GENETICS

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HEREDITY - TERMS

• GENETICS
• Study of inheritance the way in which
  information that determines characteristics are
  transmitted from parent to offspring.
• CHARACTERISTIC /TRAIT
• Distinct structural or functional feature of a
  organism.
• Characteristics inherited from parents or passed
  from parents to young.

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CHROMOSOMES

• Inheritable materials in nucleus of a cell.
• Numerous genes are located on it.
• Each gene is a small segment of DNA where a piece
  of genetic information is stored.
• Chromosomes can exist in coiled compact form or
  uncoiled extended form.
• Cells undergoing cell division will contain
  chromosomes in coiled compact form. They appear
  dark after staining, like X- shaped bodies.

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CHROMATIN THREADS

• Long thread-like structures in nucleus when cell
  is NOT undergoing cell division.
• Actually uncoiled and extended DNA molecules.

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DNA (DEOXYRIBONUCLEIC ACID)

• Molecules that contain genetic code for formation
  of all types of proteins for normal functioning
  of our body.
• Each body cell contains a fixed no. of
  chromosomes in its nucleus.
• Total no. of chromosomes is an EVEN number. Cell
  with full even number of chromosomes is called a
  diploid cell (2n).
• Some examples
• Apple tree (34), bat (44), cat (38), dog (78),
  chicken (78), housefly (12), maize plant (40),
  mouse (40), pea plant (14) and humans (46).

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DNA

• During meiosis (cell division in gonads), haploid
  gamete (n) cells with only half the number of
  chromosomes are produced.
• 2 haploid gamete cells fuse during fertilization,
  a diploid zygote (2n) cell is produced.

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EXERCISE on chromosome

• Write down the no. of chromosomes in the
  following structures
• Structure Chromosome no. 2n / n
• Human zygote 46 2n
• Cat sperm cell 19 n
• Apple ovule 17 n
• Bat wing muscle cell 44 2n
• Housefly egg 6 n
• Pea plant leaf cell 14 2n
• Maize pollen grain 20 n
• Human sperm cell 23 n

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HOMOLOGOUS CHROMOSOMES

• In diploid cell, all chromosomes exist in
  matching pairs HOMOLOGOUS PAIRS or HOMOLOGUES
• Human body cell has 46 chromosomes 23 pairs of
  homologous chromosomes.
• 2 members of the chromosome pair are exactly
  alike in shape and size/length (except sex
  chromosomes).
• 1 chromosome comes from male parent and the other
  comes from female parent.
• During meiosis, the pairs of homologues will be
  separated and only one partner of the pair will
  go into a gamete (random manner).

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HOMOLOGOUS CHROMOSOMES
centromere
chromatid
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GENE

• A segment on the chromosome that controls one
  characteristic.
• Each chromosome contains numerous genes arranged
  in a specific position and sequence along its
  length.
• Chromosomes occur in pairs, genes are also
  present in pairs.
• 1 characteristic may be controlled by a pair of
  genes or by several pairs of genes.

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GENE LOCUS

• Position or location on a chromosome where a gene
  is found.
• Genes on homologous chromosomes in each gene
  locus will have 2 genes (a pair) controlling the
  same characteristic.

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ALLELES

• Alternative/different forms of the same gene.
• 2 or more forms of a gene which control the same
  characteristic by producing different effects.
• Occupy same relative gene locus on a pair of
  homologous chromosomes.
• Dominant allele Recessive allele
• Allele controls phenotype Allele controls
• when it is present in EITHER phenotype only
• a single or double number when it is present
• in DOUBLE numbers
• Represented by CAPITAL Represented by SMALL
• letters letters

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PHENOTYPE GENOTYPE

• GENOTYPE
• Genetic combination of an expressed
  characteristic
• Genetic make-up of an organism
• PHENOTYPE
• Refers to the visible expression of hereditary
  characteristic
• A pair of alleles control a phenotype.
• Example
• Phenotype for height of pea plant, let the
  dominant allele for tallness be represented by T
  and recessive allele for dwarfness be represented
  by t.
• Usually denoted by capital or small letters, e.g.
  TT, Tt, tt

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GENOTYPE

• Combination of alleles Genotype
• 2 dominant alleles TT
• 1 dominant and 1 recessive Tt
• 2 recessive alleles tt
• Description of genotype Phenotype
• Homozygous dominant Tall
• Heterozygous Tall
• Homozygous recessive Dwarf

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Exercise Genotype (ref)

• Dominant/ Recessive allele Characteristic
• F Short finger length
• r Non-tongue roller
• A Normal skin colour
• D Curly hair (homozygous)
• B Black hair (heterozygous)
• E Unattached ears (free)
• b Blue eyes
• d Straight hair
• B Brown eyes

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Exercise - Genotype

• Genotype Phenotype
• FF, Ff Short fingers
• rr Unable to roll tongue
• AA, Aa Normal skin colour
• DD Curly hair (homozygous)
• Bb Black hair (heterozygous)
• Ee Unattached ears (heterozygous)
• bb Blue eyes
• dd Straight hair
• BB, Bb Brown eyes

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TERMS

• Homozygous
• Organism with identical alleles for a particular
  phenotype
• 2 alleles at a locus are IDENTICAL e.g. TT or tt
• True-breeding or pure-bred or pure-line
• Homozygous dominant or homozygous recessive
• Dominant allele shows its effect in both
  homozygous (TT) and heterozygous (Tt) states e.g.
  in the phenotype for height (tall)
• Always express themselves as a characteristic.
• Recessive allele has no effect on phenotype
  unless it is homozygous recessive e.g. tt short.

