MENDELIAN LAWS OF INHERITANCE

Genetics is the study of inheritance and variation in organisms.

It has various subdisciplines. Branches of Genetics

• Molecular genetics deals with DNA and gene expression and regulation.
• Cytogenetics deals with chromosome structure and behavior during cell division.
• Transmission genetics deals with different patterns of inheritance. It is also called classical genetics, is the oldest subdiscipline of genetics. It attempts to predict outcomes of reproduction.
• Population genetics deals with how forces of evolution influence genes in populations.

Gregor Mendel – Father of Genetics

Gregor Mendel is the father of genetics. He performed experiments on garden pea or Pisum sativum. This led him to formulate the laws of inheritance in his publication, Experiments on Plant Hybrids.

Mendel took the path to priesthood when he entered Augustinian monastery of St. Thomas and became a monk. This is also where he performed his pea plant studies. Mendel chose the legumes garden peas or Pisum sativum for his hybridization experiments.

What makes peas ideal for genetic studies?

• They exhibit vigorous growth.
• They can self-fertilize.
• They can cross-fertilize.

Challenges faced by Mendel Previous Notions of Inheritance

• Pangenesis was the belief that seeds are produced in different organs and will later on gather to form the offspring.
• Homunculus theory - The invention of the microscope made people believe that sperm cells bear a homunculus or little man.
• The blending theory of inheritance states that traits of parents blend every generation of offspring.

A chromosome consists of a DNA molecule, which serve as the repository of genetic information in cells. Our chromosomes occur in pairs called homologous chromosomes.

• Paternal (from the father or male parent)
• Maternal (from the mother or female parent)

A gene is the basic unit of heredity. It controls the expression of a biological characteristic. A characteristic is a heritable feature of an organism. Also, note that genes occur in pairs. Thus, a pair of genes control a particular characteristic. Alleles are the alternative forms of a gene.

Genotype refers to the set of alleles possessed by an organism.

• The genotype is homozygous if the alleles are identical.
• The genotype is heterozygous if the alleles are different.

Phenotypes refer to the actual manifestation of genotypes into observable traits.

Principle of Dominance

In a heterozygous individual, one allele (dominant) completely masks the expression of the other allele (recessive).

Law of Segregation

The two alleles of a gene in an individual segregate or separate from each other during gamete formation.

Law of Independent Assortment

The alleles from different genes are sorted into the gametes independently of each other. Thus, the inheritance of these two genes becomes independent.

SAMPLE PROBLEM

In pea plants, axial inflorescence is dominant over terminal inflorescence. If Laiza crossed a parent plant that is heterozygous for inflorescence to another plant with terminal inflorescence, what are the genotypic and phenotypic ratios of the offspring? The genotypic ratio of the cross is 1/2 AA: 1/2 aa. The phenotypic ratio is 1/2 axial: 1/2 terminal.

PEDIGREE ANALYSIS

Studying Inheritance in Peas

• Matings of individuals can be controlled.
• The generation time is relatively shorter.
• The number of offspring is relatively larger.
• Handing the organism is more convenient.

Studying Inheritance in Humans

• Matings can never be done in controlled conditions.
• The generation time is relatively longer.
• The number of offspring is relatively smaller.
• Collection of pertinent information is more tedious.

A pedigree is a pictorial representation of how a trait is transmitted among the members of a family. Pedigree analysis was devised to observe and analyze how a trait runs in families. It uses a pedigree, a pictorial representation of the transmission of a characteristic in at least two generations of families. Pedigrees are used to determine the mode of inheritance of a trait.

• Individuals with an autosomal dominant trait usually have at least one affected parent.
• Individuals with an autosomal recessive trait usually have no affected parents. Although, still, there are cases when one parent is also affected by the recessive condition.

The Purpose of Pedigree Analysis

• Identification of the mode of inheritance of an unknown genetic trait.
• Determination of the genotype of an individual with respect to a trait.
• Determination of the risk of recurrence of a genetic condition in a family,

Challenges of Pedigree Analysis

A major challenge in pedigree analysis is being able to collect substantial and accurate information regarding the history of a family with regard to a genetic condition.

