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Introduction to Genetics Slide 1 Introduction Now that we understand how chromosomes are divided to produce egg and sperm, let’s begin looking at genetics. The first branch of genetics we will study is Mendelian genetics. Mendelian genetic is named for Gregor Mendel a monk who lived in the 1800’s. He worked during a time when no one knew anything about chromosomes and DNA, but he was able to determine the very basics of how traits are inherited. He did his work in the monastery garden on pea plants. Interestingly he presented his work to the scientific community and they were unable at the time to understand how important his work was. Mendel’s work was lost for almost 50 years. Slide 2 Dominant versus recessive In order to understand Mendelian genetics, we need to define some terms. Traits can either be recessive or dominant. Dominant traits mask or cover up other traits. If a dominant trait is present, this is what we will see. In the common abbreviation used in inheritance, dominant traits are represented with capital letters. Recessive traits are masked or covered up by dominant traits. Recessive traits are presented by lower case letters. Slide 3 Mendel’s Principles What did Mendel actually figure out from studying his pea plants? Fist he found out that heredity is not blending; he found that traits come in either a recessive or dominant form. Mendel also figured out that there are units of heredity. Today we know these units of heredity as genes. He determined that every individual has a pair of these units for each trait. This means that you have two copies of each gene for each trait. Remember the homologous chromosomes? One came from your mother and one came from your father. Each has a gene for the same trait, like eye color. What we know from Mendel’s work is that you have two copies, but they don’t need to be identical. For example, your mother could have given you a gene for blue eyes, while your father gave you a gene for brown eyes. Different forms of the gene are celled alleles. During meiosis, the homologous chromosomes separate; therefore so do the alleles. Half of the gametes produced by you would have the allele for blue eyes and the other half would have the allele for brown eyes. This is why we say the egg and sperm are haploid. When the egg and sperm come together a new diploid organism is formed. Slide 4 Alleles Let’s review what an allele is. An allele is a form of a gene. Recessive alleles will be represented by a lower case letter, while dominant alleles will be represented by a capital letter. Slide 5 Homozygous versus heterozygous You have two alleles for each trait. You received one from your mother and one from your father. If the two alleles you have are the same, you are homozygous. If the alleles are of the dominant form, then you are homozygous dominant. If both your alleles are the recessive form, then you are homozygous recessive. If you have one dominant allele and one recessive allele, then you are heterozygous. The only time you can see a recessive trait is when the individual is homozygous recessive, because there are no other traits to mask the recessive trait.
Introduction to Genetics Slide 6 Genotype versus phenotype Two more term that you need to know are genotype and phenotype. The genotype is the alleles present in an individual. Unless you are homozygous recessive for a trait, you probably do not know your genotype. The term heterozygous and homozygous refer to genotype. The phenotype is what an individual looks like. For example, your phenotype would be whether you have blue or brown eyes. If you have a dominant phenotype, you can either be homozygous dominant or heterozygous. Slide 7 Check Your Understanding Now that we have learned about genes and alleles, let’s check your knowledge of the subject. The following slides will have a series of questions on the topic. Be sure to click “Submit” after answering each question. Slides 8 through 18 Genes and Alleles Interactive Quiz A nongraded assessment of your knowledge of genes and alleles. Slide 19 Punnet squares Now that we have mastered terminology, let’s look at how we can determine the genotypes and phenotypes of offspring. To determine the genotype and phenotype, we are going to use a device called a Punnet square. Slide 20 Ear lobe attachment Let’s see how to use a Punnett square by using an easily observable trait. The trait we will observe is ear attachment. Unattached ear lobes are dominant to attached ear lobes; therefore individuals with unattached ear lobes can have the genotype EE or Ee. Attached ear lobes have the genotype ee. Slide 31 Punnett square Let’s see what would happen if two people that are heterozygous for ear lobe attachment had children. Both of the parents have the genotype Ee. This means that half of the mother’s eggs would have the E allele and the other half would have the e allele. Half of the father’s sperm would have the E allele and the other half would have the e allele. Slide 22 Punnett square To make a punnett square we will place the possible alleles from the mother across the top of the square and the possible alleles from the father along the left side of the square. Now it is like cross multiplying. Slide 23 Punnett square In the first square the mother contributes an E and the father contributes an E. The child has the genotype EE. Slide 24 Punnett square In the second square in the first row, the mother contributes an e, while the father contributes an E, so the child has an Ee genotype.
