 This video will give you an overview of Mendelian genetics from Chapter 12 of OpenStacks Biology with additional content from Chapter 8 of OpenStacks Concepts of Biology. After watching this video, you should be able to answer the following study objectives to distinguish between genotype and phenotype, explain how dominant and recessive traits are inherited and result in different phenotypes, define Mendel's laws of dominance, segregation and independent assortment, and use a Punnett square to calculate the expected genotype and phenotype ratios for a monohybrid cross. Genetics is the study of heredity. Gregor Mendel set the framework for genetics long before chromosomes or DNA had been identified. Mendel selected a simple biological system and conducted methodical quantitative analyses using large sample sizes. Mendel's work was accomplished by mating garden peas that express different traits. A trait is a variation in the physical appearance of a heritable characteristic. For example, the characteristic of flower color is shown in the illustration and the two different traits for flower color are violet or white. Genpies normally fertilize themselves, so the offspring are identical to the parents, which is known as true breeding. Hybridization is the process of mating two individuals that differ with the goal of achieving certain characteristics or certain traits of a characteristic in their offspring. A monohybrid is the result of a cross between two true breeding parents that express different traits for only one characteristic. In Mendel's experiments, the parental generation was called P0, and the offspring of breeding the parental generation were called F1 for first filial generation. When Mendel bred peas with violet and white flowers, the F1 generation all had violet flowers. Then Mendel performed a self-cross, allowing the F1 generation plants to fertilize themselves, producing a second filial generation called F2. Interestingly, the white flower trait reappeared in the F2 generation. Mendel deduced that hereditary factors, which we now know as genes, must be inherited as discrete units. The violet flowers are an example of dominant traits that are inherited unchanged in hybridization. White flowers are an example of what Mendel called recessive traits, which become latent or disappear in the offspring of hybridization. We now understand that the traits Mendel observed result from different alleles, were gene variants that arise by mutation or a change in the DNA sequence of a gene that exists at the same relative location on homologous chromosomes. The phenotype of an organism is the observable trait, for example the violet and white traits for flower color. Another example of a characteristic that Mendel studied is the color of the seeds from pea plants. You can see here in the illustration there are yellow and green traits for seed color. When Mendel bred the yellow and green true breeding pea plants together, all of the offspring had yellow seeds. Then when he allowed the yellow seed F1 generation to self-fertilize, the green seed trait reappeared in the F2 generation in approximately 25%. An organism's underlying genetic makeup is called the genotype, which consists of both physically visible alleles and non-expressed alleles such as the genes that code for recessive traits in the F1 generation. The dominant alleles are represented with an uppercase letter or a capital letter and the recessive alleles are represented with a lowercase letter. For seed color, the dominant allele for the yellow trait is represented with the uppercase Y and the recessive allele for the green trait is represented with the lowercase Y. An organism's genotype is heterozygous if the organism has two different alleles for a given gene on homologous chromosomes. In contrast, a homozygous organism has two identical alleles for a given gene on the homologous chromosomes. In Mendel's experiments, all of the P0 generation were homozygous and all of the F1 generation were heterozygous. In the F2 generation, on average, 25% were homozygous for the dominant allele, 50% were heterozygous, and 25% were homozygous for the recessive allele. Because the heterozygous as well as the homozygous dominant individuals both express the dominant trait, this gave us 75% that had the dominant yellow seed color, or 3 out of 4, and 25% that had the recessive green color trait, or 1 out of 4. This characteristic 3 to 1 phenotype ratio is seen in the F2 generation, and a 1 to 2 to 1 genotype ratio of homozygous dominant, heterozygous, and homozygous recessive was seen in the F2 generation. Mendel studied seven different characteristics. This table shows the results from just four of those. You can see from the table that he worked with a large sample size in order to obtain reliable results. As we saw previously, the violet flower color is dominant and the white flower color is recessive. For seed texture, when peas grown from round seeds were bred to peas grown from wrinkled seeds, 100% of the offspring had round seeds. Therefore the round trait is the dominant trait for seed texture, and wrinkled is the recessive trait. Similarly, when yellow peas were bred with the green peas, as we previously seen, 100% of the offspring in the F1 generation were yellow, so yellow is the dominant trait. Another characteristic Mendel studied is plant height. He bred tall plants to shorter dwarf plants. In the F1 generation, 100% of the offspring were tall, demonstrating that tall is the dominant trait for plant height. Importantly, the recessive traits all reappeared in approximately 25% of the F2 generation. Although we now know that more complex forms of inheritance exist, Mendel's laws of inheritance summarize the basics of classical genetics. Mendel proposed that paired genes were transmitted faithfully from generation to generation by disassociation and re-association of paired genes during gametogenesis and fertilization. