Genes:
What are they, how they work,
and how are they inherited?
Lecture 3
Dr. Maharaj/Dr Gupta
What is Heredity?
Heredity = the passing on of traits from one
generation to the next.
Mendel the Father of Genetics and his
work
• Inheritance of traits – continuity of life forms
(genetic continuity)
• Yet parents and offspring may not look exactly
the same
• Current knowledge- chromosomes; genes, units
of inheritance; alleles; reductional division
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Gregor Johann Mendel (1822-1884)
•First person to formulate the principles
of heredity
•As a young man worked as a gardener
and attended Philosophical Institute of
Olomouc.
•1843 Entered Augustinian Abbey of St
Thomas in Brno (now Czech republic)
•1847 Ordained as a priest
•1851-1853 University of Vienna
•1853-1865 Experiments with pea crosses
in monastery garden
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1865 His paper “Experiments on Plant Hybridization” presented at
the meeting of The Natural History Society of Brünn and published
a year later (1866) in The Proceedings of The Natural History
Society of Brünn.
No one realizes the significance of his findings until 1900.
St Thomas Abbey, Brno
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Before Mendel
It was known that plants
have male and female
organs and that flowers
need to be fertilized to
develop seed.
Many domesticated plant
and animal varieties were
bred to produce desirable
traits, but no one did
careful analysis of how
those traits were passed on
from generation to
generation.
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Mendel’s experimental system
• Garden Peas:
- many varieties
- self-fertilizing (subject to
experimental cross-fertilisation)
- produce lots of seed
Simple, elegant experiments: careful choice of sharply
contrasting traits (pairs)
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Mendel’s approach
• Make sure that the varieties used were true
breeding (true strains) by selfing them for several
generations.
• True breeding means that offspring will always
exhibit same trait as parent, i.e. parent is
homozygous for the trait.
• Use of characters that have only 2 discrete
alternative traits. Traits did not exhibit continuous
variation.
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The experiment
• Cross plants (P) exhibiting different traits
(Cross between two parents with contrasting
traits)
• Follow the trait in offspring (F1)
• Self F1 offspring and observe the trait in next
generation F2.
• Count and analyze data using mathematics.
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What is a Trait?
A trait is a specific characteristic that is unique.
Traits affect the way we look
Traits affect how our bodies function
Traits are inherited
• Examples are hair color, eye color, handedness, etc.
A gene is a sequence of DNA specifying the
sequence of amino acids of a particular protein
involved in the expression of a trait.
Different forms of the same gene are called alleles. Different alleles
produce variations in inherited characteristics (traits).
• Modern understanding of these
experiments in terms of (homologous)
chromosomes, alternative forms of
genes for the same trait (alleles),
position or location (loci) of genes on
chromosomes; and meiosis.
• Unknown to Mendel. He used term
“factor”
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• Homozygotes – in a zygote, paired
factors (genes) alike on homologous
chromosomes (T/T).
• Heterozygotes– paired factors
(genes) contrasting or different on
homologous chromosomes (T/t).
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1. Monohybrid cross
Cross between
homozygous
parents that differ
at one locus (a cross
involving only one pair of
contrasting traits)
Dominant
& Recessive
First generation
offspring (F1) will be
hybrid for one
character.
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P Generation
Cross or reciprocal cross
gives the same results
F1 Generation
Tall plant
T
T
Short plant
t
Selfing of tall
(F1)
t
F2 Generation
All tall plants
T t
No blending of traits
Tall plant
T
Dominant
& Recessive
T
Tall plant
T
t
Tall plant
T
t
Short plant
t
3 tall : 1 short
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t
Character pairs
selected by Mendel
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In F1 generation all plants exhibited one trait –
called dominant.
In F2 generation the ratio between plants
exhibiting dominant and recessive traits was
approximately 3:1.
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Interpretation of results
1
2
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(Punnett square)
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Conclusion
1st Principal of Mendelian inheritance –
Segregation
“in the formation of gametes, paired factors
(heredity determinants) specifying alternative
phenotypes (visible traits) segregate
independently of one another”
Phenotypes vs Genotypes
(During meiosis, alleles for each locus
segregate in gametes.)
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Dominant trait denoted by a capital letter
and the corresponding recessive by a
lowercase.
