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Introductory Genetics
http://www.stats.gla.ac.uk/~paulj/intro_genetics.ppt
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…
mRNA
DNA
RNA
(deoxyribonucleic acid)
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
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
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%
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
Non-Mendelian inheritance: Haemophilia
• Haemophilia A
• Males with a mutant gene are
affected
• Females with one mutant gene
are unaffected carriers
Non-Mendelian inheritance: Incomplete dominate 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