Chapter 7: Patterns of Inheritance PDF

Chapter 7: Patterns of Inheritance Prof. Calkins SCB 101 Recap: What is produced by the process of meiosis? a. Duplicate cells b. Diploid gametes c. Haploid gametes d. A zygote Recap Why is it important that gametes be haploid (only have one copy of each chromosome)? a. Because they ne...

Chapter 7: Patterns of Inheritance Prof. Calkins SCB 101 Recap: What is produced by the process of meiosis? a. Duplicate cells b. Diploid gametes c. Haploid gametes d. A zygote Recap Why is it important that gametes be haploid (only have one copy of each chromosome)? a. Because they need to have extra chromosomes to develop into a complete organism. b. Because the sperm and egg cells need to be very small. c. It isn’t really important. That’s just how it works. d. Because if they weren’t, when they joined at fertilization the resulting zygote would have twice the normal number of chromosomes. Review: Fertilization Fertilization: the fusion of two haploid gametes, resulting in a zygote Zygote: a single diploid cell Contains all the DNA Undergoes mitosis to form the new complete organism Recap: What are the building blocks for nucleic acids? a. Amino acids b. Lipids c. Nucleotides d. Carbohydrates Getting to the Genes Gene The basic unit of information that codes for a genetic trait A segment of DNA The most basic function of genes is to contain the instructions on how to make a protein Allele Different version of a gene Ex: A gene can code for eye color, with different alleles such as blue, green, brown…etc. Mutations A mutation is a change in a gene Most mutations are harmless, but some can change traits for good or for bad For example, a mutation on a gene that codes for cell division may result in uncontrolled cell division, or cancer While some mutations can give us a variety of benefits such as HIV resistance, lactose tolerance, and color-vision Recap Which of the following is a stretch of DNA coding for a single protein? a. trait b. gene c. allele d. mutation Genetic Traits Genetic traits: any inherited characteristic of an organism that can be observed or detected Passed on from parents to offspring For sexual reproducing organisms, TWO copies of each gene are used to make an organism Therefore, the genetic information you get from both of your parents code for aspects of who you are The unique combination of genes makes us genetically diverse Genetic Traits Genotype The genetic makeup of an organism The alleles one has that code for their individual Phenotype The physical expression of those genes How the genotype ‘shows up’ Genotypes give rise to phenotypes Dominant and Recessive Traits A Dominant allele is a trait that is expressed, or shown even when other alleles are present We denote dominant alleles with a capital letter Ex: The allele for black fur in poodles is ‘B’ A Recessive allele is masked, or not expressed when other alleles are present We denote recessive alleles with a lower-case letter Ex: the allele for brown fur in poodles is ‘b’ So, if we have a poodle with the genotype ‘Bb’, what color fur would it have? Black Homozygous Versus Heterozygous Homozygous: consists of two copies of the same allele. a. Homozygous dominant (BB): - Consists of two copies of the dominant allele. - Phenotype is the dominant allele. b. Homozygous recessive (bb): - Consists of two copies of the recessive allele. - This is the only way a recessive allele can be displayed as the phenotype. Heterozygous (Bb): consists of one dominant allele and one recessive allele. Phenotype is the dominant allele. Recap: Which might you observe directly: the genotype, or phenotype? Phenotype Can you identify the genotype of a black poodle (B= black, b= brown)? Why or why not? No, because the allele for fur color is dominant, we know that a black poodle will have at least one copy, but we have no way of knowing on this information alone what the second allele is. What about a brown poodle? Yes. A brown poodle will have the genotype ‘bb’. Because brown is recessive, we know that the poodle must have two copies of the allele in order to be brown. Recap: When a genotype is depicted as “BB,” what phenotype would it express? a. homozygous dominant b. homozygous recessive c. heterozygous dominant d. incompletely dominant Gregor Mendel Gregor Mendel was an Austrian monk, known as the “Father of Modern Genetics.” He experimented with pea plants in his monastery garden. He performed Monohybrid genetic crosses, which are controlled mating experiments that can determine how traits are inherited, by focusing on one trait at a time Monohybrid Genetic Crosses P generation (parent generation) One parent is homozygous dominant (PP) The other is homozygous recessive (pp) F1 generation (first filial): The result of a P generation cross Offspring are all heterozygous (Pp) F2 generation (second filial): The result of an F2 generation cross Offspring are PP, Pp, and pp Mendel’s Results The experiment yielded two important results: 1. The white trait disappeared in the results of the first cross but then reappeared in the next cross. Therefore, purple must be dominant to white. 