Homework #24 (130222): Chapter 14 (due Friday 130222) Total points: 20 for Attempt on all questions ONE Concept Map: 10 points Objectives Gregor Mendel’s Discoveries 1. Explain how Mendel’s particulate mechanism differed from the blending theory of inheritance.
• Blending: parents genes mixed • Particulate: parents genes still retain identity The blending theory was that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow make green. It predicts that over generations, freely mating populations will eventually becoming a uniform population of individuals.According to Mendel’s particulate hypothesis of inheritance, the gene idea was that parents pass on discrete heritable units that retain their separate identities in offspring. 2. Define the following terms: true-breeding, hybridization, monohybrid cross, P generation, F1 generation, and F2 generation. True breeding: When true-breeding plants self-pollinate, all their offspring are of the same variety. For example, purple flowers give rise to plants with purple flowers. Hybridization: The crossing of two true-breeding varieties is hybridization.
Monohybrid cross: A monohybrid cross is a genetic cross using a single trait with two alleles.P generation: The true-breeding parents are referred to as the P generation, or parental generation. F1 generation: The hybrid of the P generation is the F1 generation, or the first filial generation. F2 generation: The offspring of the self-fertilizing F1 generation is the F2 generation, or second filial generation. 3. List and explain the four components of Mendel’s hypothesis that led him to deduce the law of segregation.
• Alleles make different characters • Two alleles inherited • Dominance and recessive • Alleles segregate in gamete formation First, alternative versions of genes account for variations in inherited characters.A gene exists in different versions. Second, for each character, an organism inherits two alleles, one from each parent. An organism inherits one set of chromosomes from each parent, giving two representations for a gene. Third, if the two alleles at a locus differ, then the dominant allele determines the organisms, while the recessive allele has no noticeable effect on the organism’s appearance.
Fourth, the law of segregation, states that the two alleles for a heritable character separate, or segregate, during gamete formation and end up in different gametes. 4.Use a Punnett square to predict the results of a monohybrid cross, stating the phenotypic and genotypic ratios of the F2 generation.
5. Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype. • Dominant masks recessive • Heterozygous is two alleles, homozygous is one • Phenotype = appearance, genotype = alleles in gene A dominant allele determines an organism’s appearance, or phenotype, and masks the recessive allele. However, the recessive allele is still present in the genotype, or genetic makeup.A homozygous gene has a pair of identical alleles for that character. An organism that has two different alleles for a gene is said to be heterozygous for that gene.
6. Explain how a testcross can be used to determine if an individual with the dominant phenotype is homozygous or heterozygous. • Determined to cross allele to find phenotype • Based on genotype A testcross, which is the breeding of a recessive homozygote with an organism of dominant phenotype is a method used to determine the genotype.They cross the two plants and observe their offspring and based on that can determine the genotype of the dominant organism with the unknown genotype of the P generation. 7. Use a Punnett square to predict the results of a dihybrid cross and state the phenotypic and genotypic ratios of the F2 generation. 8. State Mendel’s law of independent assortment and describe how this law can be explained by the behavior of chromosomes during meiosis.
Mendel’s law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.This means that each allele separates and packages into a gamete in all possible allelic combinations. 9. Use the rule of multiplication to calculate the probability that a particular F2 individual will be homozygous recessive or dominant. 10. Given a Mendelian cross, use the rule of addition to calculate the probability that a particular F2 individual will be heterozygous. 11.
Use the laws of probability to predict, from a trihybrid cross between two individuals that are heterozygous for all three traits, what expected proportion of the offspring would be: a. omozygous dominant for the three traits b. heterozygous for all three traits c. homozygous recessive for two specific traits and heterozygous for the third A. For homozygous dominant for the three traits, it is 1/64.
B. For heterozygous of all three traits, it would be 1/8. C. It would be 3/8. 12. Explain why it is important that Mendel used large sample sizes in his studies. Mendel used large sample sizes in his studies to understand the feature of inheritance with a broader dependent group, understand chance, and allow the results to accurately conform to his predictions.Extending Mendelian Genetics 13.
