BIOL 189 Chapter 8 - Patterns of Inheritance PDF

Chapter 8 Patterns of Inheritance  Chapter 8 Outline  Genetics: history and terms  Basic patterns of inheritance  Mendel’s laws  Extensions of Mendel’s laws  Human genetic disorders ◦ Autosomal recessive ◦ Autosomal dominant ◦ Sex-linked  Let’s Remembe...

Chapter 8 Patterns of Inheritance  Chapter 8 Outline  Genetics: history and terms  Basic patterns of inheritance  Mendel’s laws  Extensions of Mendel’s laws  Human genetic disorders ◦ Autosomal recessive ◦ Autosomal dominant ◦ Sex-linked  Let’s Remember Chromosomes! homologous pair  History of Genetics  Genetics: 1700s-1850 ◦ Features can be inherited from parents by offspring – see agriculture ◦ The dominant idea of the time was the theory of blended inheritance  Traits of both parents are “blended” in the offspring  History of Genetics: Blended Inheritance  However, some observations didn’t match those predictions ◦ How to explain “lost” traits that reappear in subsequent generations?  When traits “skip” a generation ◦ Features of offspring are not always intermediate (between both parents) Fr. Gregor Mendel had other ideas…  History of Genetics  Gregor Mendel - 1865 ◦ Viennese monk ◦ The “Father of Genetics” ◦ He did experimental crosses with pea plants – followed different traits/trait combinations ◦ Proposed the concept of the gene* as the basic unit of inheritance – discrete units of genetic information  *He did not use the word!  Essential Terms in Genetics Diploid: ◦ Having two copies of each chromosome, in the form of a homologous pair – one paternal, one maternal Humans: 2n = 46 Haploid: ◦ Reproductive cells have only one copy of each chromosome 23 23 Humans: n = 23  Essential Terms in Genetics Gene:  Portion of DNA that governs one or more genetic traits  Single chromosome contains many hundreds of genes  Contain instructions for the manufacture of proteins Allele:  An alternative version of a gene – each gene has two or more Genotype:  Allelic makeup responsible for the phenotype of an individual Phenotype:  Expression of a genetic trait Somatic Cells are Diploid  Alleles in an Individual Dominant allele: ◦ Exerts a controlling influence on the phenotype ◦ Denoted by upper case letters – example: A ◦ Only one copy of the allele required for individual to express the trait ◦ The letter used to describe the trait is named for the dominant allele  Example: P for purple, p for white  Alleles in an Individual Recessive allele: ◦ Has no effect on phenotype when paired w/ dominant allele ◦ Allele must be present in two copies for individual to express the trait ◦ Denoted by lower case letters – example: a  Alleles in an Individual Homozygote: ◦ Individual has two copies of the same allele ◦ If two copies dominant allele (AA), individual is homozygous dominant ◦ If two copies of recessive allele (aa), individual is homozygous recessive Heterozygote/Heterozygous ◦ Individual has two different alleles (Aa)  Alleles: From Gene Mutations  Different alleles originally arose from a mutation ◦ Change in DNA that makes up a gene ◦ Sequence of bases in a gene may be modified through random chance, damage ◦ Do not occur because they are “needed” by the organism – are a random occurrence  Mutations can be: ◦ Harmful – tend to be recessive alleles; heterozygote is not affected ◦ Neutral – new allele protein almost identical to original ◦ Beneficial – rarest of mutations; improve on function  What is a Genetic Cross?  A cross is a controlled mating experiment performed to examine inheritance of a particular trait  Mendel looked at 7 traits/characteristics in peas:  What is a Genetic Cross?  Mendel proposed that for each trait, offspring We call these “alleles” but inherit two separate he didn’t use the term “units” of genetic information – one from each parent ◦ One “unit” (=allele) inherited from the egg ◦ One “unit” inherited from the sperm ◦ These combine to make the genotype of the offspring  What is a Genetic Cross? Mendel started a cross with true-breeding plants called the P generation = offspring have same phenotype as parents Cross the parents to get the F1 generation = First ‘filial’ generation They will be heterozygous and look like the dominant trait parent Self-cross (=interbreed) the F1 to get the F2 generation = Second ‘filial’ generation See a predictable ratio of dominant to recessive phenotypes (3:1) Let’s do a Punnett square!  