Introduction to Genetics Slide 25 Punnett square In the first square of the second row, the mother contributes an E, while the father contributes an e, therefore the child has the genotype Ee. Slide 26 Punnett square In the final square, each parent contributes an e, so the child has the genotype ee. Slide 27 Genetics Questions From this punnet square, there are several questions that can be asked. I could ask what percentage of the children have unattached ear lobes. The answer to this question would be 75%. This is because three of the four boxes have least one dominant allele. Slide 28 Genetics Questions I could also ask what percentage of the children are heterozygous. The answer to this question is 50%, since two of the four boxes have the genotype Ee. Slide 29 Genetics Questions Another question that I could pose is the ratio of the dominant allele to the recessive allele. The answer would be 3:1, since three of the boxes have a dominant allele and only one has recessive alleles only. Slide 30 Genetics Questions I could also ask what the ratio of homozygous dominant to heterozygous to homozygous recessive. The answer to this question is 1:2:1. When doing genetics questions, be sure you are doing what is being asked. Slide 31 Genotypic and phenotypic ratios The discussion on the previous slide was about genotypic and phenotypic ratios. Phenotypic ratios ask you to show the number of children that would have the dominant phenotype compared to the number of children that would have the recessive phenotype. Genotypic ratios ask you show the number of children who are homozygous dominant compared to the number of children who are heterozygous compared to the number of children who are homozygous recessive. Slide 32 Theoretical ratios Looking at the ratios mentioned in the previous slides, does this mean that if a couple has four children, they will see these ratios in their family? The answer is no. Each time the parents have a child it is the luck of the draw as to which alleles will end up in the offspring. It is like tossing a coin. You may have a streak where you have several heads in a row; however if you toss the coin enough times you will end up with 50% heads and 50% tails. Punett squares show the theoretical ratios that would occur if large numbers of individuals were born from a specific pairing.
Introduction to Genetics Slide 33 Recessive trait disorders Recessive traits are traits that are inherited through a recessive allele. In order for an individual to display a recessive trait, they must inherit a recessive allele from each of their parents. Some diseases are inherited through recessive traits and can be very devastating. Among them are TaySachs disease, Cystic fibrosis, and Phenylketonuria (PKU). Individuals that inherit one recessive allele for one of these diseases, but also inherit one dominant allele that mask the recessive allele are known as carriers for the disease. If there is a history of one of these genetic diseases in the family, an individual will often be testing to see if they are a carrier. Slide 34 Pedigrees During genetic counseling it often becomes necessary to determine the family history through the collection of a pedigree. In a pedigree, males are represented by squares, females by circles, and individuals carrying a genetic disorder by shading. Looking at a pedigree can help determine how a genetic disorder is transmitted through a family. Slide 35 Check Your Understanding Now that we have learned about pedigrees, let’s check your knowledge of the subject. The following slides will have a series of questions on the topic. Be sure to click “Submit” after answering each question. Slides 36 through 41 Pedigrees Interactive Quiz A nongraded assessment of your knowledge of pedigrees Slide 42 Dominant trait disorders Dominant traits are those that are inherited by a dominant allele. People tend to think that dominant traits are not harmful, but a few genetic disorders are caused by the presence of a dominant trait. These include Marfan syndrome, hypercholesterolemia, Huntington’s disease, and achrondroplasia. In order for an individual to inherit a dominant trait, one of their parents must have has the trait. Both dominant and recessive traits in families can be traced using a pedigree analysis. Slide 43 Check Your Understanding Now that we have learned about Mendelian Genetics, let’s check your knowledge of the subject. The following slides will have a series of questions on the topic. Be sure to click “Submit” after answering each question. Slides 44 through 53 Genetics Problems Interactive Quiz A nongraded assessment of your knowledge of working through genetics problems. Slide 54 NonMendelian inheritance Not all traits are inherited in a simple Mendelian fashion. These traits are passed from one generation to the next through nonmendelian inheritance. Some examples of nonMendelian inheritance include polygenic traits, incomplete dominance, multiple alleles, codominance, sex linked traits, sex influenced traits, and environmental influenced traits. We will explore each of these is further detail.
