Heredity and Evolution

The topics covered in this chapter are:

 Heredity: Mendel's laws of inheritance, mono and dihybrid crosses, Punnett square, dominant and recessive traits, codominance, incomplete dominance, sex determination, sex-linked inheritance.

 Evolution: Origin of life, evolution of life forms, evidences for evolution, natural selection, and adaptation, speciation, human evolution, and evolutionary relationships. 

 Mendelian disorders: Sickle cell anaemia, thalassemia, phenylketonuria, and color blindness.

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Heredity  

Heredity refers to the passing down of genetic information from one generation to the next. This genetic information includes DNA, which contains the instructions for the development and function of an organism. Heredity plays a crucial role in determining many of an organism's traits, such as physical characteristics, susceptibility to disease, and behavior. The study of heredity is known as genetics, which has advanced significantly in recent years due to advances in molecular biology and the mapping of the human genome.

 Mendel's laws of inheritance

 Mendel's laws of inheritance, also known as Mendelian genetics, are the fundamental principles of genetics discovered by the Augustinian monk Gregor Mendel in the 19th century. His experiments with pea plants led to the discovery of the basic principles of inheritance, which form the foundation of modern genetics.

 Mendel's laws of inheritance are as follows:

 Law of Segregation: Mendel's first law states that during the formation of gametes (sperm or egg cells), the two alleles (or versions) of a gene separate from each other and only one allele is passed on to each offspring. This explains why traits appear to skip generations. 

 Law of Independent Assortment: Mendel's second law states that the inheritance of one trait is not dependent on the inheritance of another. This means that the distribution of alleles for one trait does not affect the distribution of alleles for another trait.

 Law of Dominance: Mendel's third law states that when two different alleles are present, one allele (the dominant allele) will be expressed in the phenotype (physical appearance) of the organism, while the other allele (the recessive allele) will be masked.

 Mono and dihybrid crosses 

Mono and dihybrid crosses are two types of genetic crosses used to study the inheritance of traits in organisms. These crosses are commonly used in genetics to predict the probability of offspring inheriting certain traits from their parents.

 A monohybrid cross is a genetic cross between two individuals that differ in only one trait. For example, if we cross two plants that differ in flower color, one with white flowers and one with purple flowers, we would perform a monohybrid cross. In this case, the offspring of the cross are called the F1 generation. The F1 generation will all have the same phenotype, in this case, they would all have purple flowers because the purple flower trait is dominant. However, they will be heterozygous for the flower color trait, meaning that they have one allele for purple flowers and one allele for white flowers. 

 A dihybrid cross, on the other hand, is a genetic cross between two individuals that differ in two traits. For example, if we cross two plants that differ in both flower color and flower shape, we would perform a dihybrid cross. In this case, the offspring of the cross are called the F1 generation. The F1 generation will all have the same phenotype, but they will be heterozygous for both traits. 

 To predict the probability of offspring inheriting certain traits in a monohybrid or dihybrid cross, we use Punnett squares. Punnett squares are grids used to represent all possible combinations of alleles that can occur in the offspring of a genetic cross. By filling in the squares with the possible alleles for each parent, we can determine the probabilities of the different genotypes and phenotypes that may result in the offspring.

Punnett Square   

• A Punnett square is a simple diagram used in genetics to predict the probability of inheritance of a particular trait or set of traits. It is named after Reginald Punnett, who first introduced the concept in 1905. 

•  The Punnett square is typically a square divided into four quadrants, with each quadrant representing a possible combination of alleles from two parents. The alleles are represented by letters or symbols, with uppercase letters indicating dominant traits and lowercase letters indicating recessive traits. 

•  To use a Punnett square, the alleles of the parents are written along the top and side of the square. The possible combinations of alleles in their offspring are then filled in within the squares, based on the rules of Mendelian genetics. The resulting probabilities can be used to predict the likelihood of specific traits appearing in the offspring.

•  The Punnett square is a simple but powerful tool that has been used for decades in genetics research and is still widely used today. It provides a clear and concise way to visualize the complex patterns of inheritance that govern genetic traits in living organisms.

