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Chapter 5

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Chapter 5

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  1. Chapter 5 Are You Only as Smart as Your Genes?Mendelian and Quantitative Genetics 0

  2. 1 The Inheritance of Traits Offspring resemble their parents, but not exactly. Siblings resemble each other, but not exactly. How much is because of environment? How much is inherited?

  3. 1 The Inheritance of Traits The human life cycle: Adults produce gametes in their gonads by meiosis. Sperm cells fertilize egg cells to form single-celled zygotes. Repeated cell divisions form the embryo.

  4. 1 The Inheritance of Traits The human life cycle, cont.: The embryo grows to become a fetus. After birth, the individual continues to grow until reaching adulthood.

  5. 1 The Inheritance of Traits Genes are segments of DNA that code for proteins. Analogous to a store for shopping or words in a paragraph; each store specializes and each paragraph has different meaning We have many genes on each of our chromosomes. Chromosomes are clusters of genes in a single piece of DNA. Analagous to groups of stores in a shopping center, or chapters in a book Different types of cells use different genes in different ways, much like each of us gets different things in the same shopping center depending on our purpose

  6. 1 The Inheritance of Traits - Producing Diversity in Offspring Mistakes in copying DNA (mutations) produce different versions of genes (alleles), with different results. Consider: “I threw a ball” vs. “I threw a bell”.

  7. 1 The Inheritance of Traits - Producing Diversity in Offspring Parent cell has two complete copies of the manual: 23-page copy from mom and 23-page copy from dad Dad’s version will be similar but NOT identical to Mom’s version (from 0.1 – 1% different at gene level) 23 pairs of homologous chromosomes

  8. 1 Producing Diversity in Offspring Segregation: in meiosis, one member of each homologous pair goes into one gamete; other member goes to other gamete Gamete gets just one copy of each page of the manual some get dad (of gamete)’s copy; the others get mom (of gamete)’s copy Independent assortment randomly determines which member of a pair of chromosomes goes into a gamete This is due to random alignment during metaphase I Think of doing 10 coin flips, over and over again, and recording each sequence; will each sequence be different? About 8 million different combinations of chromosomes (223 ~ 8 million).

  9. 1 The Inheritance of Traits - Producing Diversity in Offspring Due to independent assortment, the instructions in each single sperm cell is an essentially unique combination of pages.

  10. 1 Producing Diversity in Offspring Random fertilization of unique sperm and unique egg produces more diversity: 64 trillion possibilities from two parents! (8 million x 8 million) No two humans are genetically identical, except for monozygotic twins (identical twins). Somatic mutations during growth and development make identical twins VERY slightly different (1:10,000) Dizygotic twins (fraternal twins) are simply two siblings that develop and are born at the same time.

  11. 2 Mendelian Genetics: When the Role of Genes Is Clear Gregor Mendel: first to accurately describe rules of inheritance for simple traits His research involved controlled mating between pea plants. His pattern of inheritance occurs primarily in traits that are due to a single gene with a few clearly different alleles (e.g., A and B blood types, Rh +/-). Mendel’s principles also apply to many genetic diseases or traits (left-handedness, red-green color blindness, SCID, etc.) in humans.

  12. Genotypes and Phenotypes Alterations to nucleotides within genes (called mutations) occur all the time. Defining the specific mutation (i.e., adenine to guanine) in the gene allows one to describe the genotype of that variant gene. These gene variants are called alleles. Some alleles affect an aspect(s) of the organism that we can see(called a trait or a phenotype) because the altered gene makes a protein with an altered activity. If the genotype being studied matches a commonly observed genotype within a population (usually the most prevalent genotype), then that allele is a wild-type allele of that gene. Wild-type alleles do NOT have to be fully functional (O-type blood is more common than A-type or B-type blood There can be multiple wild-type alleles in a population for example, curly hair and straight hair are both common alleles

  13. Must wild-type traits be functional? Note that the gene does not necessarily need to be functional compared to that same gene in other species, or other alleles present in small numbers in the population (e.g., full body hair in humans vs. gorillas)

  14. Dominant and Recessive Alleles Some alterations must be present on both genes copies to change the phenotype. These altered genotypes are called recessive alleles and the changed phenotype are called recessive traits. If a disease is the result of this genotype, the disease is classified as a recessive genetic disease. Sometimes a change in only one allele changes the phenotype This altered genotype is a dominant allele and corresponding phenotype is a dominant trait. If a disease results from this genotype, then this disease is called a dominant genetic disease.

