Genetics of Organisms
Mendelian Genetics
Evolution is caused by changes in the genetic composition of populations. In the field of population genetics, scientists model thi…
An inheritance is something passed from one generation to another. In some contexts, this means stuff like houses and money. In the context of biology, however, the study of genes and how they are inherited, is called genetics. Gregor Mendel is credited with being the father of modern genetics. His work is responsible for our understanding of how noticeable physical traits, or phenotypes, are passed from one generation to the next. Famously, he studied such traits in pea plants.
The pieces of information controlling those phenotypes are called genes. As there are two copies of each gene, known as alleles, we can represent these as letters. Here, we'll use the letter P. This is the first step of Mendel's famous experiment on pea flower coloration, represented here, using a tool called the Punnett square. Mendel found that when he crossed purple flowers with white ones, all of the progeny, or the first-generation plants, had purple flowers. This is because the purple color is dominant, shown using uppercase P. And carrying even one dominant allele means the phenotype will be expressed. But interestingly, when these purple flowers were crossed again, 1/4 of them were white. Where were the white flowers in the first generation? All of the first generation of purple-flowered plants were heterozygous, meaning that they had one capital purple P and one lowercase, or white, P. When they passed on their alleles in the F2 generation cross, this meant that 1/4 of the offspring received two small P alleles, and so, expressed the white phenotype. Mendel didn't have the Punnett square tool to use, he had to figure all of this out by keeping track of thousands of plants, and then noticing patterns in their numbers.
From this evidence, and a lot more, we now know that genes are also present in two copies in other organisms, too, like humans, and flies. In following up on Mendel's work, several scientists found that not all inheritance patterns followed the simple, basic model that Mendel proposed. For example, in hemophilia, a genetic clotting disorder, unaffected mothers were capable of transmitting the disease to their male children. The reason behind this lies in chromosomes, which were studied by Thomas Hunt Morgan, using his famous fruit flies, Drosophila. Because of Morgan and others, we now know that chromosomes are long strands of DNA that typically exist in pairs. Here, we can see that Drosophila has four of them. These chromosomes have genes on them for different traits, much like how a cookbook contains lots of different recipes. Nowadays, using modern microscopy, we can actually see these chromosomes and even organize them. The product of this process is called a karyotype. Here, you can see a human one. In both humans and flies, there are autosomes, and sex chromosomes. Humans, like flies, have X and Y chromosomes controlling their sex. However, most of the genes on these chromosomes control things that have nothing to do with sex. In the rare form of hemophilia that we mentioned earlier, the reason that it occurs more frequently in males, is because the phenotype is controlled by a gene found on the X chromosome, in a section which has no partner on the Y chromosome. If a female has a bad copy of the gene, and her male child inherits this copy, he will have the disorder, he has no backup copy of the X chromosome. Since one copy of the gene is sufficient for a person to clot normally, a female must inherit two bad alleles of the gene, one from each parent, in order to exhibit the disease. Which, in this case, is not possible, because the father is unaffected. As a result, this type of hemophilia affects more males than females.
In this lab, we'll look at inheritance in Drosophila. Eye color in flies is controlled by a series of genes, some controlling what kinds of pigments are made, and one particularly important gene, called the ABC transporter, which controls transports of pigments into granules in the eye. If that gene is broken, even if the fly is making pigment, that pigment will be invisible, and the fly will have white eyes. The lab exercise is to recreate one of Thomas Hunt Morgan's most famous experiments, and explore the genetic inheritance pattern of the gene encoding the pigment transporter. Is it inherited like the purple color of Mendel's pea flowers, or is it sex-linked, like hemophilia?
An inheritance is something passed from one generation to another. In some contexts, this means stuff like houses and money. In the context of biology, however, the study of genes and how they are inherited, is called genetics. Gregor Mendel is credited with being the father of modern genetics. His work is responsible for our understanding of how noticeable physical traits, or phenotypes, are passed from one generation to the next. Famously, he studied such traits in pea plants.
The pieces of information controlling those phenotypes are called genes. As there are two copies of each gene, known as alleles, we can represent these as letters. Here, we'll use the letter P. This is the first step of Mendel's famous experiment on pea flower coloration, represented here, using a tool called the Punnett square. Mendel found that when he crossed purple flowers with white ones, all of the progeny, or the first-generation plants, had purple flowers. This is because the purple color is dominant, shown using uppercase P. And carrying even one dominant allele means the phenotype will be expressed. But interestingly, when these purple flowers were crossed again, 1/4 of them were white. Where were the white flowers in the first generation? All of the first generation of purple-flowered plants were heterozygous, meaning that they had one capital purple P and one lowercase, or white, P. When they passed on their alleles in the F2 generation cross, this meant that 1/4 of the offspring received two small P alleles, and so, expressed the white phenotype. Mendel didn't have the Punnett square tool to use, he had to figure all of this out by keeping track of thousands of plants, and then noticing patterns in their numbers.