Work this example! P What are the genotypes of the parents? F1 F1 generation? F2 F2 generation?  What is a Genetic Cross? Phenotypic Vs. Genotypic Ratios YY yy  Summary of Mendel’s Concepts 1. Alternative versions of genes cause variation in inherited traits – P allele causes purple flowers, p allele white flowers 2. Offspring inherit one copy of a gene from each parent 3. An allele is dominant if it has exclusive control over the phenotype of an organism when paired with a different allele – Law of Dominance  Summary of Mendel’s Concepts 4. Two copies of a gene (= the homologous pair) separate in meiosis and end up in different gametes – this is the Law of Segregation (see our use of Punnett squares!) 5. Genes do not influence each other with regard to the sorting of alleles into gametes – Law of Independent Assortment 6. Gametes will fuse during fertilization w/o regard to which alleles that they carry  Mendel’s First Law: Law of Segregation  Can represent this separation (and their random recombining through fertilization) by using a Punnett square  List the genotypes of each parent, segregate the alleles (Pp  P p ) to represent the gametes  A Punnett square gives the probability of the genotypes and phenotypes in the offspring – every offspring outcome is independent of the others  Law of Segregation: Each Gamete Carries One of Two Alleles  your understanding! You want to examine wing color pattern inheritance in a beetle species. For this trait, solid wings are dominant to polka-dotted wings. You cross a dad homozygous for solid wings and a mom homozygous for polka-dotted wings. Correctly identify the alleles based on rules of dominance, use those to complete the Punnett square of the offspring, and report on the their genotypes and phenotypes:  Extensions of Mendel’s Laws  Many alleles do not show complete dominance or are not under simple genetic control, so phenotype is controlled by: ◦ Incomplete dominance ◦ Multiple alleles ◦ Codominance ◦ Pleiotropy ◦ Epistasis ◦ Polygenic traits Twins Lucy and Maria Aylmer  Incomplete Dominance Incomplete Dominance:  Heterozygote is an intermediate ◦ Neither allele exerts it full effect in the phenotype ◦ This is the most like a “blended” phenotype  If these heterozygous offspring are bred, the parental features will appear again  Snapdragons: ◦ Red + white  pink flowers  Horses: ◦ Chestnut horse (dark) + cream horse (light) can be bred to produce a palomino  Multiple Alleles  Multiple alleles - When there are 3 or more alleles of a gene (but you still only inherit 2, one from each parent) Example: Human blood groups  Three alleles of the gene for blood type ◦ IA: A carbohydrate on RBCs ◦ IB: B carbohydrate ◦ i: neither A nor B  Combinations of these possible alleles produces four blood groups: A, B, AB, and O  Genetics of Blood Type  You still inherit two alleles (one from mom and one from dad) – even though there are three possible alleles of the one blood type gene  The combinations of those alleles produce the genotype and phenotype Let’s do a Punnett square! We will shorthand these alleles as A (IA), B (IB ), and i (i)  Genetics of Blood Type Example: A mom has Type O and a dad has Type AB blood Mom’s blood type alleles: Dad’s blood type alleles: s  Codominance Codominance:  The effect of both alleles are equally visible in the heterozygote  The two parental phenotypes are expressed together in the offspring Codominance is not the same as incomplete dominance!  