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. Mendel's law of dominance states that, in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Mendel's law of segregation states that alleles are randomly separated into gametes during meiosis, so all offspring have an equal likelihood of inheriting each allele. Mendel's law of independent assortment tells us that genes do not influence each other with regard to sorting of alleles into gametes, so each combination of alleles has equal probability. Crossing over of homologous chromosomes contributes to the segregation of alleles into gametes. However, we now know that alleles do not necessarily follow the law of independent assortment. A modern perspective of genetics includes the concept of linkage, in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together. However, all of the alleles that Mendel studied were far enough away on chromosomes or located on different chromosomes so that they were not significantly showing any linkage. A punnett square is a visual representation of a cross between two individuals in which the gametes of each individual are written along the top and side of a grid. And the possible zygotic genotypes from fertilization are written within each box of the grid. In this example, we can see the results of a self-cross between the heterozygous individuals of the F1 generation. And the F1 generation self-crosses, we see there's 1 in 4 or 25% homozygous dominant. In this example, the yellow seed color, 2 out of 4 or 50% heterozygous having an allele for the yellow as well as an allele for the green seed color. And 1 in 4 or 25% have the homozygous recessive alleles for the green seed color. Sickle cell anemia is an example of a human trait that follows the laws of Mendelian genetics. The sickle cell mutation is a mutation in the gene for hemoglobin which causes red blood cells to have an abnormal shape, a sickle shape, and these cells will clump together and break easily and this will decrease the oxygen carrying capacity of the blood, decrease the ability of our blood to carry oxygen throughout the body, which is known as anemia. Now let's use a Punnett square to predict the genotype and phenotype ratios. If a person with sickle cell anemia mates with a carrier that's heterozygous. And so because a carrier is heterozygous, they have one normal allele and one of the mutated sickle cell alleles and then a person with sickle cell anemia has both copies their homozygous for the recessive sickle cell allele. Then in the boxes here we can recombine their gametes to form the zygotic genotypes. We see the heterozygous genotype and the homozygous genotype, homozygous recessive genotype there, and then another heterozygous and homozygous recessive genotype. And so in the cross between a person with sickle cell anemia and a person that has the normal phenotype but carries a sickle cell allele, we see a genotype ratio of 1 to 1 or 50% of the offspring on average would have the heterozygous carrier genotype and 50% would have the homozygous recessive sickle cell anemia genotype. And similarly the phenotype ratio is 1 to 1, 50% would be carriers but would not express the sickle cell anemia phenotype and 50% would express sickle cell anemia. Another example of a gene that follows the Mendelian laws of inheritance is human blood types. You may have heard of type A blood or type O blood or even type AB blood and type B blood. For now we'll just focus on type A blood and type O blood. Type A blood has the dominant allele and type O blood has the recessive allele. The letter I is used to represent the blood type because a chemical on the surface of red blood cells called isoglutinogen is responsible for the blood type. An individual that is homozygous for the type A allele has type A blood but an individual that's heterozygous for the type A and type O alleles will also have type A blood whereas an individual that's homozygous for the type O allele will have type O blood. Now let's use the Punnett square to predict genotype and phenotype ratios formating between two heterozygous individuals with type A blood. We can see if we write up the alleles on the top and along the side and then carry across to form the recombination we find a homozygous dominant and a heterozygous then another heterozygous and a homozygous recessive and so the genotype ratio is 1 to 2 to 1 or 25% homozygous dominant 50% heterozygous and 25% homozygous recessive then the phenotype ratio is 3 to 1 that is 75% would have type A blood and 25% would have type O blood. Of course there are more complicated forms of blood type than just type A and type O we also have type AB blood which results from co-dominance of the type A allele and type B allele. Co-dominance is when a heterozygote expresses the phenotype of both alleles. So the type B allele is dominant if you have a homozygous genotype for the type B allele of course you will have the type B phenotype and if you're heterozygous for the type B allele and the recessive type O allele you'll also have type B blood but if you're heterozygous for the type A and type B alleles you'll have the phenotype of type AB blood. Now let's use a Punnett square analysis to predict the genotype and phenotype ratios formating between heterozygous type A and heterozygous type B individuals. Someone whose heterozygous type A would have one dominant A allele and one recessive O allele. Someone whose heterozygous and has type B blood would have one dominant type B allele and one recessive type O allele. When the A and B alleles combine we'll have a heterozygote with both dominant alleles and then when the B and O alleles combine we'll have a heterozygote that has the dominant B allele and the recessive O allele then here we'll have a heterozygote with the dominant type A allele and the recessive type O allele and last in the bottom right we will see an individual that's homozygous for the recessive type O allele. And so this gives us a genotype ratio of 1 to 1 to 1 to 1 or 25% for each of the four genotypes observed as well as a phenotype ratio of 25% type A, 25% type B, 25% type A, B, and 25% type O. Another form of inheritance that does not follow the law of dominance is called incomplete dominance where the heterozygote expresses two contrasting alleles that blend together producing an intermediate phenotype. The example shown here is the snapdragon plant which has a white allele for flower color, a red allele for flower color, and a heterozygote that has both the white and the red alleles expresses a pink colored flower. Until now we have only considered inheritance patterns among non-sex chromosomes or autosomes. In addition to the 22 homologous pairs of autosomes human females have a homologous pair of X chromosomes whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region similar to the X chromosome that allows them to pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X chromosome but not on the Y chromosome it is said to be X-linked. X-linked recessive disorders in humans include red-green color blindness, a mutation in the photopigment gene, the opsin gene that is important for detecting color vision. Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders like red-green color blindness are disproportionately observed in males. Females must inherit two of the recessive X-linked alleles in order to express the trait. When they inherit just one recessive X-linked allele and one dominant X-linked allele they are carriers of the trait but are typically unaffected. Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. This is called a test crossed. In the test crossed the dominant expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant expressing organism is a homozygote then all of the F1 offspring will be heterozygous expressing the dominant trait. Alternatively if the dominant expressing organism is a heterozygote the F1 offspring will exhibit a one to one ratio of heterozygotes and recessive homozygotes. Doing a test cross in humans is unethical and impractical. Instead geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases. Alcaptanuria is a recessive genetic disorder in which two amino acids phenylalanine and tyrosine are not properly metabolized. Affected individuals have darker color in their skin and urine as a result of an accumulation of a metabolite and they may also suffer joint pain and damage and other complications in the skeletal system. In this pedigree analysis individuals with the disorder are indicated in blue and have a homozygous recessive genotype. Unaffected individuals are indicated in yellow and have either the homozygous dominant or heterozygous genotype. It is often possible for us to determine a person's genotype from the genotype of their offspring and so this is the utility of a pedigree analysis. This is why a geneticist would use a pedigree analysis. For example if neither parent has the disorder but the child does we can infer that the parents must both be heterozygous. Two individuals on this pedigree have an unaffected phenotype but an unknown genotype. Since they do not have the disorder we know that they must have at least one normal allele so their genotype gets the A question mark designation. Now we should be able to predict the genotype for the individual shown as number one two and three. The convention is to use a box for males and a circle for females and here we see the yellow color for individuals that are unaffected and the blue color for individuals that are affected. Now remembering that in order to be affected you have to be homozygous for the recessive allele in order to express the phenotype of this disorder we can infer that the individual number one male shown as number one must be homozygous recessive because their their boxes colored blue indicating that they are affected. They express the phenotype of alcaptanuria the disease. When they mated with individual number two they produced a sun that was affected therefore we can infer that number two individual number two was a woman who had a heterozygous genotype because she was not affected but her offspring was she must have been a carrier for the allele. Similarly individual number three is not affected but when this man mated with a woman that was homozygous recessive they produced a daughter that expressed the homozygous recessive phenotype the express the alcaptanuria phenotype therefore individual number three must have been heterozygous. A dihybrid is the result of a cross between two true breeding parents that expressed different traits for two characteristics. We can also use a Punnett square analysis to predict the genotype and phenotype ratios of a dihybrid cross but now we need to use a matrix with 16 squares to represent all possible combinations. The f2 generation of a dihybrid cross results in a 9 to 3 to 3 to 1 phenotype ratio with approximately 56.25 percent or 9 out of 16 of the offspring expressing both dominant traits 3 out of 16 or 18.75 percent of the offspring will express one dominant and one recessive trait and 1 out of 16 or 6.25 percent express both recessive traits. Of course two organisms that may can vary in even more than two characteristics a trihybrid cross involves two true breeding parents that express different traits for three different characteristics. With three or more characteristics it becomes impractical to use Punnett square analysis instead we can apply the product rule of probability that states the probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone. Since we know the probability of the dominant trait being expressed in the f2 generation is 3 to 1 we can multiply 3 times 3 times 3 to get 27 out of the total 4 times 4 times 4 or 64 possibilities so 27 out of 64 or 42.2 percent of the f2 generation will be dominant for all three traits. On the other hand we can look at the probability of having the recessive trait for all three characteristics which would be 1 in 64 or 1.6 percent of the f2 generation will be recessive for all three traits. We can use the product rule to calculate the probability of any combination of these traits and so we can see if you have two dominant traits and one recessive trait the probability will be 9 in 64 if you have one dominant trait and two recessive traits the probability would be 3 in 64. You could also use the product rule of probability to calculate the probability of more than just three different characteristics we could calculate the probabilities for four characteristics or five characteristics really as many different characteristics as we know follow the Mendelian rules of inheritance.