Introduction to Genetics Slide 55 Incomplete dominance One of Mendel’s rules was that heredity is not blending. However, there are some traits that do show blending. These traits show incomplete dominance. As an example if you crossed a red carnation with a white carnation, based on Mendel’s rules you would expect the offspring to be red or white. This is not the case because the offspring would actually be pink. Because we see blending this is an example of incomplete dominance. Another example of incomplete dominance is the chocolate Labrador retriever, which results from a yellow Labrador crossed white a black Labrador. Slide 56 Multiple alleles Do you remember the blood types we discussed during the immune system portion of the course? There were four different blood types, A, B, AB, and O. How is this possible if blood type is on a single gene? According to Mendel’s rules there would only be three different genotypes and two different phenotypes. This does not account for the four blood types seen in human populations. The four blood types exist because there are three alleles in the population for blood type, IA, IB, and i. Both IA and IB are dominant to i. There are other human traits that are the result of multiple alleles. Another example is human leukocyte antigen (HLA) alleles, which code for recognition proteins in the cell membranes of body cells. These are important in the immune system and help cells distinguish between “self” and “foreign”. Each HLA gene can have more than 30 different alleles. The multitude of HLA alleles comes in to play when doctors are trying to find a suitable organ donor. Slide 57 Codominance In addition to having three alleles, blood type is also unusual in that neither IA nor IB is dominant to the other. In the blood type AB, the individual has the alleles IA and IB. Both of these alleles make an antigen. IA make the A antigen and IB makes the B antigen. These individuals end up with both genes making something that is seen in the phenotype of the individual. This is an example of codominance. Codominance is different from incomplete dominance in that there is no blending of the traits. Both the A antigen and the B antigen are being made. Slide 58 Blood type alleles Let’s take a moment to look at the possible genotypes and phenotypes for blood type. If a person has type A blood, they must have the IA allele. They might have another IA allele or they may have the recessive i allele. A person with type B blood must have the IB allele. Their second allele can either be another IB or a recessive i. A person with type O blood has the recessive trait and must have two i alleles. Slide 59 Blood typing and paternity Before current paternity testing methods using DNA, blood typing was the sole method of determining paternity. This was not as accurate as today’s DNA testing. If two men had the same blood type, there was no way to exclude either one of them. This method did not allow for the proving of paternity, it only excluding individuals from paternity if a punnet square between the mother and the individual in question could not produce a box with the child’s blood type.
Introduction to Genetics Slide 60 Sickle cell disease Another example of codominance can be seen in sickle cell disease. There are two possible alleles for hemoglobin, HbA and HbS. HbA is the normal allele. A person that is completely normal has the genotype HbA HbA. An individual with sickle cell disease has two copies of the HbS allele. Individuals that have sickle cell trait, also known as a carrier, have one HbA allele and one HbS allele. To test for sickle cell trait, blood cells are placed under low oxygen. If some of the cells sickle, the person has sickle cell trait. A person with sickle cell trait produced both normal hemoglobin and sickled cell hemoglobin. It has been found that the presence of some sickled cell hemoglobin protects individuals from a disease called malaria. Slide 61 Check Your Understanding Now that we have learned about some types of NonMendelian Genetics, let’s check your knowledge of the subject. The following slides will have a series of questions on the topic. Be sure to click “Submit” after answering each question. Slides 62 through 69 Interactive Quiz A nongraded assessment of your knowledge of NonMendelian Modes of Inheritance . Slide 70 Polygenic traits All of the traits that Mendel observed were inherited from a single gene. Today we know that many traits are influenced by many genes. When a phenotype is the result of the influence of many genes it is called a polygenic trait. Polygenic traits show a bell curve distribution of phenotypes. Take skin color for example. If you look around at the people in your workplace, you will notice that they all have different skin colors. Even within your own family, you will notice these differences. There is an average skin color and most people are a few shades lighter or darker than this average. Very few people have no skin colorations at all and very few people have the maximum amount of