  15. 2 Mendelian Genetics: When the Role of Genes Is Clear A genotype of an individual is described as the genotype of both alleles for a given gene Homozygous (-ote): both alleles for a gene in the genome are identical The alleles will have the same activity Both alleles could be functional; but usually they will both be defective Heterozygous (-ote): the two alleles for a gene in the genome are different Most commonly, one allele is more active than another allele, and so will be dominant to that other allele

  16. 2 Non-carriers, Healthy carriers, and the afflicted To be afflicted by a recessive genetic disease, both alleles must be defective Homozygotes of the recessive allele will suffer the disease – they are considered afflicted. Heterozygotes) will have one functioning allele, and one defective allele. Because the functioning allele will mask any deficiencies, the person is healthy, but pass the trait to his offspring. That person is called a healthy carrier of the disease. To be afflicted by a dominant genetic disease, only one allele needs to be defective Heterozygotes will therefore be afflicted, as carrying the trait will result in the disease. Homozygotes of most dominant diseases are rare (two afflicted individuals need to mate, and it is often fatal before birth in most diseases)

  17. 2 Mendelian Genetics - Genetic Diseases in Humans Cystic fibrosis: a recessive human genetic disease Defect in chloride ion transport, prevents water secretion Causes problems in lung function, sweat production, pancreatic enzyme secretion, sperm production Recurrent lung infections, digestive problems, infertility, dramatically shortened lifespans occur in the afflicted Heterozygotes posses a copy of the defective gene, and a copy of the functional gene. They carry the defect but don’t show disease symptoms. They are defined as carriers. It is the most common recessive disease among Europeans Heterozygotes show resistance to cholera infections historically. Cholera outbreaks were common before sanitary water practices were reinstates and became common.

  18. 2 Mendelian Genetics - Genetic Diseases in Humans Huntington’s disease: a dominant human genetic disease Progressive, incurable, always fatal Symptoms occur in middle age Mutant protein forms clumps inside nerve cell nuclei, killing the nerves and causing irreversible mental defects Having a normal allele cannot compensate for this; the mutant allele disrupts the function of the normal allele. Homozygous dominants and heterozygotes will express the disease. Only those wildtype for the recessive wild-type gene will be healthy All carriers must be afflicted.

  19. Why use Punnet squares to analyze inheritance? Assume there is no difference between Aa and aA. Genetically, this means that you should not worry about whether whether A or a comes from the mother or the father (known as maternal or paternal effect) Here, two homozygotes mate (one dominant; one recessive) Here, two heterozygotes mate: Punnet squares (the quartered squares shown below) are the easiest way to demonstrate and understand inheritance from diploid parent to diploid offspring through haploid intermediates (now known to be gametes, or egg and sperm). Each mini-square indicates the traits exhibited by various diploid children. Paternal (diploid) Aa Paternal (diploid) aa haploid gametes a A a a haploid gametes AA Aa Aa Aa A A Maternal (diploid) Aa Maternal (diploid) AA a A aA aa Aa Aa haploid gametes haploid gametes Note how the potential genotypes of the children can vary and with different frequencies!

  20. 2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes Punnett square: graphic way to predict possible genotypic outcomes of a cross (mother and father mating of any species) Consider a cross between two cystic fibrosis carriers “F” = normal allele; “f” = recessive disease allele The cross would be: F f x F f What offspring could result?

  21. Using Punnet squares to predict progeny phenotype Example: predict the progeny of an Aa father and an Aa mother Steps: Determine diploid state of father (usually controlling columns) Determine diploid state of the mother (usually controlling rows) Determine haploids of father; they head each column Determine haploids of mother; they head each row Fill each square with the corresponding row head and column head Determine phenotype of each possible progeny. Paternal (diploid) Aa Maternal (diploid) Aa

  22. Using Punnet squares to predict progeny phenotype Example: predict the progeny of an Aa father and an Aa mother Steps: Determine diploid state of father (usually controlling columns) Determine diploid state of the mother (usually controlling rows) Determine gametes of father; they head each column Determine gametes of mother; they head each row Fill each square with the corresponding row head and column head Determine phenotype of each possible progeny. Paternal (diploid) Aa haploid gametes a A A Maternal (diploid) Aa a haploid gametes

  23. Using Punnet squares to predict progeny phenotype Example: predict the progeny of an Aa father and an Aa mother Steps: Determine diploid state of father (usually controlling columns) Determine diploid state of the mother (usually controlling rows) Determine gametes of father; they head each column Determine gametes of mother; they head each row Fill each square with the corresponding row head and column head Determine phenotype of each possible progeny. Paternal (diploid) Aa haploid gametes a A AA Aa A Maternal (diploid) Aa a aA aa haploid gametes Note that we get 1 AA , 2 Aa’s and 1 aa, out of four possibilities.

  24. 2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes

  25. Gender-Specific Inheritance birds and snakes: males have ZZ; females have WZ (a few genes on the W chromosome are also present on the Z chromosome to allow pairing during meiosis) The effect on the progeny are as follows: live-bearing mammals: females have XX; males have XY (a few genes on the Y chromosome are also present on the X chromosome to allow pairing during meiosis) The effect on the progeny are as follows: P: XY P: ZZ X Y Z Z In mammals (and birds and snakes), sex chromosomes exist that allow two alleles per gene to be present in one gender, but only one allele per gene in the other gender. XY XX X ZZ ZZ Z P: XX P: WZ XY XX X WZ WZ W The female gamete determines the gender of each avian or serpentine zygote. Both alleles of the DMRT gene on the Z chromosome must be active to initiate maleness The male gamete determines the gender of each mammalian zygote. The SRY gene initiates maleness in the embryo

