GENETICS:
A branch of biology that studies heredity and variation in organisms. It tries to explain both the
similarities and differences between parents and their offspring.
Heredity = passing on of traits/ characteristics from one generation to the next.
Terminology:
▪ Genetics: a branch of biology that studies hereditary and variation.
▪ Genome: a complete set of genetic instructions (genes) necessary to create an organism.
▪ Gene: a section of DNA that controls a hereditary trait.
▪ Chromosome: consists of many genes which can range from several hundred to several
thousand.
▪ Gene mutation: sudden and unpredictable change in the chemical structure of a gene.
▪ Karyotype: the characteristics of a set of chromosomes (size, shape, number etc.) of a typical
somatic cell of a given species, individual etc.
▪ Allele: alternative version of a gene found in the same locus on homologous chromosomes.
▪ Locus: position of a gene on a chromosome.
▪ Somatic cell: normal body cell (diploid).
▪ Gamete: haploid sex cell.
▪ Autosome: any chromosome that is not a sex chromosome.
▪ Gonosome: sex chromosome (XX in female, XY in male).
▪ Phenotype: the physical appearance of an organism.
▪ Genotype: genetic makeup of an organism.
▪ Homozygous: if the pair of alleles at a locus are the same e.g: BB.
▪ Heterozygous: if the pair of alleles at a locus are different (hybrid) e.g. Bb.
▪ Dominant: the trait that is expressed in the offspring.
▪ Recessive: the trait that is suppressed in the presence of the dominant allele and is not
expressed in the offspring.
▪ Filial: offspring. F1 is first generation offspring, F2 is second generation offspring.
▪ Monohybrid cross: genetic cross involving only one characteristic e.g. coat colour.
▪ Punnet square: a table with columns and rows which is used to calculate the filial genotype.
▪ Test cross / back cross: used to determine an unknown genotype.
▪ Traits: characteristics.
▪ Pure bred: homozygous for that trait.
▪ Principle of segregation: characteristics of an organism which are controlled by a pair of
alleles separate when gametes are formed in meiosis i.e. each gamete contains one pair of
alleles (a principal Mendel deduced from his experiments).
Genes and Alleles:
Genes:
▪ Each chromatid is made up of one helical DNA molecule. Each DNA molecule is made up of a
series of genes.
▪ A gene can be defined as a section of DNA (series of nucleotides/bases) that controls a
hereditary characteristic (trait). ie: it is the basic unit of heredity living organisms.
• Each chromosome has between several hundred and several thousand genes. The
total number of genes in humans is thought to be between 20,000 and 25,000.
• Nearly all somatic cells have an exact copy of all the genes in that organism.
• Because there are two of each kind of chromosome (paternal and maternal), each
cell contains two of each kind of gene. These versions of a gene are known as
alleles.
, • The gene pool is the set of all genes, or genetic information, in a population of
sexually reproducing organisms.
▪ A large gene pool: indicates high genetic diversity and increased chances of
survival.
▪ A small gene pool: indicates low genetic diversity and increased possibility of
extinction.
Alleles:
One of two or more forms of a gene.
• Alleles are genes responsible for controlling different versions of a trait/characteristic
found in the same lotus (position) on homologous chromosomes.
• How are alleles represented?
• When describing characteristics, alleles are presented by means of letters.
• Capital letter – dominant allele.
• Lower case of the same letter – recessive allele.
• How are alleles passed from parent to offspring?
• By way of chromosomes in the gametes that are made by the process of meiosis in
the sex organs.
Normal body cells (somatic cells) are diploid (2n) as they have:
➢ A pair of homologous chromosomes.
➢ Two alleles which may be the same or different.
Sex cells (gametes) are haploid (n) as they have:
➢ One pair of homologous chromosomes.
➢ One of each pair of alleles.
What happens to the gametes?
➢ During fertilization, a male gamete (with its alleles)
fuses with a female gamete (with its alleles), to form a
diploid zygote.
➢ This divides repeatedly by mitosis to form an entire
new organism with the same set of chromosomes and
alleles as in the zygote.
Gregor Mendel: “The Father of Genetics”:
▪ Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor
Mendel documented inheritance in peas.
▪ Used experimental method.
▪ Used quantitative analysis.
▪ Collected data & counted them.
▪ Excellent example of scientific method.
▪ Gregor Mendel proposed the mechanism of inheritance after this scientific process of
meticulous experimentation using large numbers of pea plants, observation, further
methodical experimentation, statistical analysis, and deduction.