Codominance  Human blood (ABO) groups show codominance IAIA or IAi = Type A ◦ IBIB or IBi = Type B ◦ IAIB = Type AB ◦ ii = Type O ◦  AB blood type is not “halfway between” A and B ◦ If so, this would demonstrate incomplete dominance ◦ People with AB have both carbohydrates (A and B) on RBC surface  Epistasis  Interaction of genes affects the phenotype ◦ Phenotypic effect of alleles of one gene are dependent on which alleles are present for another gene  In mice, coat color determined by Gene B: ◦ BB or Bb = black ◦ bb = brown  But Gene C also has an influence: ◦ If mice are CC or Cc, they are the color above ◦ If mice are cc , they are white (regardless of which B alleles are present)  Therefore, Gene C is epistatic to the Gene B  Epistasis Albino mice (without coat color) are so because of epistatic effects of Gene C on Gene B  Pleiotropy When a single gene influences a variety of traits:  Albinism – different traits are affected  Absence or reduced production of melanin (most individuals have blue eyes)  Skin color Indian boys with albinism  Vision problems – range from cross-eyedness to blindness  Pleiotropy Marfan’s syndrome ◦ Fibrillin 1 gene nonfunctional in connective tissues ◦ Problems with vision, skeleton ◦ Many are tall and gangly ◦ Nervous system, lungs and skin affected  Polygenic Traits Polygenic:  Trait is determined by two or more genes  Results in a continuous distribution of the range of traits ◦ Body size ◦ Height ◦ Skin color  Are at least three genes that control the number of “units” of melanin produced  Also influenced by environment Twins Kian and Remee Human Genetic Disorders Gene (=Allele)-based disorders Whole chromosome disorders  Human Genetic Disorders  For a disorder to be inherited by offspring, the parent can: (1) Show and have the disorder (2) Be asymptomatic and a carrier for the disorder (3) Not have the disorder seen in the child because it is a whole chromosome condition and not allele-based ◦ Example: Parents of a child with Down Syndrome do not exhibit the disease, but it is still a “heritable” disorder because it has a genetic basis (associated with gametes)  Learn more about chromosomal disorders from the National Human Genome Research Institute (NIH) LINK Genetic Testing: How and What? Genetic Testing: How and What? Fetal testing occurs during a pregnancy and is used to assess the chromosomal status of the developing fetus  Human Genetic Disorders  Inherited genetic disorders can be: ◦ Mutations in individual genes, and then located:  On autosomes or sex chromosomes Can be dominant Usually on the or recessive X chromosome ◦ Abnormalities in chromosome number or structure (examples?) Human Genetic Disorders Gene (=Allele)-based disorders Whole chromosome disorders  Autosomal Recessive Diseases  Represents several thousand disorders  Can be mild to lethal, depending on the disease  Examples: ◦ Cystic fibrosis – thick mucus in lungs, GI tract ◦ Tay-Sachs disease – lethal brain deterioration ◦ Sickle cell anemia – RBC’s are misshapen  Autosomal Recessive Diseases  Usually both parents are carriers  Each is heterozygous (Aa), with a single copy of the recessive allele  Parents don’t show the condition (e.g., phenotype), but can pass it to offspring  Two carriers have 25% chance (1 in 4) of producing an affected offspring  This pattern holds for all autosomal recessive conditions Human Genetic Disorders Gene (=Allele)-based disorders Whole chromosome disorders Autosomal Dominant Conditions  One copy of this gene causes the disease  Do not exhibit “carrier status,” as parent with the allele shows the condition  More rare than recessive conditions ◦ Affected individuals usually don’t survive to reproduce ◦ Recur due to new mutations ◦ Example: Achondroplastic dwarfism The Roloff Family Autosomal Dominant Conditions Achondroplasia inheritance Human Genetic Disorders Gene (=Allele)-based disorders Whole chromosome disorders A Note on Sex Determination  By definition males only inherit one X chromosome ◦ Male sex chromosomes not a homologous pair (are in females)  During reproduction, male gamete contribution determines sex ◦ Sperm has an X = girl ◦ Sperm has a Y = boy  Sex Determination: The Y Chromosome  Y chromosome has the SRY gene (Sex-determining Region of the Y chromosome) - male “master switch”  Has ~100 genes (vs. 1000s)  The Y chromosome follows a purely paternal inheritance ◦ Remains entirely unaffected by influences or exchanges