Pedigree Analysis in Genetic Counseling

• Performed in accredited health institutions
• Advice is also given to families at risk of having a genetic condition
• Advice is given to a family with a genetic condition
• Elaborates on the genetic, psychological, developmental aspects

Constructing Pedigrees

• Use a “square” for male individuals, while use a “circle” for female individuals.
• Use a “diamond” for individuals whose specific sex cannot be identified.
• Use a “shaded” symbol if an individual is confirmed to have the genetic condition.
• Sometimes, a “slash” is used to specify whether an individual is already deceased.
• Use an “arrow” for the proband: the individual who first sought medical attention.
• Use a “half-shaded” symbol to denote that an individual is a carrier.
• Use a “horizontal line” to connect the members of a couple.
• Use a “double horizontal line” to connect the members of a couple who are genetically related.
• Use a “common branching point” for the members of twins.
• Connect them if they are monozygotic. Do not connect them if they are dizygotic.
• Use Roman numerals to represent the order of generations.
• Use Arabic digits to number the individuals from left to right every generation.

REMEMBER:

In problems involving pedigree analysis, it is rare that the specific trait or characteristic is provided in the problem. This will somehow defeat the purpose of the analysis.

Cystic fibrosis is an autosomal recessive condition that is characterized by the buildup of mucus in respiratory and digestive organs, which results in various other complications.

Sickle cell anemia is a recessive mutation of the hemoglobin gene. Gas transport is impaired, and the sickle-shaped red blood cells may become stuck in small blood vessels.

SEX LINKAGE AND RECOMBINATION

Replication produces another copy of chromosomes.

Recombination increases genetic diversity. This is the reason why you and your sibling look different even if your genes are inherited from the same parents.

Recombination is a mechanism where the homologous pairs of chromosomes exchange genes. The 1st to 22nd pairs are called the autosomes, and the 23rd pair is termed as the sex chromosome. Genes that go along with either sex chromosome are said to be sex-linked.

Sex Linkage

• Sex linkage is termed as such because their phenotypic manifestation and inheritance patterns vary between males and females. Therefore, genes that go along with either sex chromosome are said to be sex-linked.
• When the trait is linked to the X chromosome, it is called an X-linked trait.
• If the trait is linked to the Y chromosome, it is called a Y-linked trait.

X-linked Trait

• The X-linked trait is more common in males than in females.
• Even if the mother is just a carrier of the trait and the father is normal, there is still a possibility that they will have an offspring with an X-linked trait.
• Generally, males have a 1/2 or 50% chance (50% chance of being normal or 50% chance of possessing the X-linked trait) to express the trait.
• Females only have 1/3 or 33.3% chance (33.3% chance of being normal, 33.3% chance of being a carrier, and 33.3% chance of manifesting the X-linked trait) of acquiring the trait.

Color Blindness Is an X-linked Trait

• Colorblindness is the inability to distinguish certain colors.
• The Ishihara chart is used as a test for color blindness. Hemophilia is an X-linked Trait
• Hemophilia is a rare inherited bleeding disorder.
• People with this condition bleed longer after an injury as compared to a normal person.

Y-linked Trait

• The Y-linked trait is only common in males since only males have the Y chromosome.
• An example is the hypertrichosis pinnae auris trait, which is characterized by having a hairy ear.

Sex-Related Inheritance

As opposed to Mendelian inheritance wherein the heterozygous genotype expresses the dominant trait, the heterozygous genotype of an X-linked trait in females will result in a carrier female.

Sex-influenced Trait

• Sex-influenced traits are controlled by autosomal genes.
• The genes are found on both sexes, but one expresses it more than the other.
• Human pattern baldness is a sex-influenced characteristic.