Dominant and recessive traits,

•  In genetics, traits refer to observable physical or biochemical characteristics that are determined by an individual's genes. A trait may be controlled by one or more genes, and each gene may have different versions or variants, called alleles.

•  Dominant and recessive traits are two types of inheritance patterns that describe how traits are passed down from one generation to the next.

•  A dominant trait is a trait that is expressed or observed when an individual has one or two copies of the dominant allele. In other words, if an individual inherits at least one copy of the dominant allele, the dominant trait will be expressed, and the recessive trait will be masked. For example, if an individual inherits a dominant allele for brown eyes and a recessive allele for blue eyes, the dominant brown eye trait will be expressed.

•  On the other hand, a recessive trait is a trait that is expressed only when an individual has two copies of the recessive allele.In other words, if an individual inherits two copies of the recessive allele, the recessive trait will be expressed, and the dominant trait will be masked. For example, if an individual inherits two recessive alleles for blue eyes, the recessive blue eye trait will be expressed.

•  It is important to note that dominant and recessive traits are not always determined by a single gene or allele. Some traits may be controlled by multiple genes or have more complex inheritance patterns, such as incomplete dominance or codominance.

Codominance 

• Codominance is a type of inheritance pattern in which two alleles of a gene are expressed equally in a heterozygous individual, resulting in the production of a distinct phenotype that is different from either homozygous phenotype. This means that both alleles are fully expressed and contribute to the phenotype, without one allele being dominant over the other. 

•  For example, in the case of blood type, the A and B alleles are codominant, meaning that if an individual has one copy of the A allele and one copy of the B allele, they will have a blood type that expresses both A and B antigens, resulting in blood type AB. In contrast, individuals who have two copies of the A allele have blood type A, and individuals with two copies of the B allele have blood type B. 

•  In codominance, neither allele is dominant or recessive, and both are expressed equally in the phenotype. This is in contrast to dominant-recessive inheritance, in which the dominant allele is expressed, and the recessive allele is only expressed in homozygous individuals.

Incomplete Dominance  

• Incomplete dominance is a genetic phenomenon where the dominant allele does not completely mask the recessive allele in the phenotype of the heterozygote. This means that the phenotype of the heterozygous individual is an intermediate blend of the phenotypes associated with the homozygous dominant and homozygous recessive genotypes.

•  For example, in a flower, the allele for red color (R) is dominant over the allele for white color (r). In incomplete dominance, a heterozygous flower with one R allele and one r allele would produce pink flowers, which is a blend of red and white.

•  This is different from complete dominance, where the dominant allele completely masks the recessive allele, and only the dominant phenotype is expressed in the heterozygote.

•  Incomplete dominance is one of the mechanisms that contributes to genetic diversity, as it allows for new phenotypes to emerge in the population.

Sex Determination 

•   Sex determination refers to the process by which an organism develops as male or female. In many species, including humans, sex is determined by genetic factors. Specifically, humans have 23 pairs of chromosomes, with one pair being the sex chromosomes. Females have two copies of the X chromosome (XX), while males have one X and one Y chromosome (XY).

•  During fertilization, the sex of the offspring is determined by which sex chromosome the sperm carries. If the sperm carries an X chromosome, the resulting offspring will be female (XX). If the sperm carries a Y chromosome, the resulting offspring will be male (XY).

•  There are also other mechanisms of sex determination in different species. For example, some reptiles have temperature-dependent sex determination, where the temperature of the environment during embryonic development determines the sex of the offspring. In some fish, the sex of the individual can change throughout its lifetime, based on environmental cues.

Sex-Linked Inheritance 

•  Sex-linked inheritance refers to the inheritance pattern of genes located on the sex chromosomes, which are the X and Y chromosomes in mammals. In humans, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). 

•  Because males have only one X chromosome, they will inherit any gene on that X chromosome from their mother. Therefore, if a woman is a carrier of a recessive X-linked gene, there is a 50% chance that her son will inherit the mutated gene and express the associated trait, and a 50% chance that her daughter will inherit one normal and one mutated X chromosome and be a carrier like her mother.