Mendel made a good choice of pea plants to study – why?
▪ Pea plants are good for genetic research.
▪ Relatively simple genetically.
▪ Most characters are controlled by a single gene.
, ▪ Each gene has only 2 alleles, one of which is completely dominant over the
other.
▪ Available in many varieties with distinct heritable features with different variations.
▪ Flower colour, seed color, seed shape, etc.
▪ Mendel had strict control over which plants mated with which.
▪ Each pea plant has male & female structures.
▪ Pea plants can self-fertilize.
▪ Mendel could also cross-pollinate plants: moving pollen from one plant to another.
Mendel’s work:
▪ Bred pea plants:
▪ Cross-pollinated true breeding parents(P)
▪ Raised seed & then observed traits (F1)
▪ Filial
▪ Allowed offspring to cross-pollinate & observed next generation (F2)
▪ For 7 years between 1856 and 1863, Mendel
crossed and produced about 28 000 pea plants.
Did Mendel share his findings with other scientists?
▪ Mendel read his paper at two meetings of the
Natural History Society of Brunn in Bohemia in
1865.
▪ It was published in 1866 and circulated to many
academic institutions.
▪ However, the significance of Mendel's work was
not recognized and validated until scientists
rediscovered his paper and replicated his
experiments in the early 1900s.
▪ Mendel's work was not accepted by most scientists when he was alive for three main
reasons:
▪ When he presented his work to other scientists, he did not communicate it well, so
they did not really understand it.
▪ It was published in a scientific journal that was not well known.
▪ He could not explain the science behind why characteristics were inherited.
What did Mendel’s findings mean?
▪ Traits come in alternative versions:
▪ purple vs. white flower color
▪ alleles
▪ different alleles vary in the sequence of nucleotides at the specific locus of a
gene.
▪ Traits are inherited as discrete units:
▪ For each characteristic, an organism inherits 2 alleles, 1 from each parent.
▪ Diploid organism
• Inherits 2 sets of chromosomes, 1 from each parent.
• Homologous chromosomes.
• Like having 2 editions of encyclopedia:
o Encyclopedia Britannica.
o Encyclopedia Americana.
A branch of biology that studies heredity and variation in organisms. It tries to explain both the
similarities and differences between parents and their offspring.
Heredity = passing on of traits/ characteristics from one generation to the next.
Terminology:
▪ Genetics: a branch of biology that studies hereditary and variation.
▪ Genome: a complete set of genetic instructions (genes) necessary to create an organism.
▪ Gene: a section of DNA that controls a hereditary trait.
▪ Chromosome: consists of many genes which can range from several hundred to several
thousand.
▪ Gene mutation: sudden and unpredictable change in the chemical structure of a gene.
▪ Karyotype: the characteristics of a set of chromosomes (size, shape, number etc.) of a typical
somatic cell of a given species, individual etc.
▪ Allele: alternative version of a gene found in the same locus on homologous chromosomes.
▪ Locus: position of a gene on a chromosome.
▪ Somatic cell: normal body cell (diploid).
▪ Gamete: haploid sex cell.
▪ Autosome: any chromosome that is not a sex chromosome.
▪ Gonosome: sex chromosome (XX in female, XY in male).
▪ Phenotype: the physical appearance of an organism.
▪ Genotype: genetic makeup of an organism.
▪ Homozygous: if the pair of alleles at a locus are the same e.g: BB.
▪ Heterozygous: if the pair of alleles at a locus are different (hybrid) e.g. Bb.
▪ Dominant: the trait that is expressed in the offspring.
▪ Recessive: the trait that is suppressed in the presence of the dominant allele and is not
expressed in the offspring.
▪ Filial: offspring. F1 is first generation offspring, F2 is second generation offspring.
▪ Monohybrid cross: genetic cross involving only one characteristic e.g. coat colour.
▪ Punnet square: a table with columns and rows which is used to calculate the filial genotype.
▪ Test cross / back cross: used to determine an unknown genotype.
▪ Traits: characteristics.
▪ Pure bred: homozygous for that trait.
▪ Principle of segregation: characteristics of an organism which are controlled by a pair of
alleles separate when gametes are formed in meiosis i.e. each gamete contains one pair of
alleles (a principal Mendel deduced from his experiments).
Genes and Alleles:
Genes:
▪ Each chromatid is made up of one helical DNA molecule. Each DNA molecule is made up of a
series of genes.