Sex-limited Trait

• Sex-limited traits are also controlled by autosomal genes.
• The genes are also found on both sexes, but only one sex expresses it.
• Lactation is a female-limited trait.

DNA, RNA, AND PROTEINS

The Discovery of DNA

Friedrich Miescher (1844–1895) isolated the material “nuclein” when studying pus cells from surgical bandages.

• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)

The Role of DNA Molecules

• primary genetic material in almost all living organisms
• capacity to be replicated and inherited during reproduction of cell
• storage of genetic information
• provide avenues for mutation to enhance genetic diversity

Monomers of DNA

The sugar in DNA is a deoxyribose. The nucleotides in DNA are named after the nitrogenous base they have.

The Polynucleotide Chain

The uniqueness of a polynucleotide chain of DNA is found on the sequence of the nitrogenous bases. By convention, it is written in the 5’ to 3’ direction.

• For example, 5’-ATGGCCTTAATGAG-3’ or simply, ATGGCCTTAATGAG

Tautomers of Nitrogenous Bases

The cyclic bases are classified to purines and pyrimidines.

The DNA Helix

• Two strands of DNA polynucleotide chains are bound together via complementary base pairing.
• The two strands of DNA are said to be antiparallel.
• One strand runs in 5′→3′ direction, while the other runs in 3′→5′ direction.

Rosalind Franklin (1920–1958) and her colleague Maurice Wilkins (1916–2004) performed x-ray crystallography technique to analyze the structure of DNA.

Remember the acronyms PuAG (read as poo-wag) and PyCT (read as pee-sit) to remember the classification of bases. PuAG stands for Purine, Adenine, and Guanine, while PyCT stands for Pyrimidine, Cytosine, and Thymine.

The Structure of RNA

The RNA Molecule RNA, a nucleic acid, is known to

• be single-stranded,
• have the base uracil instead of thymine in DNA,
• have ribose instead of deoxyribose in DNA, and

4. a product of transcription of DNA

RNA molecules may also assume enzymatic functions:

• Ribozymes are a group of catalytic RNA molecules.
• RNase P is a ribozyme that catalyzes tRNA synthesis.

The Structure of Proteins

The Amino Acids

Amino acids are the subunits or monomers of proteins. Each amino acid consists of:

• a carboxyl group (COOH)
• an amino group (NH2)
• a proton
• a varying R group

Levels of Protein Structure

Amino acids are organized into polypeptides and proteins via different levels.

• Primary structure
• Secondary structure
• Tertiary structure
• Quaternary structure

Primary Structure

This level is attained by the formation of a peptide bond between each of the amino acids.

Secondary Structure

Secondary protein structures may be in the form of a helix or a pleated sheet.

Tertiary Structure

Tertiary structure consists of polypeptides and both helices and sheets

Quaternary Structure

This level is attained whenever multiple folded protein subunits form a single complex to confer greater functionality.

DNA REPLICATION AND PROTEIN SYNTHESIS

The duplication of the genetic material is a key process in the reproduction of living cells. The Meselson-Stahl experiment proved the semiconservative nature of DNA replication.

CLASSICAL AND MODERN BREEDING TECHNIQUES

Breeding is the controlled process of subjecting two parent organisms to sexual reproduction to produce offspring.

Objectives of Plant Breeding

• Increased yield of vegetative parts and grains
• Improved processing quality
• Improved nutrient quantity
• Stress resistance (salinity, drought, frost)
• Diseases resistance (bacterial, viral, fungal)
• Pest/Insect resistance

Captive Breeding

Objectives of Animal Breeding

• Increased milk production
• Increased egg production
• Improved docility or less aggression
• Improved docility or less aggression
• Improvement in meat quantity and quality
• Improved resistance against diseases
• Good mothering ability
• Increased wool production

Captive Breeding

Objectives of Animal Breeding

• Increased milk production
• Increased egg production
• Improved docility or less aggression
• Improved docility or less aggression
• Improvement in meat quantity and quality
• Improved resistance against diseases
• Good mothering ability
• Increased wool production

Crocodylus mindorensis (Philippine crocodile)

• It is a critically endangered species of freshwater crocodile that was once believed to have been extinct.
• A captive breeding facility for C. mindorensis was established at the Silliman University of Negros Oriental.

