16.2 Genetics and Inheritance Patterns

4 min read•august 28, 2024

Genetics and inheritance patterns form the foundation of how traits are passed from parents to offspring. These principles, discovered by Gregor Mendel, explain how DNA carries genetic information and how it's expressed in living organisms.

Understanding genetics helps us grasp how traits are inherited, why some diseases run in families, and how genetic variations contribute to diversity. This knowledge is crucial for fields like medicine, agriculture, and evolutionary biology.

Mendelian Genetics Principles

Gregor Mendel's Discoveries

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  • Gregor Mendel, known as the "father of modern genetics," discovered the fundamental laws of inheritance through experiments with pea plants in the mid-1800s
  • Mendel's Law of Segregation states that during the formation of gametes (egg and sperm cells), the two alleles for each gene separate, with each gamete receiving only one allele
  • The alleles then recombine randomly during fertilization
  • Mendel's Law of Independent Assortment states that the inheritance of one trait is independent of the inheritance of other traits
  • This law applies to genes located on different chromosomes or far apart on the same chromosome

Alleles, Genotypes, and Phenotypes

  • Alleles are different versions of a gene that can be passed on to offspring
  • Alleles can be dominant or recessive, with dominant alleles masking the expression of recessive alleles
    • Dominant alleles are typically represented by a capital letter (A), while recessive alleles are represented by a lowercase letter (a)
  • Genotype refers to an individual's genetic makeup, the specific combination of alleles inherited from their parents
  • Phenotype is the observable physical or biochemical characteristics of an organism, determined by both the genotype and environmental influences
  • Punnett squares are used to predict the probability of offspring inheriting specific genotypes and phenotypes based on the genotypes of the parents

DNA Structure and Function

DNA Composition and Structure

  • DNA (deoxyribonucleic acid) is the hereditary material found in nearly all living organisms
  • It carries the genetic instructions for the development, functioning, growth, and reproduction of organisms
  • DNA is a double-stranded molecule composed of nucleotides
  • Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C)
  • The two strands of DNA are held together by hydrogen bonds between complementary base pairs: A always pairs with T, and G always pairs with C
  • This complementary base pairing is crucial for DNA replication and the storage of genetic information

Genetic Code and Chromosomes

  • The sequence of nucleotides in DNA determines the genetic code, which provides instructions for the synthesis of proteins
  • Three nucleotides (a codon) code for a specific amino acid or a stop signal
  • DNA is organized into structures called chromosomes, which are found in the nucleus of eukaryotic cells
  • Humans have 23 pairs of chromosomes, for a total of 46
  • During cell division (mitosis and meiosis), DNA is replicated and passed on to daughter cells, ensuring the transmission of genetic information from one generation to the next

Autosomal vs Sex-Linked Inheritance

Autosomal Inheritance

  • Autosomal inheritance refers to the transmission of traits controlled by genes located on autosomes (non-sex chromosomes)
  • Autosomes are numbered from 1 to 22 in humans
  • Autosomal dominant inheritance occurs when a single copy of a dominant allele is sufficient to express the trait (Huntington's disease)
  • Autosomal recessive inheritance requires two copies of the recessive allele (one from each parent) for the trait to be expressed (cystic fibrosis, sickle cell anemia)

Sex-Linked Inheritance

  • Sex-linked inheritance involves traits controlled by genes located on the sex chromosomes (X and Y)
  • The inheritance patterns of sex-linked traits differ between males and females due to their different sex chromosome combinations (XY in males, XX in females)
  • X-linked inheritance occurs when the gene controlling the trait is located on the X chromosome
  • Males are more likely to be affected by X-linked recessive disorders because they have only one X chromosome (color blindness, hemophilia)
  • Y-linked inheritance is rare and involves traits controlled by genes on the Y chromosome
  • These traits are passed from father to son and only affect males (male infertility caused by deletions in the Y chromosome)

Probability in Inheritance

Monohybrid and Dihybrid Crosses

  • The principles of probability, such as the product rule and sum rule, can be used to calculate the likelihood of inheriting specific genotypes and phenotypes
  • Monohybrid crosses involve the inheritance of a single gene with two alleles
  • The probability of each genotype in the offspring can be determined using a Punnett square
    • For a monohybrid cross between two heterozygous individuals (Aa x Aa), the probability of each genotype in the offspring is 25% AA, 50% Aa, and 25% aa
  • Dihybrid crosses involve the inheritance of two genes, each with two alleles
  • A Punnett square with 16 possible genotype combinations is used to determine the probabilities
    • For a dihybrid cross between two individuals heterozygous for both genes (AaBb x AaBb), the probability of each genotype in the offspring is 6.25% AABB, 12.5% AABb, 12.5% AaBB, 25% AaBb, 12.5% Aabb, 12.5% aaBb, 6.25% aabb, and 12.5% aaBB

Pedigree Analysis and Hardy-Weinberg Principle

  • Pedigree analysis is used to study the inheritance patterns of traits within a family
  • Pedigrees can help determine the mode of inheritance (autosomal dominant, autosomal recessive, or sex-linked) and the probability of an individual inheriting or passing on a specific trait
  • The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of disturbing factors (mutation, natural selection, genetic drift)
  • This principle can be used to calculate the frequency of alleles and genotypes in a population at genetic equilibrium
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