2. Flower color must be regulated by two hereditary factors that are sorted individually. We now refer to these “factors” as genes One from each parent, which are sorted during meiosis into gametes  Punnett Squares Let’s cross two heterozygous poodles. Both have the black fur phenotype, and both have ‘Bb’ genotype Probabilities The A Punnett Outcomes Probabilities probability of square predicts are never become an event is the the ratio of chance that it offspring certain. much more will occur. expected by accurate as knowing the sample sizes parents’ increase. genotypes.  Punnett Squares Let’s cross two heterozygous poodles. Both have the black fur phenotype, and both have ‘Bb’ genotype What probability that puppies will be: a) Homozygous dominant: 25% _______ b) Homozygous recessive: 25% _______ c) Heterozygous: 50% _______ Which percent of puppies have 75% a) Black fur: ____________ 25% b) Brown fur: ____________ Rules of Inheritance (thanks Mendel!) Mendel’s research revealed much of the basic information about inheritance: 1) Genes can come in multiple forms, or alleles 2) The alleles of a gene are sorted individually into gametes during meiosis. The sorting of these genes are independent of eachother. Fertilization pairs them up again. 3) Certain traits exhibit a dominant/recessive relationship. The dominant trait is the one that is seen even when only one allele is present. The recessive trait is the one that is not seen when only one allele is present. Rules of Inheritance The law of segregation: the two alleles of a gene are separated during meiosis and end up in different gametes— egg or sperm cells. One of the two alleles is found on one of the chromosomes in a homologous pair. The other allele is found on the other chromosome in the pair. Remember, homologous chromosome pairs are partitioned into separate daughter cells during meiosis I. Rules of Inheritance The law of independent assortment: when gametes form, the two alleles of any given gene segregate during meiosis independently of any two alleles of other genes. Different genes are found on different chromosomes. Each chromosome randomly moves into daughter cells during meiosis I without regard to movement of other chromosomes. Independent Assortment Mendelian Traits Mendelian traits: traits controlled by a single gene and unaffected by environmental conditions When Mendel described his laws of inheritance, he had no idea what genes were made of, where they were located within a cell, or how they segregated and independently assorted. Now we know that genes are located on chromosomes and that these chromosomes are the basis for all inheritance. Mendelian Traits Mendel’s rules do not completely explain inheritance, but his discoveries were great start Inheritance can be much more complicated: Alleles can interact, so can genes Genes may affect more than one characteristic Gene expression depends on the environment Alleles Interact Incomplete dominance: when neither allele is able to exert its full effect A heterozygote displays an intermediate phenotype. Alleles Interact Codominance: when the effect of the two alleles is equally visible in the phenotype of the heterozygote Neither allele is diminished or diluted by the presence of the other allele. As in the case of incomplete dominance Neither allele is suppressed by a dominant allele. As in the case of dominant and recessive alleles Example: In some flowers, petal color can be codominant Genes Affect Multiple Characteristics Pleiotropy: when a single gene influences a number of different traits Pleio- “many”; -tropy “change” - Examples: The shape of a dog’s head and the shape of its limb bones are controlled by a single gene. In the domestication experiment with foxes, tamer foxes also had floppier ears, shorter legs, and curlier tails. As foxes became tamer, these traits were magnified. Environmental Factors External and internal conditions can affect genetic expression of genes, including: Ambient or body temperature CO2 levels Sunlight Example: Siamese cat coat color The production of fur pigment depends on temperature. Because the paws, nose, ears, and tail are cooler than the rest of the body, melanin is produced there. Results in darker pigment. Recap Do Mendel’s rules explain inheritance perfectly? a. Yep. b. Nope, but we don’t exactly know why…. inheritance is a mystery c. No because alleles interact, genes affect multiple characteristics and change with environmental cues d. No, because genes change and don’t really affect our traits anyway. e. Yes, genes are genes and are straightforward Recap: When the effect of the two alleles is equally visible in the phenotype of the heterozygote, the pair of alleles shows which mode of inheritance? a. pleiotropy b. polygenic c. epistasis d. codominance Recap Your genes partly determine your physical characteristics, but the environment you live in also plays a role. a. True---Some genes can be altered by environmental features. b. False---Your genes completely determine your physical characteristics. c. Scientists aren’t sure what determines an individual’s characteristics. d. False---The environment plays no role. e. True---In fact the environment always alters genetic characteristics.