Like this presentation? Why not share!

# 05 Inheritance

## by Martin Jellinek, Teacher at Box Hill High School on Jul 27, 2011

• 378 views

### Views

Total Views
378
Views on SlideShare
378
Embed Views
0

Likes
1
5
0

No embeds

### Categories

Uploaded via SlideShare as Microsoft PowerPoint

## 05 InheritancePresentation Transcript

• Genetics & Inheritance
• INHERITANCE OF TRAITS
• Allele 1 Allele 2 GENE
• 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 TT tt tt Tt Tt Tt Tt Tt Tt TT tt
• Punnet Squares for: Monohybrid Crosses 1 st Generation 2 nd Generation All Tt or Tt All Tall 1 TT : 2 Tt : 1 tt or ½ TT: Tt : ½ tt 3 Tall : 1 Dwarf Genotypic ratio Phenotypic ratio T T t Tt ¼ Tt ¼ t Tt ¼ Tt ¼ T t T TT ¼ Tt ¼ t Tt ¼ tt ¼
• 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
• 1 WWDD: 1 WWdd: 2 WWDd: 2 wwDd: 4 WwDd: 2 WwDD: 2 Wwdd: 1 wwDD: 1 wwdd
• 9 straight/detached: 3 straight/attached: 3 widow/detached: 1 widow attached
WD Wd wD wd WD WWDD straight/detached WWDd straight/detached WwDD straight/detached WwDd straight/detached Wd WWDd straight/detached WWdd straight/attached WwDd straight/detached Wwdd straight/attached wD WwDD straight/detached WwDd straight/detached wwDD widows/detached wwDd widows/detached wd WwDd straight/detached Wwdd straight/attached wwDd widows/detached wwdd widows/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 WWDD straight/detached WWDd straight/detached WwDD straight/detached WwDd straight/detached Wd WWDd straight/detached WWdd straight/attached WwDd straight/detached Wwdd straight/attached wD WwDD straight/detached WwDd straight/detached wwDD widows/detached wwDd widows/detached wd WwDd straight/detached Wwdd straight/attached wwDd widows/detached wwdd straight/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
• 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
• 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 )
• 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
• 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%
• 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?
• Linked, with no crossing over
• Now draw a representation of the heterozygote’s chromosomes
A a B b 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
• 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 bb bb bb Bb Bb Bb Bb Bb Can you work out the genotypes for each individual? Bb
• Is this genetic pedigree possible, if so, explain how
• 1 2 4 3 Yes it is, known genotypes are shown below 5 6 7 8 bb bb bb bb bb Bb Bb Bb Bb Bb Bb Bb Bb
• Autosomal Recessive
• Key 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 Dominant
• Key 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