Introductory Genetics. What is a gene? A gene is a stretch of DNA whose sequence determines the...
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Transcript of Introductory Genetics. What is a gene? A gene is a stretch of DNA whose sequence determines the...
Introductory Genetics
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
Protein
…GAATTCTAATCTCCCTCTCAACCCTACAGTCACCCATTTGGTATATTAAAGATGTGTTGTCTACTGTCTAGTATCC…
Computer program
Specific function
…function sf(){document.f.q.focus()}…
Working copymRNA
Genes are located in the cell nucleus on chromosomes
Karyotype
Down syndrome karyotype (trisomy 21)
DNA(deoxyribonucleic acid)
mR
NA
Protein
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.
B b
B BB Bb
b Bb bbEggs
Sperm
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.
B b
B BB Bb
b Bb bbEggs
Sperm
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.
B b
B BB Bb
b Bb bbEggs
Sperm
Terminology…
• Haploid: containing one copy of each chromosome (n=23)
B b
B BB Bb
b Bb bbEggs
Sperm
• 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 Y/y
SY Sy sY sy
25% 25% 25% 25%
Mendel’s law of independent assortment
S Y
s y
Gametophytes(gamete-producing cells)
S Y
s yGametes
A b
a BRecombinants
Segregation
Mendel’s law of independent assortment
S Y
s y
Gametophytes(gamete-producing cells)
S Y
s yGametes
S y
s YRecombinants
Recombination
Segregation
• Simplified view of eye colour inheritance: biallelic Mendelian trait
– Brown dominant: BB, Bb
– Blue recessive: bb
Human eye colour
B b
B BB Bb
b Bb bbEggs
Sperm
Human eye colour
?
What is the probability of a child being born with blue eyes?
Human eye colour
?
Human eye colour
?
B?
B?B?B? bb
bb B?
Human eye colour
?
Bb
BbBbB? bb
bb B?
Human eye colour
?
Bb
BbBb
B?
Human eye colour
?
BbP(BB)=1/3
BbBb
P(Bb)=2/3
Human eye colour
?
BbP(BB)=1/3
BbBb
P(Bb)=2/3
P(b)=2/3x1/2=1/3 P(b)=1/2
Human eye colour
?
BbP(BB)=1/3
BbBb
P(Bb)=2/3
P(b)=2/3x1/2=1/3 P(b)=1/2
P(bb)=1/3x1/2=1/6
• Haemophilia A• Males with a mutant gene are
affected• Females with one mutant gene
are unaffected carriers
Non-Mendelian inheritance: Haemophilia
Summary
• Mendel deduced three simple laws of inheritance:– Segregation– Dominance– Random assortment
• The majority of traits don’t follow these rules but Mendel’s laws are nevertheless crucial to understanding almost all genetic inheritance
Frequency
Case 0.200
Control 0.165
Odds ratio: 1.26
1. Eye-catching headline of the form “Gene for…”
2. Highly qualified factual paragraph