8.2-Heredity

8.2-Heredity Important Formulae

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Heredity is the biological process through which traits and characteristics are passed from parents to their offspring. This transmission of genetic information is fundamental to the study of genetics and plays a crucial role in determining the phenotypic traits of organisms.

Basics of Heredity

Heredity operates through the transfer of genes, which are units of heredity located on chromosomes. Each gene can exist in different forms, known as alleles. The combination of alleles inherited from both parents influences an organism’s traits.

Mendelian Genetics

Gregor Mendel, often referred to as the "Father of Genetics," conducted experiments with pea plants in the 19th century that laid the groundwork for modern genetics. His key contributions include:

  • Law of Segregation: This law states that during gamete formation, the two alleles for a trait segregate from each other so that each gamete carries only one allele for each gene.
  • Law of Independent Assortment: This law states that alleles for different traits are distributed to gametes independently of one another, leading to genetic variation.
Genotype and Phenotype

In genetics, two important concepts are genotype and phenotype:

  • Genotype: The genetic constitution of an organism; it refers to the specific alleles inherited from the parents. For example, a plant may have a genotype of TT (homozygous dominant), Tt (heterozygous), or tt (homozygous recessive).
  • Phenotype: The observable characteristics or traits of an organism, resulting from the interaction of the genotype with the environment. For example, a plant may be tall or short, depending on its genotype and environmental factors.
Inheritance Patterns

Heredity can follow various patterns of inheritance, including:

  • Dominant and Recessive Inheritance: In this pattern, dominant alleles mask the effect of recessive alleles. For example, in pea plants, the allele for tallness (T) is dominant over the allele for shortness (t). A plant with genotypes TT or Tt will be tall, while only tt will be short.
  • Co-dominance: In this case, both alleles express themselves equally in the phenotype. An example is the ABO blood group system, where individuals with genotype IAIB exhibit both A and B antigens on their red blood cells.
  • Incomplete Dominance: This occurs when the phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes. For example, in snapdragon flowers, crossing red (RR) and white (WW) flowers results in pink (RW) flowers.
Chromosomal Theory of Inheritance

The chromosomal theory of inheritance, developed in the early 20th century, states that genes are located on chromosomes, which are passed from parents to offspring during reproduction. Key points include:

  • Chromosomes exist in pairs in diploid organisms, with one chromosome of each pair inherited from each parent.
  • During meiosis, the process of gamete formation, chromosomes segregate, ensuring each gamete receives one chromosome from each pair.
Human Genetics

In humans, heredity influences a wide range of traits, including physical characteristics (like eye color and height), genetic disorders (like cystic fibrosis or hemophilia), and behavioral traits. Understanding heredity in humans involves:

  • Pedigree Analysis: A diagram that represents family relationships and inheritance patterns for specific traits, helping to track genetic disorders across generations.
  • Genetic Counseling: Providing information to prospective parents about the risks of inherited conditions based on family history and genetic testing.
Environmental Influences

While heredity plays a significant role in determining traits, environmental factors can also influence the expression of those traits. For example, factors such as nutrition, climate, and lifestyle can affect the growth and development of individuals.


Father and son with jug ears.
Xubor, CC BY-SA 3.0, via Wikimedia Commons

8.2-हेरिडिटी

हेरिडिटी का अर्थ है जीवों के गुणसूत्रों में मौजूद गुणों का एक पीढ़ी से दूसरी पीढ़ी में स्थानांतरित होना। इसे वंशागति भी कहा जाता है। यह प्रक्रिया कोशिकाओं में गुणसूत्रों (Chromosomes) द्वारा नियंत्रित होती है, जो हमारे शरीर के हर गुणसूत्र में स्थित जीन (Genes) के द्वारा विशिष्ट लक्षणों का निर्धारण करते हैं।

हेरिडिटी के अध्ययन में दो मुख्य पहलू होते हैं: (1) गुणसूत्रों का संचरण और (2) लक्षणों का आनुवंशिकता द्वारा पीढ़ी दर पीढ़ी संचरण।

गुणसूत्र और जीन

मानव कोशिकाओं में 46 गुणसूत्र होते हैं, जो 23 जोड़ों में होते हैं। इनमें से आधे गुणसूत्र माता से और आधे पिता से मिलते हैं। इन गुणसूत्रों पर जीन होते हैं, जो विशिष्ट लक्षणों के लिए जिम्मेदार होते हैं। उदाहरण के लिए, आंखों का रंग, बालों का प्रकार आदि।

डॉ. ग्रेगोर मेंडल का योगदान

हेरिडिटी के सिद्धांत को सबसे पहले डॉ. ग्रेगोर मेंडल ने 19वीं सदी में खोजा। उन्होंने मटर के पौधों पर प्रयोग किए और यह पाया कि लक्षणों का संचरण निश्चित नियमों के अनुसार होता है। उनके द्वारा निर्धारित दो मुख्य नियम थे:

  1. मोनोहाइब्रिड क्रॉस (Monohybrid Cross): यह एकल लक्षणों का अध्ययन करता है।
  2. डिहाइब्रिड क्रॉस (Dihybrid Cross): यह दो लक्षणों का एक साथ अध्ययन करता है।
साधारण वंशागति के सिद्धांत