•  In contrast, if the father has a recessive X-linked gene, he can only pass it on to his daughters because they receive his X chromosome. Sons will always inherit their father's Y chromosome and will not receive any X-linked genes. 

•  Some examples of X-linked genetic disorders include color blindness, hemophilia, and Duchenne muscular dystrophy. Because females have two X chromosomes, they are less likely to express X-linked recessive traits because the normal X chromosome can compensate for the mutated one. However, females can still be carriers of X-linked traits and pass them on to their offspring.

 Evolution 

Evolution is the process by which species of organisms change over time, often through the gradual accumulation of small genetic variations that are passed on from one generation to the next. This process occurs through natural selection, which favors traits that increase an organism's chances of survival and reproduction in its environment, while eliminating traits that decrease its chances of success. Over long periods of time, these genetic changes can lead to the development of new species and the extinction of older ones. Evolution is a fundamental concept in biology and has been supported by extensive evidence from a wide range of scientific disciplines.

 Origin of life

• The origin of life is a scientific question that seeks to understand how life began on Earth. While there is no definitive answer to this question, there are several scientific theories and hypotheses that attempt to explain the origin of life. One popular hypothesis is the "RNA world" hypothesis, which suggests that RNA (ribonucleic acid) was the first self-replicating molecule on Earth. According to this hypothesis, RNA molecules could have arisen spontaneously from simple organic molecules present on the early Earth. These RNA molecules could then have evolved the ability to replicate themselves, eventually leading to the formation of the first living cells.

•  Another hypothesis is the "metabolism first" hypothesis, which suggests that metabolism, or the chemical reactions necessary for life, arose before self-replicating molecules. In this hypothesis, simple organic molecules could have combined to form more complex molecules capable of catalyzing chemical reactions. Over time, these molecules could have evolved into the first living cells.

•  There are also other hypotheses, such as the "panspermia" hypothesis, which suggests that life may have originated elsewhere in the universe and arrived on Earth via meteorites or other means. 

•  While the exact origin of life on Earth remains a mystery, scientists continue to study this topic through a variety of approaches, including laboratory experiments, computer simulations, and studies of the early Earth and other planets in our solar system.

 Evidences for evolution 

 Evolution is a well-supported scientific theory that explains the diversity of life on Earth. There are numerous lines of evidence that support the theory of evolution, including: 

•  Fossil Record: The fossil record provides evidence for the evolution of species over time. Fossils show the progression of life forms from simple to complex, and they provide a record of the appearance and disappearance of species.

•  Comparative Anatomy: The comparison of the anatomy of different species shows that they share common structures that have evolved from a common ancestor. This is known as homology. 

•  Embryology: The study of embryos shows that many species share common developmental patterns that suggest a common ancestor. 

•  Molecular Biology: The comparison of DNA and protein sequences between different species provides evidence for their evolutionary relationships. Similarities in DNA sequences indicate that species are related and have a common ancestor.

•  Biogeography: The distribution of species across different regions of the world provides evidence for evolution. Similar species are found in geographically close regions, and the distribution of species can be explained by their evolutionary history.

•  Experimental Evolution: Experimental evolution in the laboratory provides direct evidence for evolution. It has been demonstrated that species can evolve in response to changes in their environment over a relatively short period of time. 

•  All of these lines of evidence support the theory of evolution, and together they provide a strong case for the evolution of life on Earth.

Natural selection

 Natural selection is a process by which certain traits become more or less common in a population over time, based on their ability to enhance or diminish an organism's ability to survive and reproduce. Individuals with advantageous traits are more likely to survive and pass on their genes to the next generation, while those with less advantageous traits are less likely to do so. 

Adaptation 

 Adaptation is the process by which an organism becomes better suited to its environment over time, often through the process of natural selection. Adaptations can be physical, such as the development of a thicker coat of fur in response to colder temperatures, or behavioral, such as the ability to recognize and avoid predators.