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TERMS

• Heterozygous
• Organism with different alleles for a particular
  phenotype.
• 2 alleles at a locus are different
• It has 1 dominant allele and 1 recessive allele.
• Heterozygous dominant
• Normally known as a hybrid if offspring arise
  from 2 different pure-line of organisms
• Offspring
• Resulting individuals of a crossing or mating

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TERMS

• Complete dominance
• Co-dominance when both alleles express
  themselves (50-50)
• Example human blood groups
• Multiple alleles IA, IB , IO
• IA, IB are dominant alleles IO is recessive
• IA IB (co-dominant)
• Gene status Blood grp (phenotype) Genotype
• IA A IAIA IAIO
• IB B IBIB IBIO
• IO O IOIO
• IA and IB AB IAIB

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TERMS

• Incomplete dominance not the same as
  co-dominance
• 2 alleles do not express themselves equally
• e.g. sickle-cell anaemia 3 phenotypes
• Normal (none) HNHN
• Very slight HNHS
• Severe - HSHS

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TERMS

• Test Cross
• Unknown genotype
• homozygous or heterozygous dominant???
• Crossing or mating of an organism of unknown
  genotype with homozygous recessive organism.
• Carried out for purpose of finding out genotype
  of an organism whose phenotype is controlled by a
  dominant gene.

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DNADeoxyribonucleic acid
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DNA

• Chromosome is made up of a molecule of DNA
  wrapped around protein bundles.

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DNA
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DNA

• Double helix molecule
• 2 linear strands run parallel and twisted around
  each other
• Long, twisted, spiral ladder
• Basic unit Nucleotide
• Nucleotide
• Deoxyribose sugar,
• Phosphate group
• Nitrogenous bases A, T, G, C
• A adenine G guanine
• T thymine C - cytosine

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"the sugar-phosphate backbone"
The backbone of the DNA molecule (coloured pink
in this picture) are made up of a chain of sugar
molecules, each bearing a phosphate group.
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Nucleotide
Key
sugar-phosphate backbone"
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• Nucleotide
• 1 base (A, G, T or C)
• 1 phosphate
• 1 sugar (a ribose which lacks an O deoxy-ribose)

H
H
H
H
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Complementary Base Pairing A base is one of the
smallest units of genetic information inside a
gene. 4 bases adenine (A), guanine (G),
cytosine (C), and thymine (T). They are arranged
in pairs. T always pairs with A. C always pairs
with G.
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• Hydrogen bonding
• between bases
• in opposite DNA strands
• Sequence of bases
• Types of amino acids
• Genetic code

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DNA replication the making of self-copy to
maintain and propagate genetic entities ?
Semi-conservative
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DNA Genes -- Proteins

• Written in the DNA are 30,000-40,000 genes which
  human cells use as templates to make proteins.
• DNA controls production of proteins
• 1 gene ? 1 protein/ enzyme
• 1 enzyme controls a chemical change
• 1 or more chemical change helps to determine a
  characteristic
• In some characteristics, environment influences
  expression of the genes.

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VARIATION

• Differences that are observed among individuals
  within a species of organism.
• E.g. in humans, in plants we have differences in
  flower colour, height and fruit size.
• 2 types of variations
• - Continuous variation
• - Discontinuous variation.

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CONTINUOUS VARIATION

• The features or characteristics vary gradually
  from one extreme to the other.
• Shows a gradual change between 2 extreme forms.
• Affected by environmental factors.
• Examples
• Human height (range of heights between tallest
  and shortest humans).
• Skin colour (range of pigmentation between
  darkest and fairest - skin colour of a naturally
  fair person could be tanned as a result of
  exposure to the sun or height can be stunted due
  to malnutrition.
• Palm span

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DISCONTINUOUS VARIATION

• Features or characteristics show no in-between
  (intermediate) forms.
• A distinct difference exists between 2 extreme
  forms with no gradual changes between the 2
  extremes.
• Under natural conditions characteristics in this
  type of variation are normally not affected by
  environmental factors or conditions
• Examples
• Handedness (person is born either left or
  right-handed)
• Ability to roll tongue (either a roller or
  non-roller)
• Blood grouping (either A, B, O or AB)