Pithecophaga jefferyi (Philippine eagle)

• It is a critically endangered species of eagle that is threatened due to continuous habitat loss.
• A captive breeding facility for P. jefferyi, the Philippine Eagle Center, was established at Davao City.

Historical Progress of Plant Breeding

• First (archeological) evidence of crop domestication was found at Tigris River.
• Camerarius proposed that new plant types can be produced from crossing.
• Fairchild conducted the first reported hybridization (between sweet William and carnation).
• Koelreuter of Germany produced the first hybrid tobacco.
• Mendel laid foundation to heredity of traits through his garden pea hybridization.
• Pioneer Hi-Bred Corn Company became the first seed company in the world.
• During the Green Revolution, Borlaug introduced cultivation practices that boosted global production.

Historical Progress of Animal Breeding

• Sir Robert Bakewell introduced the method of introducing records of the “performance” of livestock.
• Morgan received a Nobel Prize for his work related to the breeding of fruit flies (Drosophila) for genetic studies
• Fisher, the father of Statistics, applied statistical methods in the breeding of mouse and livestock.
• Lush proposed the use of genetic data in breeding rather than subjective evaluation of animal appearance.
• Hazel proposed the selection index theory where multiple traits of an animal must be considered for selection.

Selective Breeding

• Mass selection aims to improve the qualities of a crop by selecting good-quality offspring every generation and allowing them to open pollinate (both self-pollination and cross-pollination are allowed).
• Pure-line selection aims to establish a breed that is homozygous for particular traits; thus, self-pollination is most applicable.
• Clonal selection is applicable for good-quality hybrids that usually cannot reproduce (e.g., seedless); thus, only the vegetative or asexual reproduction of the plant is harnessed.

Crossbreeding

It aims to combine two superior traits from different breeds or species of plants. Sometimes, it also results in eliminating the inferior traits present in the parent plants. This process involves the emasculation of one of the parent plants.

Inbreeding

• Accumulation of superior traits
• Reinforcing of a trait to a group or herd of animals
• Allowing superior male and female offspring to mate
• Mating between related individuals
• Mating between a superior individual and its offspring
• Mating a superior male or female with the opposite sex

Crossbreeding

• Individuals from two different breeds are bred together.
• Desirable traits from different breeds of animals are combined.
• Two individuals must be reproductively compatible.

Radish (Raphanus sativus) Cabbage (Brassica oleracea)

Modern Breeding Methods

Somatic Hybridization

Protoplasts from two different plants with desirable traits are fused.

Mutation Breeding

Crops, such as soybeans, are induced to mutate (e.g., exposure to gamma radiation).

Artificial Reproductive Technologies

Some female cattle are artificially inseminated by using semen collected from superior bulls.

Genetic Engineering

Genetic engineering transcends classical breeding techniques and allows introduction of genes from a totally different organism.

PROCESSES INVOLVED IN GENETIC ENGINEERING

Genetic engineering involves the direct manipulation of genes of organisms in laboratory for them to express the desired traits. Recombinant DNA technology is the primary technique used in the genetic engineering of organisms. The Macedonian Academy of Sciences and Arts has devoted their resources to establish a genetic engineering facility.

Brief History of Genetic Engineering

• Stanley Cohen and Herbert Boyer introduced a plasmid segment into the plasmid of another bacterium.
• With their attempt to perform recombinant DNA technology, Escherichia coli became the first transgenic organism.
• Jaenisch successfully created the first transgenic animal—a mouse.
• The company Genentech was able to produce somatostatin, a human protein, through GM E. coli.
• Richard Flavell (left) and Mary Dell-Chilton (right), alongside Michael Bevan, successfully created the first transgenic crop, a tobacco.