Speciation 

•  Speciation is the process by which new species arise from existing ones. This can happen when populations of the same species become isolated from each other and evolve independently, developing genetic differences that make them distinct from one another. Over time, these differences may become so significant that the populations can no longer interbreed and produce viable offspring, leading to the formation of two or more separate species.

• Human evolution explained. Pushpa Grover human evolution Human evolution refers to the biological and cultural development of the Homo sapiens species over time. It is believed that humans evolved from primates, specifically from a common ancestor with chimpanzees and bonobos, about 6-7 million years ago.

•  There have been several stages of human evolution, each characterized by significant changes in anatomy, behavior, and cognitive abilities. The earliest hominins, such as Sahelanthropus tchadensis, lived around 6-7 million years ago and had ape-like features. Over time, hominins evolved to walk upright on two legs (bipedalism), developed larger brains, and made and used tools.

•  The genus Homo, which includes modern humans, evolved around 2.5 million years ago. Early members of this genus, such as Homo habilis and Homo erectus, had larger brains and more sophisticated tool-making abilities. Homo sapiens, the species to which modern humans belong, evolved about 300,000 years ago in Africa. Modern humans have larger brains, a more complex language, and advanced cultural and technological abilities.

•  The study of human evolution involves a variety of disciplines, including genetics, anthropology, archaeology, and paleontology. Scientists use a range of methods, such as DNA analysis, fossil evidence, and comparative anatomy, to reconstruct the evolutionary history of humans.

Evolutionary Relationships. 

•  Evolutionary relationships refer to the genetic and ancestral connections between different species or groups of organisms. These relationships are determined by studying their physical and genetic characteristics, as well as their geographic distribution and fossil records.

•  The study of evolutionary relationships is called phylogenetics, and it involves constructing evolutionary trees or phylogenetic trees that show the relationships between different organisms. These trees are usually based on shared physical characteristics or genetic sequences, and they allow scientists to understand the evolutionary history of different groups of organisms.

•  Phylogenetics has revealed many surprising connections between different groups of organisms. For example, it has shown that birds are closely related to dinosaurs, and that humans share a common ancestor with chimpanzees. Understanding these evolutionary relationships can help scientists understand the origins of different species, and how they have adapted to their environments over time.

 Mendelian disorders

•   Mendelian disorders are genetic disorders that are caused by variations (mutations) in a single gene. These mutations are usually inherited in a predictable manner from parents to their offspring, following the principles of inheritance discovered by Gregor Mendel, a 19th-century Austrian monk who is considered the father of modern genetics.

•  There are different types of Mendelian disorders, depending on the mode of inheritance of the mutated gene. Some of the most common types are: 

•  Autosomal dominant disorders: These are caused by a mutation in one copy of an autosomal gene (i.e., a gene located on one of the 22 pairs of non-sex chromosomes). Examples include Huntington's disease and Marfan syndrome. 

•  Autosomal recessive disorders: These are caused by mutations in both copies of an autosomal gene, one inherited from each parent. Examples include sickle cell anemia and cystic fibrosis. 

•  X-linked dominant disorders: These are caused by a mutation in a gene located on the X chromosome, one of the two sex chromosomes. The inheritance pattern is more complex in females than in males, as females have two X chromosomes. Examples include Rett syndrome and vitamin D-resistant rickets.

•  X-linked recessive disorders: These are also caused by mutations in a gene located on the X chromosome, but the inheritance pattern is more common in males, who have only one X chromosome. Examples include hemophilia and Duchenne muscular dystrophy.

•  Mitochondrial disorders: These are caused by mutations in the mitochondrial DNA, which is inherited only from the mother. Examples include Leigh syndrome and mitochondrial myopathy.

•  Mendelian disorders can have a wide range of symptoms and severity, depending on the specific gene affected and the type of mutation. Some disorders are lethal in infancy, while others may not cause symptoms until adulthood. Genetic testing can help diagnose Mendelian disorders and provide information about the risk of passing them on to future generations.

