Date post: | 11-May-2015 |
Category: |
Education |
Upload: | martin-jellinek |
View: | 437 times |
Download: | 0 times |
INHERITANCE OF TRAITS
The Genetic Lottery
• As every individual carries two alleles for a particular gene, the chance of inheriting a particular one of these alleles is 1 in 2 or ½
• So the chance of if inheriting your genotype for any trait is ½ x ½ = ¼
Mendel’s Pea Plants
TT TTtt tt
Tt Tt Tt Tt
Tt TtTT tt
Punnet Squares for:Monohybrid Crosses
T T
t
Tt
¼
Tt
¼
t
Tt
¼
Tt
¼
T t
T
TT
¼
Tt
¼
t
Tt
¼
tt
¼
1st Generation 2nd Generation
All Tt or TtAll Tall
1 TT : 2 Tt : 1 tt or ½ TT: Tt : ½ tt 3 Tall : 1 Dwarf
Genotypic ratio Phenotypic ratio
Dihybrid Crosses• Consider two traits• W = straight brow, w = widows peak• D = detatched earlobes, d = attached• If we cross 2 people who are heterozygous
for both traits, how would the punnet square look and what would be the phenotypic ratios?– NOTE: Genotypic / phenotypic ratios can be
represented as fractions, decimals, ratios or percentages
Dihybrid Crosses
• 1WWDD:1WWdd:2WWDd:2wwDd:4WwDd:2WwDD:2Wwdd:1wwDD:1wwdd
• 9straight/detached:3straight/attached:3widow/detached:1widow attached
WD Wd wD wd
WD WWDDstraight/detached
WWDdstraight/detached
WwDDstraight/detached
WwDdstraight/detached
Wd WWDdstraight/detached
WWddstraight/attached
WwDdstraight/detached
Wwddstraight/attached
wD WwDDstraight/detached
WwDdstraight/detached
wwDDwidows/detached
wwDdwidows/detached
wd WwDdstraight/detached
Wwddstraight/attached
wwDdwidows/detached
wwddwidows/attached
Dihybrid Crosses
• 1WWDD:1WWdd:2WWDd:2wwDd:4WwDd:2WwDD:2Wwdd:1wwDD:1wwdd
• 9straight/detached:3straight/attached:3widow/detached:1widow attached
WD Wd wD wd
WD WWDDstraight/detached
WWDdstraight/detached
WwDDstraight/detached
WwDdstraight/detached
Wd WWDdstraight/detached
WWddstraight/attached
WwDdstraight/detached
Wwddstraight/attached
wD WwDDstraight/detached
WwDdstraight/detached
wwDDwidows/detached
wwDdwidows/detached
wd WwDdstraight/detached
Wwddstraight/attached
wwDdwidows/detached
wwddstraight/attached
Test Crosses
• Prior to it being possible to establish an individual’s genotype via molecular means, a test cross was used.
• Mono or dihybrid test crosses involve breeding the unknown individual with a homozygous recessive.
• Eg. An unknown black cat (DD or Dd) is mated with a grey cat (dd).
• If just one grey kitten is produced, a Dd genotype of the unknown is confirmed
• The general rule is that 16 consecutive black kittens will provide reasonable confidence that the unknown is DD
Linked Genes• If two genes are said to be linked, this
means that they are located in a simillar position on the same chromosome and have a higher chance of being passed on together.
• Eg. RBC shape and RH blood group are linked– RH gene: D = Rh+, d = Rh-
– EL1 gene: E = eliptical RBCs, e = normal RBCs
Linked Genes
• The only way the alleles of linked genes can be separated is through crossing over.
• Based on the above, Sarah is far more likely to produce parental eggs (DE or de) than recombinant eggs (De or dE)
Linked Genes• Based on the spacing between linked genes, a
probability of recombination can be expressed.• In the case of RH and EL1, the probability of
recombination is 0.01 for each type
Detecting Linkage
• Do a punnet square for a test cross with a known heterozygote (aabb x AaBb).– Genotypic ratio for unlinked genes is approx. 1:1:1:1
• If the genes are linked, instead you should see a greater proportion of parental rather than recombinant genotypes.
Genotype AaBb aabb Aabb aaBb
Unlinked 25% 25% 25% 25%
Linked 44% 44% 6% 6%
Detecting Linkage• A simple calculation can be used to either
predict distance between loci or outcomes for linked genes
– So, in a litter of 16 mice, if 7 have brown, curly hair, 7 have black, straight hair, 1 has brown straight hair and 1 has black curly hair ...
– We can estimate these gene loci to be
= 12.5 map units apart
Distance between loci =
What can you predict from the following test cross results?
• Independent assortment (unlinked)
• Linked, with no crossing over
• Linked with crossing over
• Now draw a representation of the heterozygote’s chromosomes
Genotype AaBb Aabb aaBb aabb
Ratio 1.1 0.9 1.2 1.0
Genotype AaBb aabb
Ratio 1.0 1.0
Genotype AaBb Aabb aaBb aabb
Ratio 1.0 0.45 0.55 1.0
A aB b
Pedigrees – guide to symbols
Mr Jellinek’s family tree – EYE COLOUR
Can you work out the
genotypes for each
individual?
Mr Jellinek’s family tree – EYE COLOUR
bb
bb bbbb
bb
Bb
BbBb Bb
Bb
Can you work out the
genotypes for each
individual?
Bb
Is this genetic pedigree possible, if so, explain how
1 2
43
Yes it is, known genotypes are shown below
5 6 7 8
bbbb bb
bb
bb
Bb Bb
Bb Bb BbBbBbBb
Autosomal RecessiveKey Features
• Presence of affected individuals can frequently skip a generation.• In large samples there will be an even distribution of affected males and
females• It is possible for two normal individuals to have an affected child
Autosomal DominantKey Features
• It is not possible for the disease to skip a generation • Homozygous affecteds are rare as they will always have affected offspring,
meaning that lineage is often short-lived.• In large samples there will be an even distribution of affected males and females
X-Linked Recessive• That disease will frequently skip a generation as
females can be carriers• Majority of affecteds are usually male• All sons of an affected female will be affected• All daughters of an affected male will be carriers
Key features
X-Linked DominantKey Features
• Affected male will always pass on the trait to his daughters, but not his sons• In large samples there will be an even distribution of affected males and
females• Can not skip a generation
Construct this pedigree
Susan
Did yours look like this?
Jim Jean
Scott James Natasha AlanKylie
AlisonPaul
Anne Emma Colin