वह दो प्रकार के गुणसूत्रों में अंतर बताते हैं - डोमिनेंट गुणसूत्र और रीससिव गुणसूत्र। डोमिनेंट गुणसूत्र वह होते हैं जो एक पीढ़ी से दूसरी पीढ़ी में प्रमुख रूप से दिखाई देते हैं, जबकि रीससिव गुणसूत्र दबे रहते हैं जब तक दो समान रीससिव गुणसूत्र एक साथ न मिल जाएं।

जीन के विभिन्न रूपों को एल्ली (Allele) कहा जाता है। जब किसी व्यक्ति के पास एक ही प्रकार के जीन होते हैं, तो इसे होमोजाइगस (homozygous) कहा जाता है, और जब विभिन्न प्रकार के जीन होते हैं, तो इसे हेटेरोजाइगस (heterozygous) कहा जाता है।

पुनः संयोजन और क्रॉसिंग ओवर

जीनों के पुनः संयोजन की प्रक्रिया में गुणसूत्रों का एक दूसरे से क्रॉसिंग ओवर होता है, जिससे नए संयोजन उत्पन्न होते हैं और आनुवंशिक विविधता पैदा होती है। यह प्रक्रिया विशेष रूप से युग्मजनक कोशिकाओं (Gametes) में होती है।

आनुवंशिकता के नियम

मेंडेल के नियमों के आधार पर हम वंशागति को तीन प्रकार से समझ सकते हैं:

  1. प्रथम नियम (Law of Dominance): डोमिनेंट एल्ली हमेशा रीससिव एल्ली के ऊपर प्रभाव डालती है।
  2. द्वितीय नियम (Law of Segregation): प्रत्येक गुणसूत्र अपनी एक ही प्रति युग्मजनक कोशिका में भेजता है।
  3. तृतीय नियम (Law of Independent Assortment): विभिन्न गुणसूत्र एक दूसरे से स्वतंत्र रूप से संप्रेषित होते हैं।
हेरिडिटी में प्रमुख अवधारणाएं

इसमें जीन, एल्ली, गुणसूत्र और गुणसूत्रों के बीच अनुवांशिक जानकारी का वितरण शामिल होता है। यदि किसी जीन का एक एल्ली डोमिनेंट है, तो वह लक्षण प्रकट होगा, जबकि रीससिव एल्ली तभी प्रकट होती है जब दोनों एल्ली रीससिव होते हैं।

आनुवंशिकता और आनुवंशिक विकार

हेरिडिटी केवल शारीरिक लक्षणों तक सीमित नहीं रहती, बल्कि आनुवंशिक विकार भी इसमें समाहित होते हैं, जैसे हेमोफिलिया और थैलेसीमिया। इन विकारों का संचरण भी आनुवंशिकता के द्वारा होता है।

हेरिडिटी में जीन के प्रकट होने के तरीके और उसके प्रभावों का अध्ययन जीवविज्ञान के लिए महत्वपूर्ण है, क्योंकि यह हमें जीवन के विभिन्न पहलुओं को समझने में मदद करता है।

How do Mendel’s experiments show that traits may be dominant or recessive?

Solution:

Mendel’s Experiments on Dominant and Recessive Traits

Gregor Mendel, through his pea plant experiments, established the concept of dominant and recessive traits. He crossed purebred tall plants with purebred short plants. The first generation (F1) showed only tall plants, indicating that the tall trait is dominant. When he self-pollinated the F1 generation, the second generation (F2) exhibited a 3:1 ratio of tall to short plants. This revealed that the short trait was recessive, as it reappeared despite being masked in the F1 generation. Mendel's work laid the foundation for understanding inheritance patterns in genetics.

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How do Mendel’s experiments show that traits are inherited independently?

Solution:

Mendel’s Experiments on Independent Inheritance

Mendel conducted experiments using pea plants, focusing on traits like seed shape and color. He crossbred plants with different traits and observed the offspring. In dihybrid crosses, he found a 9:3:3:1 phenotypic ratio in the F2 generation, indicating that traits segregate independently. For example, the inheritance of seed shape did not affect the inheritance of seed color. This suggested that alleles for different traits are distributed to gametes independently during

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A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

Solution:

Dominance of Blood Group Traits

The information provided is not sufficient to determine which blood group trait is dominant. Blood group A is controlled by the A allele, while blood group O is controlled by the O allele. The daughter has blood group O, which indicates that she received the O allele from her mother. Since the father has blood group A, he could either be genotype AA or AO. If he were AA, all offspring would inherit A, leading to blood group A. However, since the daughter is O, it suggests he is AO, confirming O as a recessive trait.

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How is the sex of the child determined in human beings?

Solution:

Determination of Sex in Humans

In humans, sex is determined by the combination of sex chromosomes inherited from the parents. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). During fertilization, the sperm from the male, carrying either an X or Y chromosome, combines with the egg from the female, which always carries an X chromosome. If the sperm carries an X chromosome, the resulting child will be female (XX). If the sperm carries a Y chromosome, the child will be male (XY). Thus, the father determines the sex of the child.

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