  26. NOTICE ONLY THE GENDER SPECIFIC EVENTS! NOTHING HERE NEEDS TO BE MEMORIZED! Inheritance of sex-linked traits P: XgY (SCID) Xg Y X-linked SCID is a recessive genetic disease caused by mutations in a gene called IL2RG resulting in a protein that cannot promote the maturity and communication of cells in the immune system; we’ll use the symbol G for functioning IL-2Rg, and g for the defective protein. Note that it is superscripted to the X to indicate X-linkage. An afflicted father and a non-carrier mother will produce only carrier females, but will never produce afflicted males. XGXg (healthy) XGY (healthy) XG P: XGXG (healthy) XGY (healthy) XG XGXg (healthy) Let’s analyze some of the unique aspects of X-linked inheritance… P: XGY (healthy) A healthy non-carrier male and a carrier female will never produce afflicted females, but can produce afflicted males. Stated differently, an afflicted male must come from a carrier female. P: XgY (SCID) XG Y Xg Y XGXG (healthy) To produce an afflicted female, there must be progeny between an afflicted father and a carrier female; this must be a more rare event than what is displayed on the left. XGY (healthy) XG XGXg (healthy) XGY (healthy) XG P: XgXG (healthy) P: XgXG (healthy) XgY (SCID) Xg XgXG (healthy) XgY (SCID) Xg XgXg (SCID) This is the inheritance of male-pattern baldness!

  27. 2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes Dihybrid crosses are crosses that involve two traits. The first step in a dihybrid is to determine the possible gametes. For peas, Yellow color peas (Y) are dominant to green color peas (y) and Round peas (R) are dominant to wrinkled peas (r). If you cross YyRr x YyRr, Possible gametes for parent 1 are YR, Yr, yR, yr Possible gametes for parent 2 are YR, Yr, yR, yr

  28. 2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes • The results of the cross results in a 9:3:3:1 phenotypic ratio. • Due to the appearance of two recessive traits, 7 of the 16 possible children look different than either parent! • Also, only 4 of 16 children have the same genotype as the parents

  29. 3 Quantitative Genetics: When Genes and Environment Interact Quantitative traits show continuous variation: Large range of phenotypes E.g., height, weight, intelligence Variation due to both genetic and environmental differences

  30. Genes are segments of DNA that code for ________. proteins centromeres carbohydrates karyotypes

  31. Genes are segments of DNA that code for ________. proteins centromeres carbohydrates karyotypes

  32. Genes are comparable to________. words in an instruction manual pages in an instruction manual copy of pages in an instruction manual appendix in an instruction manual

  33. Genes are comparable to________. words in an instruction manual pages in an instruction manual copy of pages in an instruction manual appendix in an instruction manual

  34. Which of these events does not contribute to unique combinations of alleles? mutations independent assortment random fertilization cell cycle checkpoints

  35. Which of these events does not contribute to unique combinations of alleles? mutations independent assortment random fertilization cell cycle checkpoints

  36. True or False: Monozygotic twins occur when two separate eggs fuse with different sperm. True. False.

  37. True or False: Monozygotic twins occur when two separate eggs fuse with different sperm. True. False.

  38. A pea plant has one recessive allele for wrinkled seeds and one dominant allele for smooth seeds. What will the pea plant look like? wrinkle smooth half wrinkled, half smooth not enough information to tell

  39. A pea plant has one recessive allele for wrinkled seeds and one dominant allele for smooth seeds. What will the pea plant look like? wrinkle smooth half wrinkled, half smooth not enough information to tell

  40. Two heterozygotes mate. What are the odds that their offspring will be homozygous recessive? 100% 75% 50% 25%

  41. Two heterozygotes mate. What are the odds that their offspring will be homozygous recessive? 100% 75% 50% 25%

  42. Which trait is a quantitative trait that does not show continuous variation? height skin color presence or absence of a widow’s peak intelligence

  43. Which trait is a quantitative trait that does not show continuous variation? height skin color presence or absence of a widow’s peak intelligence

  44. Does nature or nurture play a bigger role in determining who we are? nature nurture they both play a large role

  45. Does nature or nurture play a bigger role in determining who we are? nature nurture they both play a large role

  46. The Punnett square shown here illustrates the outcome of a cross between a man who carries a single copy of the dominant Huntington’s disease allele and an unaffected woman. What are the odds that Huntington’s disease will not be passed to this offspring? 100% 75% 50% 25%

  47. The Punnett square shown here illustrates the outcome of a cross between a man who carries a single copy of the dominant Huntington’s disease allele and an unaffected woman. What are the odds that Huntington’s disease will not be passed to this offspring? 100% 75% 50% 25%

  48. This Punnett square illustrates the likelihood that a woman who carries the cystic fibrosis allele would have a child with cystic fibrosis if the sperm donor were also a carrier. What are the odds that this offspring will have cystic fibrosis? 100% 75% 50% 25%

  49. This Punnett square illustrates the likelihood that a woman who carries the cystic fibrosis allele would have a child with cystic fibrosis if the sperm donor were also a carrier. What are the odds that this offspring will have cystic fibrosis? 100% 75% 50% 25%