from the maternal X chromosome The entire human ◦ Can be used to study human genome has evolution, migration, prehistory 3 billion base pairs  Sex-linked Traits  Refers to those genes on the X chromosome ◦ 1,100 genes present ◦ Can also be called X-linked ◦ Examples:  Red-green color blindness  Hemophilia – blood disorder, not enough clotting factors  Duchenne muscular dystrophy – muscular degeneration Sex-linked (X-linked) Inheritance  Inheritance of this type of condition occurs through carrier mothers ◦ They can pass on the affected allele to both male and female offspring ◦ The ratio of affected females and affected males are both sex-specific ◦ Since males receive only one copy of the X chromosome, if they inherit the faulty allele (50% chance they do), then they are affected Sex-linked Inheritance of Red-Green Color Blindness What number do you see?  Sons are 50% unaffected, 50% affected  Daughters are 50% unaffected, 50% carriers  Work this example! What would be the outcome in the offspring with a colorblind dad and a mom with normal vision? Girls are ________ Boys are ________  Effects of Inbreeding  Inbreeding is the mating of organisms closely related by ancestry  Increases the risk that recessive alleles remain within the population ◦ This doesn’t create the disease, just the reproductive opportunities to allow the alleles to be perpetuated ◦ Most likely to happen in a small or isolated population (of humans or other organisms) ◦ Q: What are some ways that human populations may have worked to safeguard against inbreeding?  Effects of Inbreeding  Examples: ◦ Dog breeds – mating of parent and offspring, or of siblings, to continue desirable characteristics The pug skull has changed considerably over ~150 years  Effects of Inbreeding  Examples: ◦ Humans – The Hapsburg Family (1440-1740, Austria & Spain); ◦ Queen Victoria of England and hemophilia in her descendants Charles II of Spain ◦ Racehorses: (with ‘Hapsburg Jaw’)  Effects of Inbreeding https://www.bbcearth.com/blog/?article=what-are-the-effects-of-inbreeding Chapter 8 Review Questions  Self-Check Concept Quiz A red carnation and a white carnation produce offspring that are all pink. What type of inheritance pattern is this? A. Complete dominance B. Incomplete dominance C. Codominance B  Self-Check Concept Quiz If an allele for tall plants (T) is dominant to short plants (t), what offspring would you expect from a TT x Tt cross? A. ½ tall; ½ short B. ¾ tall; ¼ short C. All tall C  Self-Check Concept Quiz An autosomal recessive disease for which two carriers are needed to produce the disease in offspring means that: A. both parents are homozygous dominant B. both parents are heterozygous C. this disease can’t exist in the offspring B  Self-Check Concept Quiz An example of a sex-linked inheritance pattern is seen in: A. Skin color B. Red-green color blindness C. Achondroplasia B  Short Answer Review Questions 1.) Be able to set up and complete a Punnett square (cross) like the example we completed in class (slide here). Work through this example. Identify the phenotypic (characteristic) and genotypic (alleles) ratios of the offspring, and how dominant and recessive alleles are named and then identified in the cross. 2.) During sexual reproduction, which gamete (egg or sperm) determines the gender of an offspring? Explain your response in terms of inheritance patterns of sex chromosomes.  Short Answer Review Questions 3.) a.) What is meant by the term carrier? b.) What is the implication of this term to human disease inheritance? c.) Provide an example of this type of disease or condition. 4.) a.) In which sex (males or females) are sex-linked recessive genetic disorders more commonly seen, and why? b.) Provide an example of a sex-linked disorder. Punnett Square Practice Problems: Punnett Square Practice Problems: Punnett Square Practice Problems: Punnett Square Practice Problems: Punnett Square Practice Problems: See next slide  Punnett Square Practice Problems: Punnett Square Practice Problems: See next slide  Punnett Square Practice Problems: Punnett Square Practice Problems: