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Introductory Genetics
http://www.stats.gla.ac.uk/~paulj/intro_genetics.ppt
How genes work
What is a gene?
• A gene is a stretch of DNA whose sequence determines
the structure and function of a specific functional
molecule (usually a protein)
DNA
mRNA
Protein
…GAATTCTAATCTCCCTCTC
AACCCTACAGTCACCCATTT
GGTATATTAAAGATGTGTTG
TCTACTGTCTAGTATCC…
Genes are located in the cell nucleus on chromosomes
Karyotype
Down syndrome karyotype (trisomy 21)
DNA
(deoxyribonucleic acid)
mRNA
Protein
Transcription movie
Translation
Translation
Translation
Translation movie
Gene expression movie
Summary
• A gene is a length of DNA that contains instructions for
making a specific protein
• Genes are arranged along 23 pairs of chromosomes in
the cell nucleus
• Genes work by specifying the amino acid sequence of a
protein
Mendel’s laws
Genetic knowledge used for 1000s of years: agriculture
Patterns of disease inheritance known for 1000s of years,
e.g. haemophilia
Mendel deduced the underlying principles of genetics from
these patterns
1. Segregation
2. Dominance
3. Independent assortment
Mendel’s experiments
Mendel’s data
Mendel’s law of segregation
•
•
•
A normal (somatic) cell has two variants (alleles) for a Mendelian
trait.
A gamete (sperm, egg, pollen, ovule) contains one allele,
randomly chosen from the two somatic alleles.
E.g. if you have one allele for brown eyes (B) and one for blue
eyes (b), somatic cells have Bb and each gamete will carry one
of B or b chosen randomly.
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
Mendel’s law of dominance
• If your two alleles are different (heterozygous, e.g. Bb), the trait
associated with only one of these will be visible (dominant) while the
other will be hidden (recessive). E.g. B is dominant, b is recessive.
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
Mendel’s law of dominance
• If your two alleles are different (heterozygous, e.g. Bb), the trait
associated with only one of these will be visible (dominant) while the
other will be hidden (recessive). E.g. B is dominant, b is recessive.
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
Terminology…
• Haploid:
containing one
copy of each
chromosome
(n=23)
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
• Diploid:
containing two
copies of each
chromosome
(2n=46)
Terminology…
• Genotype: the states of the two alleles at one or more locus
associated with a trait
• Phenotype: the state of the observable trait
Genotype
Phenotype
BB (homozygous)
Brown eyes
Bb (heterozygous)
Brown eyes
bb (homozygous)
Blue eyes
Mendel’s law of independent assortment
• Knowledge of which allele has been inherited at one locus gives no
information on the allele has been inherited at the other locus
S/s
SY
25%
Y/y
Sy
sY
sy
25%
25%
25%
Mendel’s law of independent assortment
Gametophytes
(gameteproducing
cells)
S
Y
s
y
Segregation
Gametes
S
Y
s
y
A
b
Recombinants
a
B
Mendel’s law of independent assortment
Gametophytes
(gameteproducing
cells)
S
Y
s
y
Recombination
Segregation
Gametes
S
Y
s
y
S
y
Recombinants
s
Y
Human eye colour
• Simplified view of eye colour inheritance: biallelic
Mendelian trait
– Brown dominant:
BB, Bb
– Blue recessive:
bb
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
Human eye colour
?
Human eye colour
B?
B?
B?
bb
bb
B?
B?
?
Human eye colour
Bb
B?
Bb
bb
bb
B?
Bb
?
Human eye colour
Bb
Bb
B?
Bb
?
Human eye colour
Bb
Bb
P(BB)=1/3
P(Bb)=2/3
?
Bb
Human eye colour
Bb
Bb
P(BB)=1/3
P(Bb)=2/3
P(b)=2/3x1/2=1/3
Bb
P(b)=1/2
?
Human eye colour
Bb
Bb
P(BB)=1/3
P(Bb)=2/3
P(b)=2/3x1/2=1/3
Bb
P(b)=1/2
?
P(bb)=1/3x1/2=1/6
Non-Mendelian inheritance: Haemophilia
• Haemophilia A
• Males with a mutant gene are
affected
• Females with one mutant gene
are unaffected carriers
Non-Mendelian inheritance: additive traits
Brown eye colour is dominant
Dominant vs additive inheritance
Trait value
100%
Dominant
50%
Additive
0%
0
1
Number trait alleles inherited
2
Non-Mendelian inheritance: additive traits
Snapdragon red colour is additive
Dominant vs additive inheritance
Trait value
100%
Dominant
50%
Additive
0%
0
1
Number trait alleles inherited
2
Non-Mendelian inheritance: polygenic traits
Distribution of trait measures for single gene additive trait
0.6
0.5
Frequency
0.4
0.3
0.2
0.1
0
0
1
Trait value
2
Non-Mendelian inheritance: polygenic traits
For example, height