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Beyond mendel

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DinDin Horneja
DinDin Horneja

Gregor Mendel was an Austrian monk who conducted experiments on pea plants in the 1850s and 1860s that formed the basis of modern genetics. Through meticulous experiments involving over 28,000 pea plants, he identified two principles of heredity that later became known as Mendel's laws of inheritance. However, his work was not widely recognized until 1900. Mendel made important contributions to our understanding of inheritance, including the concepts of dominance, recessiveness, and independent assortment.

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Beyond mendel
Mendel, Gregor Johann (1822-1884), Austrian monk, whose experimental work became the basis of modern hereditary theory. Mendel was born on July 22, 1822, to a peasant family in Heinzendorf (now Hynčice, Czech Republic). He entered the Augustinian monastery at Brünn (now Brno, Czech Republic), which was known as a center of learning and scientific endeavor. He later became a substitute teacher at the technical school in Brünn. There Mendel became actively engaged in investigating variation, heredity, and evolution in plants at the monastery's experimental garden. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants, carefully analyzing seven pairs of seed and plant characteristics. His tedious experiments resulted in the enunciation of two generalizations that later became known as the laws of heredity. His observations also led him to coin two terms still used in present-day genetics: dominance, for a trait that shows up in an offspring; and recessiveness, for a trait masked by a dominant gene.
Mendel published his important work on heredity in 1866. Despite, or perhaps because of, its descriptions of large numbers of experimental plants, which allowed him to express his results numerically and subject them to statistical analysis, this work made virtually no impression for the next 34 years. Only in 1900 was his work recognized more or less independently by three investigators, one of whom was the Dutch botanist Hugo Marie de Vries, and not until the late 1920s and the early '30s was its full significance realized, particularly in relation to evolutionary theory. As a result of years of research in population genetics, investigators were able to demonstrate that Darwinian evolution can be described in terms of the change in gene frequency of Mendelian pairs of characteristics in a population over successive generations. Mendel's later experiments with the hawkweed Hieracium proved inconclusive, and because of the pressure of other duties he ceased his experiments on heredity by the 1870s. He died in Brünn on January 6, 1884.
Beyond mendel
In cases of incomplete dominance, the inheritance of a dominant and a recessive allele results in a blending of traits to produce intermediate characteristics. For example, four-o’clock pink plants may have red, white, or pink flowers. Plants with red flowers have two copies of the dominant allele R for red flower color (RR). Plants with white flowers have two copies of the recessive allele r for white flower color (rr). Pink flowers result in plants with one copy of each allele (Rr), with each allele contributing to a blending of colors.
Another exception to Mendelian genetics involves genes with multiple alleles. Certain traits are controlled by multiple alleles that have complex rules of dominance. In humans, for example, the gene for blood type has three alleles: IA, IB, and i. With three alternatives for each member of a gene pair, there are six possible combinations of these genes (IAIA, IBIB, ii, IAi, IBi, IAIB). Although there are six possible combinations, humans have only four major blood types: A, B, AB, and O. This results because both IA and IB dominate over i, but not over each other, so a person with a gene combination of IAIA or IAi has blood type A. The gene combinations IBIB and IBi both produce blood type B. IAIB results in a blood type AB, and ii results in blood type O.
A significant number of human traits, such as eye color, skin color, height, weight, and muscle strength are typically regulated by more than one allele in a pattern known as polygenic inheritance. Several thousand alleles, for example, may combine to determine a person’s potential for pole-vaulting, and several hundred may play a role in establishing a person’s normal weight. Certain diseases may result from mutations in one or more alleles involved in polygenic inheritance. Researchers have identified nearly a dozen mutated alleles that are associated with diabetes mellitus, and a similar number are linked to asthma. Heart disease may be linked to two or three times that number. Some types of cancer may be correlated with more than 100 different genes. Polygenic inheritance is quite complex, and the ways in which multiple genes interact to produce traits are not fully understood.
In his experiments, Mendel was careful to study traits in pea plants where one trait did not appear to influence another, such as the plant’s height or the pea’s texture. These two phenotypes (height and texture) occur randomly with respect to one another in a manner known as independent assortment. Today scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes. An exception to independent assortment develops when genes appear near one another on the same chromosome. When genes occur on the same chromosome, they are inherited as a single unit. Genes inherited in this way are said to be linked. For example, in fruit flies the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.
But in many cases, genes on the same chromosome that are inherited together produce offspring with unexpected allele combinations. This results from a process called crossing over. Sometimes at the beginning of meiosis, a chromosome pair (made up of a chromosome from the mother and a chromosome from the father) may intertwine and exchange sections of chromosome. The pair then breaks apart to form two chromosomes with a new combination of genes that differs from the combination supplied by the parents. Through this process of recombining genes, organisms can produce offspring with new combinations of maternal and paternal traits that may contribute to or enhance survival.
There are two main categories of gene-mapping techniques: linkage, or genetic, mapping, a method that identifies only the relative order of genes along a chromosome; and physical mapping, more precise methods that can place genes at specific distances from one another on a chromosome. Both types of mapping use markers in the DNA sequence, detectable physical or molecular characteristics that differ among individuals and that are passed from one generation to the next.
Linkage mapping was developed in the early 1900s by American geneticist Thomas Hunt Morgan. By observing how frequently certain characteristics were inherited in combination in numerous generations of fruit flies, he concluded that traits that were often inherited in combination must be associated with genes that were near one another on the chromosome. From his studies, Morgan was able to create a rough map showing the relative order of these associated genes on the chromosomes, and in 1933 he was awarded the Nobel Prize in physiology or medicine for his work. Human linkage maps are created mainly by following inheritance patterns in large families over many generations. Originally, these studies were limited to inherited physical traits that could be observed easily in each family member. Today, however, sophisticated laboratory techniques allow researchers to create more detailed linkage maps by comparing the position of the target gene relative to the order of genetic markers, or specific known segments of DNA.