 Sickle cell anaemia

•   Sickle cell anemia is a genetic disorder characterized by the production of abnormal hemoglobin, the protein in red blood cells that carries oxygen throughout the body. In individuals with sickle cell anemia, the abnormal hemoglobin causes red blood cells to become crescent-shaped or sickle-shaped, which can lead to a variety of complications. 

•  Symptoms of sickle cell anemia can include fatigue, jaundice, pain, and organ damage. The severity of the symptoms can vary widely between individuals, with some experiencing mild symptoms and others experiencing severe, life-threatening complications. 

•  Treatment for sickle cell anemia may include blood transfusions, pain management, and medication to prevent complications. In some cases, bone marrow or stem cell transplants may be used to cure the disease.

•  There is currently no cure for sickle cell anemia, but advances in treatment and management have improved the outlook for individuals with the condition. It is important for individuals with sickle cell anemia to receive ongoing medical care and support to manage their symptoms and prevent complications.

Thalassemia 

•  Thalassemia is an inherited blood disorder characterized by abnormal production of hemoglobin, the protein in red blood cells that carries oxygen throughout the body. Thalassemia is caused by mutations in the genes that control the production of hemoglobin, resulting in reduced or absent production of one or more of the hemoglobin subunits. 

•  The severity of thalassemia depends on the type and number of gene mutations. Individuals who inherit one mutated gene are carriers of the condition, and are typically asymptomatic but can pass the condition on to their children. Individuals who inherit two mutated genes have thalassemia and may experience a range of symptoms, including anemia, fatigue, jaundice, delayed growth and development, bone deformities, and an enlarged spleen. 

•  There are two main types of thalassemia: alpha thalassemia and beta thalassemia. Alpha thalassemia is caused by mutations in the genes that produce the alpha globin subunits of hemoglobin, while beta thalassemia is caused by mutations in the genes that produce the beta globin subunits. 

• There are different subtypes of both alpha and beta thalassemia, which can have varying degrees of severity. Treatment for thalassemia depends on the type and severity of the condition. Mild cases may not require treatment, while more severe cases may require blood transfusions, iron chelation therapy to remove excess iron from the body, and bone marrow transplantation. Genetic counseling and prenatal testing are also available for individuals and couples who are carriers of thalassemia and are considering having children.

  Phenylketonuria  ( PKU )

• Phenylketonuria (PKU) is a rare genetic disorder that affects the way the body processes a specific amino acid called phenylalanine. Individuals with PKU cannot properly metabolize phenylalanine, which can lead to a buildup of this amino acid in the blood and brain. If left untreated, this can cause intellectual disability, seizures, and other neurological problems.

•  PKU is caused by a deficiency of the enzyme phenylalanine hydroxylase, which is needed to convert phenylalanine into another amino acid called tyrosine. PKU is typically diagnosed through newborn screening tests, which check for elevated levels of phenylalanine in the blood. 

•  The primary treatment for PKU involves following a strict low-phenylalanine diet. This involves avoiding high-protein foods like meat, fish, and dairy products, as well as certain fruits and vegetables. Individuals with PKU also need to take special medical formula supplements to ensure they get enough of the other essential amino acids and nutrients they need to support growth and development. 

•  Early and ongoing treatment is critical to prevent the serious complications of PKU. With proper management, individuals with PKU can lead normal, healthy lives.

Color Blindness. 

• Color blindness is a condition in which a person is unable to distinguish between certain colors, usually red and green or blue and yellow. This condition is caused by a problem with the pigments in the cones of the eye that perceive color. There are different types of color blindness, ranging from mild to severe, and it can affect both men and women, although it is more common in men. 

•  People with color blindness may have difficulty distinguishing between different colors or may see certain colors differently than people without color blindness. For example, someone with red-green color blindness may see the colors red and green as the same color, or may have trouble telling the difference between shades of red and green.

•  Color blindness is typically a genetic condition that is inherited from one's parents. There is currently no cure for color blindness, but there are tools and resources available to help people with color blindness, such as special glasses or apps that can help identify colors. It is also important for people with color blindness to be aware of their condition and to take any necessary precautions, such as using labels or other markers to identify colors.