T/T (Homo.), T/t (Hetero), t/t (Homo.),
(on homologs chromosomes)
F2 generation particularly clearly
illustrates the ‘segregation’ that forms
the basis of this law.
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• Mendel further self-fertilised plants from F2
generation and obtained pure and hybrid plants.
• Clear indication that no blend; factors passed on
as units: in pure breeding both alike and in a
hybrid, different; individual germ cell or gamete
always pure with respect to paired factors
(alleles).
• Thus Mendel’s quantitative approach to
inheritance (particulate inheritance), a great
contribution to beginnings of Genetics.
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Testcross
• Phenotypes (with dominant trait visible)
were crossed with pure recessives to
determine whether hetero- or
homozygotes.
• Testcross still applied to assess the
genotype.
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Intermediate Inheritance
Phenotype of heterozygote falls
between these two homozygotes
– Neither allele is
completely dominant
over the other
– The heterozygous
phenotype is distinct
from those of the
parents, often
intermediate between
them
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Second experiment
Dihybrid cross:
Parental varieties differing in two
traits (at two loci)
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Cross plants with different seed phenotypes
Yellow – Round
X Green Wrinkled
Monohybrid ratio
for each trait= 3:1
3:1x3:1= 9:3:3:1
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Conclusion
Mendel’s 2nd law:
‘Independent assortment’
“factors (genes) located on different pairs of
homologous chromosomes assort
independently during meiosis”
(Alleles of different loci distributed randomly
into the gametes independently of each
other.)
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Mendelian inheritance in modern
terms
Factors (heredity
determinants)?
Traits?
Today we know that
the gene loci for some
of these traits are
located on different
chromosomes and are
not linked.
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Crosses Mendel performed
•
Monohybrid cross
– Cross between homozygous parents
– Differ at one locus
• Dihybrid cross
– Differ at two loci
Test cross
• Between individual of unknown
genotype and homozygous recessive
individual
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Monohybrid
cross in
guinea pigs
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Dihybrid
cross
in
guinea pigs
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Test cross in
guinea pigs
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• Mendel’s principles of inheritance
– Segregation
•During meiosis, alleles for each locus
segregate
– Independent assortment
•Alleles of different loci distributed
randomly into the gametes
•Results in recombination
•Production of new gene combinations
not found in parent
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Application of Mendel’s Principles
• Many characters in humans, animals and
plants are inherited in Mendelian
fashion.
• Useful in plant and animal breeding.
• Many human genetic disorders behave
like Mendelian characters and can be
traced through pedigree analysis.
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What is a Pedigree?
A pedigree is a diagram that shows the history of a trait
as it is passed from one generation to the next.
Pedigrees indicate patterns
Pedigrees identify carriers of genetic disorders
Pedigrees are useful for genetic counseling
• Example: a pedigree is like a family tree for one trait.
Rules For Making A Pedigree
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Females are represented by circles
Males are represented by squares
Mother/Father couples are connected by a line
Offspring are shown oldest on the left to youngest on the
right
Half-shaded circle represents a female carrier for the trait
Half-shaded square represents a male carrier for the trait
Full-shaded circle represents a female with the trait
Full-shaded square represents a male with the trait
Symbols used in a pedigree
recorded ancestry or lineage of a person or family.
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Pedigree showing pattern of inheritance of a
recessive trait
Here affected individuals are not found in each generation.
The affected offspring comes from unaffected parents.
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Pedigree showing pattern of inheritance of a
dominant trait
Every affected child has an affected parent — no
generations are skipped in dominant inheritance.
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Pedigree
for albinism
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Human diseases that are inherited like
Mendelian characters
Recessive Disorders:
1) Hemophilia- blood clotting problems, due to
sex-linked recessive allele
2) Sickle-cell anemia - lack proper blood
proteins and RBC's are sickle shaped versus normal,
unable to carry oxygen efficiently, allele causes a
simple substitution
3) Phenylketonuria - unable to produce
enzymes to breakdown chemicals in diet, can lead
to toxic buildup and death
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Dominant disorders
• Huntington's Disease - deterioration of
nervous system
• Achondroplastic dwarfism - small
stature, disproportionate limbs, short
broad hands and feet, waddling gait,
affects bone growth beginning at
birth
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Hemophilia in the pedigree of Queen Victoria of
England
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