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Beyond mendel

  • 2. Mendel, Gregor Johann (1822-1884), Austrian monk, whose experimental work became the basis of modern hereditary theory. Mendel was born on July 22, 1822, to a peasant family in Heinzendorf (now Hynčice, Czech Republic). He entered the Augustinian monastery at Brünn (now Brno, Czech Republic), which was known as a center of learning and scientific endeavor. He later became a substitute teacher at the technical school in Brünn. There Mendel became actively engaged in investigating variation, heredity, and evolution in plants at the monastery's experimental garden. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants, carefully analyzing seven pairs of seed and plant characteristics. His tedious experiments resulted in the enunciation of two generalizations that later became known as the laws of heredity. His observations also led him to coin two terms still used in present-day genetics: dominance, for a trait that shows up in an offspring; and recessiveness, for a trait masked by a dominant gene.
  • 3. Mendel published his important work on heredity in 1866. Despite, or perhaps because of, its descriptions of large numbers of experimental plants, which allowed him to express his results numerically and subject them to statistical analysis, this work made virtually no impression for the next 34 years. Only in 1900 was his work recognized more or less independently by three investigators, one of whom was the Dutch botanist Hugo Marie de Vries, and not until the late 1920s and the early '30s was its full significance realized, particularly in relation to evolutionary theory. As a result of years of research in population genetics, investigators were able to demonstrate that Darwinian evolution can be described in terms of the change in gene frequency of Mendelian pairs of characteristics in a population over successive generations. Mendel's later experiments with the hawkweed Hieracium proved inconclusive, and because of the pressure of other duties he ceased his experiments on heredity by the 1870s. He died in Brünn on January 6, 1884.
  • 5. In cases of incomplete dominance, the inheritance of a dominant and a recessive allele results in a blending of traits to produce intermediate characteristics. For example, four-o’clock pink plants may have red, white, or pink flowers. Plants with red flowers have two copies of the dominant allele R for red flower color (RR). Plants with white flowers have two copies of the recessive allele r for white flower color (rr). Pink flowers result in plants with one copy of each allele (Rr), with each allele contributing to a blending of colors.
  • 6. Another exception to Mendelian genetics involves genes with multiple alleles. Certain traits are controlled by multiple alleles that have complex rules of dominance. In humans, for example, the gene for blood type has three alleles: IA, IB, and i. With three alternatives for each member of a gene pair, there are six possible combinations of these genes (IAIA, IBIB, ii, IAi, IBi, IAIB). Although there are six possible combinations, humans have only four major blood types: A, B, AB, and O. This results because both IA and IB dominate over i, but not over each other, so a person with a gene combination of IAIA or IAi has blood type A. The gene combinations IBIB and IBi both produce blood type B. IAIB results in a blood type AB, and ii results in blood type O.
  • 7. A significant number of human traits, such as eye color, skin color, height, weight, and muscle strength are typically regulated by more than one allele in a pattern known as polygenic inheritance. Several thousand alleles, for example, may combine to determine a person’s potential for pole-vaulting, and several hundred may play a role in establishing a person’s normal weight. Certain diseases may result from mutations in one or more alleles involved in polygenic inheritance. Researchers have identified nearly a dozen mutated alleles that are associated with diabetes mellitus, and a similar number are linked to asthma. Heart disease may be linked to two or three times that number. Some types of cancer may be correlated with more than 100 different genes. Polygenic inheritance is quite complex, and the ways in which multiple genes interact to produce traits are not fully understood.
  • 8. In his experiments, Mendel was careful to study traits in pea plants where one trait did not appear to influence another, such as the plant’s height or the pea’s texture. These two phenotypes (height and texture) occur randomly with respect to one another in a manner known as independent assortment. Today scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes. An exception to independent assortment develops when genes appear near one another on the same chromosome. When genes occur on the same chromosome, they are inherited as a single unit. Genes inherited in this way are said to be linked. For example, in fruit flies the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.
  • 9. But in many cases, genes on the same chromosome that are inherited together produce offspring with unexpected allele combinations. This results from a process called crossing over. Sometimes at the beginning of meiosis, a chromosome pair (made up of a chromosome from the mother and a chromosome from the father) may intertwine and exchange sections of chromosome. The pair then breaks apart to form two chromosomes with a new combination of genes that differs from the combination supplied by the parents. Through this process of recombining genes, organisms can produce offspring with new combinations of maternal and paternal traits that may contribute to or enhance survival.
  • 10. There are two main categories of gene-mapping techniques: linkage, or genetic, mapping, a method that identifies only the relative order of genes along a chromosome; and physical mapping, more precise methods that can place genes at specific distances from one another on a chromosome. Both types of mapping use markers in the DNA sequence, detectable physical or molecular characteristics that differ among individuals and that are passed from one generation to the next.
  • 11. Linkage mapping was developed in the early 1900s by American geneticist Thomas Hunt Morgan. By observing how frequently certain characteristics were inherited in combination in numerous generations of fruit flies, he concluded that traits that were often inherited in combination must be associated with genes that were near one another on the chromosome. From his studies, Morgan was able to create a rough map showing the relative order of these associated genes on the chromosomes, and in 1933 he was awarded the Nobel Prize in physiology or medicine for his work. Human linkage maps are created mainly by following inheritance patterns in large families over many generations. Originally, these studies were limited to inherited physical traits that could be observed easily in each family member. Today, however, sophisticated laboratory techniques allow researchers to create more detailed linkage maps by comparing the position of the target gene relative to the order of genetic markers, or specific known segments of DNA.