Genes

• Deoxyribonucleic Acid (DNA)

  • The deoxyribonucleic acid (or DNA) is a very long chain of connected nucleotides.  
  • Each nucleotide contains a phosphate group, a sugar group, and a nitrogen base.
  • double stranded chain of nucleotides
• DNA Nitrogen Bases

  1. adenine (A)  
  2.  thymine (T)  
  3.  guanine (G)  
  4.  cytosine (C).
• Deoxyribonucleic Acid (DNA)

  • DNA is considered the blueprint of life because it works the same way as the blueprint of houses made by engineers. Infrastructures would not be successfully created without a blueprint. Similarly, without the blueprint of life, humans and other living organisms will not exist.
  • DNA not only dictates the physical characteristics of an organism but also all the other processes that are necessary for the organism’s survival.
• Humans, for example, have many characteristics, such as eye color, height, type and color of hair, skin color, and so on. All of these are encoded in DNA. These characteristics are also called traits, which vary from one individual to another.
• Gene

  • A gene is a segment of the DNA that serves as a unit of heredity.  
  • A gene contains all the needed information in controlling the expression of different traits in an organism.
• The Structure of a Gene

  • Exons
  • Introns
  • Promoter Region
  • Enhancer Region
• Exons


  coding regions
• Introns


  non-coding regions
• Promoter region


  regulatory sequence that regulates the activation of genes
• Enhancer region


  augments the promoter region to be activated
• The location of the genetic material varies between eukaryotic and prokaryotic organisms.
• Structure of a Prokaryotic Cell Gene

  • the nucleoid region is the part of the cell where the genetic material is suspended.
  • this is due to the absence of a real nuclear membrane.
  • the genetic material is suspended in the cytoplasm
• Structure of a Eukaryotic Cell Gene


  the genetic material is stored within the nucleus bound by the nuclear membrane; the genetic material is all stored within the nucleus
• Chromosome

  • condensed version of chromatin, made of coiled DNA and histones
  • during prophase, the chromatin will condense into chromosomes, which makes them visible during metaphase
  • there are 46 chromosomes in humans
  • a chromosome is made of two sister chromatids
• Chromatin

  • DNA is present as chromatin only during interphase and prophase
• Chromatid

  • one version of the duplicated chromosome
  • the number of chromatids is 92
• The expression of biological traits is influenced and controlled by a set of genes that we refer to as the genotype.
• A phenotype is the observable traits expressed in an individual
• Genes


  basic unit of genetic inheritance
• Gregor Johann Mendel

  • the Father of Genetics
  • was an Austrian monk
  • Influenced by his great love for science and works of previous scientists, such as Charles Darwin (Origin of Species), he started with his pea plant experiment. 
• Peas were an excellent option for Mendel to use because they are commercially available, quick to grow and mature, their reproductive organs are entirely enclosed in the flower, and their characteristics are very distinguished, which made them easy to manipulate.
• Before Mendel’s time, people believed the blending theory of inheritance wherein the genes of the parents are mixed as they pass them to the offspring. Mendel’s pea plant experiment disproved this theory.
• Parental generation (P generation)


  This is the initial generation
• First filial generation (F1 generation)


  The first set of offspring from the parent generation. The F1 generation can reproduce to make the F2 generation and so on.
• Purebred plants


  These refer to plants that "always" produce an offspring with identical traits as the parent for many generations. For example, a parent plant with a tall trait crossed by a plant with the same trait will produce a **100% offspring** with the tall trait.
• Self-fertilization


  Some plants can fertilize by themselves. It is possible because some plants, such as pea plants, possess both reproductive organs (stamen and pistil).
• The seven traits in pea plants that were observed by Mendel were:

  • height
  • seed shape
  • seed color
  • seed coat color
  • pod shape
  • pod color
  • flower position.
• Mendelian Laws

  • The modes of inheritance developed by Mendel can characterize some patterns of heredity:
  • law of dominance  
  • law of segregation  
  • law of independent assortment.
• Law of Dominance

  • A dominant trait exists when a dominant allele masks the expression of the recessive allele
  • A recessive trait exists if the dominant allele is not present and has a pair of recessive alleles.
• Homozygous Dominant/Recessive


  a genotype represented by two dominant alleles or two recessive alleles
• Heterozygous Dominant

  • The genotype consists of one dominant, one recessive allele
  • There is no such thing as heterozygous recessive as recessive traits could only be expressed if and only if both recessive alleles are present.
  •  For example, tall is dominant over short. Therefore, TT or Tt could represent tall traits. Short, on the other hand, could be represented by tt
• In solving problems under Mendelian laws

  • Only one form of the trait will be present in the next generation
  • A cross involving a dominant homozygous trait will always appear in all of the offspring in the next generation, and the recessive trait will just be masked.
• Law of Segregation|

  • From this, Mendel constructed the idea that alleles must segregate somewhere between the production of sex cells and fertilization.
  • In his pea experiments, Mendel knew that crossed parent plants have one tall allele (T) that dominates the short allele (t), causing it to grow tall (Tt).
  • To get the short plants (tt) from these parents, the alleles from the short and tall plants should separate; otherwise, the generation of short plants is not possible.
• The law of segregation is described as the separation of the two alleles for each trait, during gamete formation (oogenesis and spermatogenesis), and during the formation of zygotes, the alleles will combine at random with other alleles.
• Law of Independent Assortment

  • Mendel derived a conclusion that different traits are inherited independently.
  • With Mendel’s work on several crossbreeds of pea plants, he observed that the height of the plant (T), color (Y), and shape (R) of the seeds had no effect on the inheritance of one another. 
• Punnett Square


  a graphical representation for predicting all possible resulting genotype combinations of a specific cross or breeding experiment.
• Making a Punnett Square

  To predict the resulting genotype combination, follow the steps below.  

  1. Draw a Punnett square by setting up a grid of perpendicular lines.  
  2. The genotype of one parent is then placed on the top.  
  3. The genotype of the other parent is placed down the left side.  
  4. The spaces at the center are filled by copying the letters on the row and column heading across or down into the empty squares.
• Tips in Making a Punnett Square

  • Any letter could represent genotypes. However, for the sake of the clarity of discussion, always use the first letter of the dominant trait.
  • Use the same letter but lowercase to represent recessive traits.
  • In addition, use the same letters if you are talking about the same trait. For example, tall is dominant over short. Therefore, tall should be represented by T and the short trait should be t. Do not use s or any letter to represent the short trait. Use the same letter but lowercase as you are talking about the same trait, which is height.
• Structure of a Gene

  A) Enhancers

  B) Promoter

  C) Exon

  D) Intron