aa Genotype Homozygous recessive = aa 2

3 Phenotype Genotype is useful scientifically/medically, but what does the organism look like? Phenotype describes observable characteristics based on expression of the genotype Homozygous dominant = AA = brown eyes Heterozygous = Aa = brown eyes Homozygous recessive = aa = blue eyes Gregor Mendel Much of what we know about patterns of inheritance started with experiments done by this man Mendel s Pea Plants Mendel s Pea Plants Mendel observed 7 characteristics let s just look at seed color Examined patterns of inheritance of phenotype Experiment: cross plant with yellow seeds by plant with green seeds Result: all offspring had yellow seeds G Parent F1 Mendel s Pea Plants Experiment: self-pollinated one of the new yellow-seeded plants Result: 25% of new plants had green seeds! Mendel s Pea Plants Experiment: self-pollinated all of the F2 generation G F2 G F1 F2 4:0 G 3:1 G G G G 0:4 G 3:1 F3 3

4 Mendel s Conclusions 1. Factors for traits come in pairs only one will be passed from parent to offspring Carry 2 alleles for each gene Separated during meiosis Inherit one allele from each parent Mendel s Conclusions 2. If factors are identical, only that factor can be passed to offspring Homozygous Mendel s Conclusions 3. If factors are different, there is a 50/50 chance of each trait being passed on Heterozygous Another of Mendel s Conclusions 4. Some factors are inherited as a group, others are inherited randomly When genes are on the same chromosome, they are often inherited together Chromosomes are sorted randomly, so genes on different chromosomes are not inherited together (More on this later) Punnett Square Once diploidity was discovered, Mendel s observations were easily explained Punnett Square: a box diagram used to determine the probability of a given genotype ellow seed color = dominant allele Green seed color = recessive allele Punnett Square Possible offspring genotypes? Phenotypes? g g 4

5 Explaining Mendel g g g g Parent F1 Possible offspring genotypes? Phenotypes? Explaining Mendel g g Explaining Mendel Explaining Mendel g g g F1 F2 g g g g g g g g Mendel s Pea Plants g g F2 F3 Enough with the peas! Let s look at a human disease: Huntington s Disease Autosomal dominant, 100% penetrance Neurodegenerative disorder Decrease in physical coordination Mental decline Behavioral symptoms Symptoms usually do not appear until after age 35, after the gene may have been passed on to offspring 5

6 Scenario: A male is diagnosed with Huntington s Disease. His wife is tested for the disease gene and has two healthy alleles. They have three children. Disease is autosomal dominant How many disease alleles must be present to cause? Let s assume he is heterozygous: Hh His wife is homozygous: hh What is the probability that any one of their children will develop? Possible offspring genotypes? Phenotypes? H h h h Based on this information, the affected individual s children decided to be tested, and to have their children tested This information was compiled into a pedigree Pedigrees A phenotypic family tree Used to determine genotype and track alleles Females are circles Males are squares Darkened individuals have the condition or trait being tracked Pedigrees Note that there is at least one affected individual in every generation Hallmark of a dominant trait Assign a genotype to all individuals in the family Step 1: Assign a genotype to anyone we know is homozygous (remember: dominant disease) Step 2: Assign all offspring of healthy individuals one healthy allele Step 3: Assign all affected individuals one disease allele Step 4: Work from siblings or offspring to fill in any missing information (if possible some alleles may remain unknown)

7 Punnett Squares Hh 5 2 hh Let s look at a another human disease : Tay- Sach s Disease (TSD) Autosomal recessive Affects the enzyme hexosaminidase A Lysosomal enzyme Fatty substance builds up in brain Mental, physical deterioration; death by age Punnett Squares Scenario: 2 healthy individuals have a child with Tay-Sach s Autosomal recessive disease so child must be homozygous One allele inherited from each parent, yet each parent is healthy Both parents must be heterozygous We call these individuals carriers Have the disease gene, but do not have the disease Possible offspring genotypes? Phenotypes? Tay-Sach s Disease T t T t Deafness Let s do a pedigree for an autosomal recessive condition: hereditary deafness (dd) Trait may skip a generation Assign a genotype to each individual Deafness Step 1: Assign a genotype to anyone we know is homozygous Step 2: Give all unaffected individuals one D Step 3: Give all offspring of affected individuals one d Step 4: Work backwards look at affected individuals; d must be present in both parents Step 5: Double check, but some will remain a mystery 7

8 Deafness Inheritance In reality, inheritance is much more complicated Many factors at play that can alter expected inheritance patterns More than two alleles for one gene More than one gene affects a trait One gene modifies expression of another gene (epistasis) We will look at 2 factors here: Incomplete dominance Codominance Incomplete Dominance Sometimes there is not one clear dominant allele In a heterozygous individual, both alleles are expressed Phenotype is a blend of both traits Incomplete Dominance Example: snapdragon color Both red (RR) and white (rr) are dominant Heterozygous (Rr) = pink Use a Punnett square to predict the ratio of red:pink:white offspring if 2 pink snapdragons are crossed Incomplete Dominance Genotype? Phenotype? Incomplete Dominance Example in humans: hair Both curly (CC) and straight (SS) are dominant Heterozygous (CS) = wavy Use a Punnett square to predict the probability of a child with wavy hair from a father with wavy hair and a mother with straight hair 8

9 Incomplete Dominance Genotype? Phenotype? C S S S Codominance Commonly seen when more than 2 alleles exist for the same gene Both dominant alleles are expressed at once Not a blend of the 2 traits both distinct traits can be seen at the same time Incomplete vs. Codominance Codominance Incomplete dominance and codominance are NOT the same thing!! Incomplete dominance: phenotype is a blend of the two traits Codominance: both traits are seen at the same time Dominant Dominant Human example: A, B, O blood types Both type A and type B are dominant (I A and I B ) Make different glycoproteins on the membrane of red blood cells Type O is recessive Makes no such glycoprotein If I A and I B are both present, both will be expressed Blood Type Codominance Consider the following genotypes, and determine the phenotype (blood type) that would be present in each individual Genotype I A I A I A i ii Phenotype I B I B I B i I A I B 9

10 Chaplin Paternity Case Before the days of DNA testing, blood type was used to settle paternity suits Doesn t always work though Charlie Chaplin was involved in such a case in 1942 with actress Joan Barry Chaplin Paternity Case Charlie Chaplin s blood type: AB Joan Barry s blood type: O Child s blood type: O Use a Punnett square to determine whether Charlie Chaplin could have been the child s father Chaplin Paternity Case Chaplin Paternity Case Charlie Chaplin s blood type: AB Only possible genotype: Joan Berry s blood type: O Only possible genotype: Child s blood type: O Only possible genotype: I A I B i i Chaplin Paternity Case Could Charlie Chaplin have been the child s father? 10