Give an example of incomplete dominance and explain why it does not support the blending theory of inheritance. • Doesn’t support because gives phenotypic ration and confirms segregation An example of incomplete dominance is in snapdragon, their color becoming pick. This does not support the blending hypothesis because the alleles that segregate in the F1 generation can than make red and white flowers. 14. Explain how phenotypic expression of the heterozygote differs with complete dominance, incomplete dominance, and codominance.
Complete dominance- phenotypes of heterozygote and dominant same • Codominance- both affect phenotype • Incomplete dominance- between two alleles Complete dominance phenotype is that the heterozygote and the dominant is indistinguishable between the two. Codominance is that both alleles affect the overall phenotype while incomplete dominance is that the organism has a phenotype that is between the extremes of the two alleles. 15.
Explain why Tay-Sachs disease is considered recessive at the organismal level but codominant at the molecular level. • Recessive because need two copies Make equal numbers of normal and dysfunctional molecules. At an organismal level, it is recessive because an organism needs to inherit two copies of the allele to have the disease. However, if an individual is heterozygote, at a molecular level, it is codominant because both code for molecule, making equal number of normal and dysfunctional enzyme molecules.
16. Explain why genetic dominance does not mean that a dominant allele subdues a recessive allele. Illustrate your explanation with the use of round versus wrinkled pea seed shape. • Does not disappear forever Can still appear in later generations • Coexist • Example- sugar can still make seed round even with recessive allele A genetic dominance does not mean that a dominant allele subdues a recessive allele because even though they do not interact, they coexist and they come into play from genotype to phenotype. For example, in dominant allele, round, it codes for an enzyme, while wrinkled do not.
However, a wrinkled seed can become round because if the seed is heterozygote, it can become round because the sugar is converted to starch, with enough enzymes produced. 17.Explain why dominant alleles are not necessarily more common in a population. Illustrate your explanation with an example. • example polydactyly.
• Dominant • 399/400 homozyote recessive An example of a dominant allele codes for a condition known as polydactyly, which means that a baby has extra fingers or toes, but only 1/400 have it, to not have it, 399/400 Americans are homozygous recessive. 18. Describe the inheritance of the ABO blood system and explain why the IA and IB alleles are said to be codominant. • By various combinations of three different enzymes • Multiple allees • Codominant in AB blood Four possible phenotypes • Variety of combinations The ABO blood group is established through several alleles of a single gene that has four possible phenotypes: AB, O, A, and B. the four blood groups result from a variety of combinations of three alleles that either attach an A carbohydrate, a B carbohydrate, or neither. Everyone carries two alleles, and consequentially, there are six genotypes and four p henotypes possible. IA and IB alleles are dominant I alleles; IAIA or IAi genotypes have type A, IBIB or IBi have type B, and ii have type O blood. IA and IB are codominant, and type for AB blood.
9. Define and give examples of pleiotropy and epistasis. • Pleiotropy: when genes affect more than one phenotypic trait o Example: sickle-cell disease • Epistasis: when a gene at one locus affects the phenotype of another gene with a different locus o Example: coat colors of mice 20.
Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms. • Cannot be either-or • Vary along continuum Most polygenic characters are described in quantitative terms because they cannot be classified as either-or.They vary along a continuum based on combined effects of at least two genes on one phenotypic character. An example would be of human skin color, which is a simple model for polygenic inheritance. 21.
Describe how environmental conditions can influence the phenotypic expression of a character. Explain what is meant by “a norm of reaction. ” • Health • Temperature sensitive genes The environment can have powerful effects on an organism, especially to the health; simple examples include temperature sensitive genes.The “norm of reaction” refers to the phenotypic range of a genotype depending on its environment.
