Introduction to Genetics

Introduction

Genetics – This refers to the study of variations (differences between individuals) and how they are inherited from parents to offspring’s

Heredity is the transmission of characteristics of parents to offspring’s

Variation is the differences among living organisms that are caused by differences in their genome

Variation can be categorized as either continuous or discontinuous

Continuous variation – This variation has a wide range of differences from one extreme to another. For example, height from the shortest to the tallest has so many intermediates between two points.

Discontinuous variation – This variation has definite distinct groups of individuals and no intermediate forms exist. Example, one is either male or female and no intermediate between the two

Causes of variations

Gamete formation

1. Crossing over
2. This occurs during prophase I of meiosis I, whereby the exchange of chromatids segments occurs at the chiasmata. 2. Independent assortment

The homologous chromosomes are arranged randomly at the equator of the spindle during the metaphase of the first meiotic division. This means that the separated homologous chromosomes can go into different daughter cells which cause variation.

Fertilization

Fertilization permits variation by combining parental genes randomly.

Mutation

A mutation refers to the spontaneous change in the genetic make-up of an organism. Mutations may be inherited or triggered by mutagens

The chromosome

• The chromosome is a thread-like structure that is found within the nucleus of the cell
• Each chromosome is made up of two chromatids, both connected by a centromere
• Body cells are referred to as somatic cells
• Homologous chromosomes are a pair of chromosomes that share the same appearance
• During sexual reproduction, each parent contributes one of the chromosomes in a homologous pair
• Genes encode proteins within the chromosome
• Chromosomes are located in the nucleus and appear only during cell division as viewed under a microscope

Genes and DNA

• Genes – The hereditary material in homologous chromosomes
• Gene loci – This is a definite location on the chromosome where each gene is located
• The first scientists to discover the double helix structure of DNA were James Watson and Francis Crick.
• DNA contains three components: A five-carbon sugar (pentose), a nitrogenous base, and a phosphate backbone.
• There are four types of bases in DNA – Adenine, Guanine, Cytosine, and Thymine
• The bases in RNA – Adenine, Guanine, Cytosine, and Uracil
• When a nitrogenous base, five-carbon sugar and phosphate combine, they form a nucleotide
• In the double helix structure of DNA, Adenine pairs with Thymine, and Guanine pairs with Cytosine
• The phosphate is joined to the sugar molecule using a phosphodiester bond
• The nitrogenous base is joined to the sugar molecule using a weak hydrogen bond
• The DNA is a twisted ladder, with the nitrogenous base forming the rungs or steps (inside of the double helix).

Roles or functions of DNA

• Stores genetic information in nucleotides (the basic unit of DNA molecule)
• Enables transfer of genetic information to daughter cells through replication
• Enables translation of the genetic material to an amino acid which forms protein in the ribosomes

DNA replication

Role of DNA in protein synthesis

• DNA is responsible for protein synthesis inside the cell
• Protein synthesis begins when DNA is used to synthesize a messenger RNA (mRNA) by a process called transcription, then the mRNA is used to synthesize amino acids inside the ribosomes through a process namely translation
• The sequence of bases in the DNA determines which amino acid is added
• Every three bases make up a codon, which in turn encodes for a specific amino acid
• Replication takes place inside the nucleus
• Transcription takes place inside the nucleus as well, forming mRNA

Messenger RNA (mRNA) formed during transcription is transported through the nuclear membrane to the cytoplasm and specifically into the ribosomes

Inside the ribosomes, protein synthesis takes place, a process called translation

The difference between DNA and mRNA is that mRNA is single-stranded while DNA is usually double-stranded. mRNA contains bases A, U, G, and C while DNA lacks Uracil hence contains A, G, C, and T.

Transfer RNA (tRNA) is found in the ribosomes and is responsible for adding amino acids on the growing protein-peptide

Protein molecules determine the inherited characteristics in organisms

The first law of thermodynamics

• Characteristics of an organism are determined by hereditary factors that occur in pairs and only one of a pair is represented in a single gamete
• Why Mendel experiment was successful
• Used favorable materials like a self-fertilized pea plant
• Pea plant used had several observable contrasting characteristics
• His study focused on specific traits rather than the whole heredity of each organism
• He kept accurate data from his experiments and was able to formulate a definite hypothesis

Monohybrid inheritance

• According to Mendel’s Experiments, some characteristics are controlled by a single pair of heredity factors contributed from both parents
• An allele is an alternative form of the same gene that controls the development of a pair of contrasting trait
• Allelic pairs describe the two pairs of an allele in a gene

Genotype – This is the genetic constitution of an organism

Phenotype – This is the observable characteristics of an organism which expresses the genotype of an individual

When representing genotype using letters, capital letters are used to show the dominant allele, while small letters are used to show the recessive allele

F1 (First filial generation) – This is the first generation of a cross between two genotypes

F2 (Second filial generation) – This is formed by a cross between F1 population

Complete dominance

• There is a dominant gene and a recessive gene
• The dominant gene expresses itself in both homozygous and heterozygous
• The recessive gene expresses itself only in the homozygous state
• When the dominant gene completely masks the expression of the recessive (other) gene in the heterozygous state, this condition is called complete dominance.
• When the allelic genes are identical e.g. TT or tt, this is described as homozygous and an individual with the homozygous condition is called a homozygote
• When allelic genes are not identical e.g., this is called the heterozygous state and an individual with such genotypic condition is called heterozygote.
• Mendel’s First law states that: Characteristics of an organism are determined by internal factors that occur in pairs and only one of a pair of such factors can be represented in a single gamete.

Revision

dominant allele
an allele that produces the same phenotype whether its paired allele is identical or different (capital letter)

recessive allele
an allele that produces its characteristic phenotype only when its paired allele is identical (lowercase letter)

genotype
the combination of alleles located on homologous chromosomes that determines a specific characteristic or trait (the allelic combination such as Bb)

phenotype
the observable physical or biochemical characteristics of an organism, as determined by the genotype (the expressed trait such as brown eyes)

homozygous
term used to refer to an organism that has two identical alleles for the same trait (ex. BB or bb)

heterozygous
term used to refer to an organism that has two different alleles for the same trait (ex. Bb)

Punnett square
diagram showing the gene combinations that might result from a genetic cross

gamete (sex cell)
specialized cell involved in sexual reproduction (sperm or egg)

probability
the possibility of different outcomes (percentage or ratio)

monohybrid cross
a one-trait cross (ex. color)

dihybrid cross
a two-trait cross (ex. color & shape)