skin color. The distribution of skin color is therefore bell shaped, with most of the individuals falling around the average skin color. Slide 71 Environmental Influences Other traits are influenced by the external environment. A good example of this is the Siamese cat. Siamese cats produce more melanin in the parts of their body that are cooler; therefore Siamese cats are darker on their nose, ears, tail, and feet. Areas that stay warmer do not produce as much melanin and remain a lighter color. Slide 72 Sex influenced traits Some traits that are not carried on the sex chromosomes are influenced by the hormones produced by a certain sex. These are called sex influenced traits. These traits are found on the autosomal chromosomes and are inherited in a normal Mendelian way, but they are only expressed when certain hormones are present. Sex influenced traits include male pattern baldness, and fraternal twins. Slide 73 Sex chromosomes The X chromosome contains information that does not related to sexual characteristics; therefore there are some traits that will be inherited on the X chromosome. Recall that females have two copies of the X chromosome, while males only have one copy of the X chromosome. Males also have a Y chromosome. Females are homozygous for their sex chromosomes, while males are heterozygous for their sex chromosomes. Where do men get there X chromosomes? They
Introduction to Genetics inherit their X chromosome from their mother; they will inherit the Y chromosome from their fathers. Slide 74 Sex chromosomes Since males only have one copy of the X chromosome, if they get a chromosome with a certain trait they are going to show that trait. Women have two copies of the X chromosome, so they can mask a recessive trait just like in regular Mendelian genetics. These women are considered carriers for these traits, since they do not display the trait but have the ability to pass it on. Slide 75 Xlinked traits Examples of Xlinked traits are hemophilia and color blindness. Men are more likely to display Xlinked traits compared to females. For women to be color blind, they need to inherit two color blind X chromosomes. Males will show color blindness with only one color blind X chromosome. Since males get their X chromosome from their mothers, the mother of a color blind male will have to be at least a carrier for the color blind trait. Males cannot be carriers for an Xlinked trait. Slide 76 Check Your Understanding Now that we have learned about Xlinked traits, let’s check your knowledge of the subject. The following slides will have a series of questions on the topic. Be sure to click “Submit” after answering each question. Slides 77 through 86 Xlinked Traits Interactive Quiz A nongraded assessment of your knowledge of Xlinked traits. Slide 87 Y chromosome analysis There are few examples of Ylinked traits. This is because the Y chromosome contains a small amount of genetic information and the majority of that information encodes traits that relate to being male. One example of a Ylinked trait is hairy pinna. Hairy pinna is the excessive growth of hair on the external part of the ear. The Ychromosome can be used to trace human ancestry. The Y chromosome is passed from father to son with no exchange of genetic information with any other chromosome. Because of this, there is very little change in the DNA on the Y chromosome over many generations. Individuals can trace paternal lines through the Y chromosome. Slide 88 mtDNA and Deep Ancestry Is the nucleus the only place you find DNA in a cell. The answer is no. Recall the mitochondria and in the case of plants the chloroplasts. These organelles are believed to have at one time been free living organisms that developed a symbiotic relationship with the cell. A symbiotic relationship is one in which both parties benefit. In the case of the mitochondria, the mitochondria got a nice place to live inside the cell and access to food and nutrients from the cell. In return, the cell got ATP energy from the mitochondria through the process of cell respiration. Mitochondria still have some of their own DNA. This DNA is inherited directly from your mother, since all of the mitochondria you have came from the egg that was fertilized. There are some traits on this DNA; however the big use of mitochondrial DNA is in determining maternal
Introduction to Genetics relationships. Since your mitochondrial DNA came from your mother, all of those individuals that you are related to maternally would have similar mitochondrial DNA. Just as Y chromosomes can be used to trace paternal lines, mtDNA can be used to trace maternal lines. The lineage that is traced with mtDNA is historical and has been used to trace human populations to their origins approximately 200,000 years ago. Slide 89 mtDNA and Deep Ancestry YouTube video Slide 90 Summary This slide is a summary of all of the “Check Your Understanding” questions from this lecture. Be sure to review the questions you answered incorrectly.

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