22. Distinguish between the specific and broad interpretations of the terms phenotype and genotype. • Phenotype: o Specific: specific characteristics of an organism o Broad: entirety of the organism, which includes physical aspects • Genotype: o Specific: a single locus o Broad: an organism’s entire genetic makeup Mendelian Inheritance in Humans 23. Explain why studies of human inheritance are not as easily conducted as Mendel’s work with his peas. • Life span • Fewer offspringStudies of human inheritance are not as easily conducted as Mendel’s work with his peace because of their life span when dealing with generations. They also produce fewer offspring, and breeding is seen as a widely unaccepted process.
24. Given a simple family pedigree, deduce the genotypes for some of the family members. 25. Explain how a lethal recessive allele can be maintained in a population. • Heterozygote carriers • Recessive individuals Lethal recessive alleles can be maintained in a population because individuals who might not phenotypically show the disease may be carriers and have the allele as a heterozygote.
The heterozygous recessive individuals could then reproduce to produce offspring that display the deadly phenotype. 26. Describe the inheritance and expression of cystic fibrosis, Tay-Sachs disease, and sickle-cell disease.
• 1 out of 2500 • 1 out of 3600 • 1 out of 400 One out of every 2500 Americans of European descents are affected with cystic fibrosis; of these, 25 are carriers for the disease. The normal allele for the gene is a membrane protein that transports Cl- between the cells and the extracellular fluid. Cystic fibrosis affects these channels and causes high levels of chloride in the extracellular fluid.Mucus coats certain cells in the pancreas, lungs, digestive tract, and more. Eventually, this causes poor nutrient absorption, chronic bronchitis, and bacterial infections. Children can die from this disease if not treated early.
Tay-Sachs disease is caused by a dysfunctional enzyme that fails to breakdown specific brain lipids. Symptoms include seizures, blindness, and degenerative motor and mental performances after birth. Tay-Sachs disease is common among Ashkenazic Jews and affects one out of every 3600 births.
Sickle-cell disease is most common among those of African descent.It affects one out of 400 African-Americans, and is caused by the substitution of a single amino acid in hemoglobin. Red blood cells become deformed into a sickle shape when oxygen levels are low, causing numerous problems such as clumping cells and clogged arteries. 27. Explain why lethal dominant genes are much rarer than lethal recessive genes. • Kill host • Heterozygous carriers Lethal dominant genes are much rarer than lethal recessive genes because they tend to kill their “host” because their allele can be passed on. Lethal recessive genes, however, can be passed down by heterozygous carriers. 8.
Give an example of a late-acting lethal dominant gene in humans and explain how it can escape elimination by natural selection. • Huntington’s disease • Dormant until reproduction An example of a late-acting lethal dominant gene in humans is Huntington’s disease. It is a degenerative disease of the nervous system, and can escape elimination by natural selection because it does not kill its “host” early. On. Phenotypes do not generally appear until the “host” is at least 35 years old, and it stays dormant long enough for the “host” to reproduce and pass down the allele. 9. Define and give examples of multifactorial disorders in humans. • Genes and environment • Heart disease Multifactorial disorders in humans are caused by genes and the environment.
Examples of such disorders include heart disease, diabetes, cancer, and some mental illnesses; some disorders are also polygenic. The family history of the individual indicates the susceptibility of the disorder, or when environmental factors are isolated. 30. Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling. Prospective parents • Choice of termination Carrier recognition can be used in genetic screening and counseling by alerting prospective parents through tests, and defining preparations and strategies.
Fetal testing can be used through ultrasound, amniocenetesis, and chorionic villus sampling; they can show risk of fetal death and allows the choice of termination to parents. Newborn screeing, which includes heel sticks, drops on filter paper, and mass spectrometry, allow the earliest treatment, often with the most complete correction. Key Terms llele amniocentesis carrier character chorionic villus sampling (CVS) codominance complete dominance cystic fibrosis dihybrid dominant allele epistasis F1 generation F2 generation genotype heterozygous homozygous Huntington’s disease hybridization incomplete dominance law of independent assortment law of segregation monohybrid multifactorial norm of reaction P generation pedigree phenotype pleiotropy polygenic inheritance Punnett square quantitative character recessive allele sickle-cell disease Tay-Sachs disease testcross trait true-breeding