From this evidence, and a lot more, we now know that genes are also present in two copies in other organisms, too, like humans, and flies. In following up on Mendel's work, several scientists found that not all inheritance patterns followed the simple, basic model that Mendel proposed. For example, in hemophilia, a genetic clotting disorder, unaffected mothers were capable of transmitting the disease to their male children. The reason behind this lies in chromosomes, which were studied by Thomas Hunt Morgan, using his famous fruit flies, Drosophila. Because of Morgan and others, we now know that chromosomes are long strands of DNA that typically exist in pairs. Here, we can see that Drosophila has four of them. These chromosomes have genes on them for different traits, much like how a cookbook contains lots of different recipes. Nowadays, using modern microscopy, we can actually see these chromosomes and even organize them. The product of this process is called a karyotype. Here, you can see a human one. In both humans and flies, there are autosomes, and sex chromosomes. Humans, like flies, have X and Y chromosomes controlling their sex. However, most of the genes on these chromosomes control things that have nothing to do with sex. In the rare form of hemophilia that we mentioned earlier, the reason that it occurs more frequently in males, is because the phenotype is controlled by a gene found on the X chromosome, in a section which has no partner on the Y chromosome. If a female has a bad copy of the gene, and her male child inherits this copy, he will have the disorder, he has no backup copy of the X chromosome. Since one copy of the gene is sufficient for a person to clot normally, a female must inherit two bad alleles of the gene, one from each parent, in order to exhibit the disease. Which, in this case, is not possible, because the father is unaffected. As a result, this type of hemophilia affects more males than females.
In this lab, we'll look at inheritance in Drosophila. Eye color in flies is controlled by a series of genes, some controlling what kinds of pigments are made, and one particularly important gene, called the ABC transporter, which controls transports of pigments into granules in the eye. If that gene is broken, even if the fly is making pigment, that pigment will be invisible, and the fly will have white eyes. The lab exercise is to recreate one of Thomas Hunt Morgan's most famous experiments, and explore the genetic inheritance pattern of the gene encoding the pigment transporter. Is it inherited like the purple color of Mendel's pea flowers, or is it sex-linked, like hemophilia?
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Q1: What is the difference between dominant and recessive alleles in Mendelian inheritance?
Dominant alleles are expressed in the phenotype even when only one copy is present, while recessive alleles are expressed only when two copies are inherited. In Mendel's pea experiments, purple flower color is dominant and requires only one uppercase P allele to appear. White flowers, the recessive trait, only appear when an organism inherits two lowercase p alleles.
Q2: Why did white flowers reappear in Mendel's second generation of pea plants?
First-generation purple flowers were heterozygous, carrying one dominant purple allele and one recessive white allele. When these plants self-crossed, offspring could inherit two recessive alleles, expressing the white phenotype. The Punnett square shows this produces a 3:1 ratio of purple to white flowers in the second generation.
Q3: How does sex linkage affect inheritance patterns differently than autosomal traits?
Sex-linked traits, like hemophilia, are controlled by genes on the X chromosome where males have no corresponding allele on the Y chromosome. Males need only one recessive allele to express the phenotype, while females require two copies. This causes sex-linked recessive disorders to appear more frequently in males than females.
Q4: What role do chromosomes play in organizing genetic information?
Chromosomes are long strands of DNA that typically exist in pairs and contain genes for different traits, similar to recipes in a cookbook. Organisms like Drosophila and humans have autosomes and sex chromosomes. A karyotype is the organized visual representation of an organism's complete chromosome set, revealing chromosome structure and number.
Q5: Why is the ABC transporter gene important in Drosophila eye color?
The ABC transporter gene controls the transport of pigments into eye granules. If this gene is broken, pigments cannot be transported into granules and remain invisible, causing white eyes even when pigment is being produced. This gene's inheritance pattern demonstrates whether traits follow Mendelian or sex-linked inheritance models in the genetics of organisms ref and wef fly genetics procedure.
Q6: How did Thomas Hunt Morgan use fruit flies to advance our understanding of genetics?
Morgan studied Drosophila to investigate inheritance patterns and discovered sex-linked genes, demonstrating that not all traits follow Mendel's simple model. Fruit flies are ideal laboratory organisms because they are easy to maintain, breed, and manipulate. Morgan's work with these organisms revealed that genes are located on chromosomes and helped reconcile Mendelian theory with observed inheritance patterns.
Q7: What makes heterozygous females carriers of X-linked recessive disorders?
Heterozygous females carry one normal allele and one recessive disease allele on their two X chromosomes. The normal allele is typically dominant, so the female does not express the disorder. However, she can pass the recessive allele to offspring. Statistically, half her sons will inherit and express the recessive phenotype, while daughters require two copies to be affected.