▪ A gene can be defined as a section of DNA (series of nucleotides/bases) that controls a
hereditary characteristic (trait). ie: it is the basic unit of heredity living organisms.
• Each chromosome has between several hundred and several thousand genes. The
total number of genes in humans is thought to be between 20,000 and 25,000.
• Nearly all somatic cells have an exact copy of all the genes in that organism.
• Because there are two of each kind of chromosome (paternal and maternal), each
cell contains two of each kind of gene. These versions of a gene are known as
alleles.
, • The gene pool is the set of all genes, or genetic information, in a population of
sexually reproducing organisms.
▪ A large gene pool: indicates high genetic diversity and increased chances of
survival.
▪ A small gene pool: indicates low genetic diversity and increased possibility of
extinction.
Alleles:
One of two or more forms of a gene.
• Alleles are genes responsible for controlling different versions of a trait/characteristic
found in the same lotus (position) on homologous chromosomes.
• How are alleles represented?
• When describing characteristics, alleles are presented by means of letters.
• Capital letter – dominant allele.
• Lower case of the same letter – recessive allele.
• How are alleles passed from parent to offspring?
• By way of chromosomes in the gametes that are made by the process of meiosis in
the sex organs.
Normal body cells (somatic cells) are diploid (2n) as they have:
➢ A pair of homologous chromosomes.
➢ Two alleles which may be the same or different.
Sex cells (gametes) are haploid (n) as they have:
➢ One pair of homologous chromosomes.
➢ One of each pair of alleles.
What happens to the gametes?
➢ During fertilization, a male gamete (with its alleles)
fuses with a female gamete (with its alleles), to form a
diploid zygote.
➢ This divides repeatedly by mitosis to form an entire
new organism with the same set of chromosomes and
alleles as in the zygote.
Gregor Mendel: “The Father of Genetics”:
▪ Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor
Mendel documented inheritance in peas.
▪ Used experimental method.
▪ Used quantitative analysis.
▪ Collected data & counted them.
▪ Excellent example of scientific method.
▪ Gregor Mendel proposed the mechanism of inheritance after this scientific process of
meticulous experimentation using large numbers of pea plants, observation, further
methodical experimentation, statistical analysis, and deduction.
Mendel made a good choice of pea plants to study – why?
▪ Pea plants are good for genetic research.
▪ Relatively simple genetically.
▪ Most characters are controlled by a single gene.
, ▪ Each gene has only 2 alleles, one of which is completely dominant over the
other.
▪ Available in many varieties with distinct heritable features with different variations.
▪ Flower colour, seed color, seed shape, etc.
▪ Mendel had strict control over which plants mated with which.
▪ Each pea plant has male & female structures.
▪ Pea plants can self-fertilize.
▪ Mendel could also cross-pollinate plants: moving pollen from one plant to another.
Mendel’s work:
▪ Bred pea plants:
▪ Cross-pollinated true breeding parents(P)
▪ Raised seed & then observed traits (F1)
▪ Filial
▪ Allowed offspring to cross-pollinate & observed next generation (F2)
▪ For 7 years between 1856 and 1863, Mendel
crossed and produced about 28 000 pea plants.
Did Mendel share his findings with other scientists?
▪ Mendel read his paper at two meetings of the
Natural History Society of Brunn in Bohemia in
1865.
▪ It was published in 1866 and circulated to many
academic institutions.
▪ However, the significance of Mendel's work was
not recognized and validated until scientists
rediscovered his paper and replicated his
experiments in the early 1900s.
▪ Mendel's work was not accepted by most scientists when he was alive for three main
reasons:
▪ When he presented his work to other scientists, he did not communicate it well, so
they did not really understand it.
▪ It was published in a scientific journal that was not well known.
▪ He could not explain the science behind why characteristics were inherited.
What did Mendel’s findings mean?
▪ Traits come in alternative versions:
▪ purple vs. white flower color
▪ alleles
▪ different alleles vary in the sequence of nucleotides at the specific locus of a
gene.
▪ Traits are inherited as discrete units:
▪ For each characteristic, an organism inherits 2 alleles, 1 from each parent.
▪ Diploid organism
• Inherits 2 sets of chromosomes, 1 from each parent.
• Homologous chromosomes.
• Like having 2 editions of encyclopedia:
o Encyclopedia Britannica.
o Encyclopedia Americana.
