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Punnet Squares, Linked Genes and Pedigrees

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Presentation on theme: "Punnet Squares, Linked Genes and Pedigrees"— Presentation transcript:

1 Punnet Squares, Linked Genes and Pedigrees

2 Predicting offspring genotype
If we know the genotype of the parents, we can predict the genotype of the offspring, using test crosses (punnett squares) Alternatively, parental genotype may be able to be determined if the offspring genotype is known

3 Example John Tracey Phenotypes: Phenotypes: Blood group B
Normal skin pigment Genotypes: IBi ? Tracey Phenotypes: Blood group B Normal skin pigment Genotypes: IBi ? Samson Phenotypes: Blood group O Albino Genotypes: ?

4 Working out the possibilities
Step 1: work out the possible genes within the sperm and egg cells Step 2: perform monohybrid cross

IB i

6 What is the probability of heterozygotes producing different phenotypes?
PUNNETT SQUARE: HETEROZYGOTES A a Genotype ratio: Phenotype ratio:

7 Complete the following:
Punnett Squares made easy worksheet

8 What if we want to work with combinations?
We want to find out all the combinations of skin pigment and blood types that Tracey and John’s children could have. Step 1: work out all the genotype possibilities within the egg and sperm cells Step 2: perform dihybrid test cross

9 Dihybrid cross IBA IBa iA ia IBIBAA IBIBAa IBiAA IBiAa IBIBaa IBiaa
iiAA iiAa iiaa Genotype ratio: Phenotype ratio:

10 Linked genes Genes are said to be “linked” when their loci are found on the same chromosome. It means that those alleles are usually inherited together… but not always!! The offspring of this couple gets one of each parental chromosome. The genes are intact, so the alleles that are inherited are of the “parental type”

11 Recombination Remember we said that in meiosis, genetic material can move from one chromosome to another? This is called recombination. It results in offspring having chromosomes that are not identical to parental chromosomes.

12 Recombination You can see that the probability of genes that are close together (a and b) being separated during crossing over is less than that of genes that are further apart (a and c)

13 Recombination In the example below, the mother is heterozygous at both loci, while the father is homozygous at both. Parental types: Ga, gA (the codes found on the parental chromosomes) If crossing over occurs in maternal meiosis, there is no change, as the alleles are the same on each. HOWEVER, if there is crossing over in the paternal chromosomes, and the G/g alleles swap places, there will be RECOMBINANT offspring with chromosomes with the following allelic combinations: ga or GA g G G G A a a a

14 Detecting linkage If you know the number of recombinant offspring (those without the parental type chromosomes), you can work out how far apart the genes are from each other. This is the equation to use: 100 x number of recombinant offspring total number of offspring

15 Detecting linkage example
Paternal genotype Parental type Recombinant Ga gA GA ga GaGa GagA GaGA gaGa 44% 6% Maternal genotype So, using our formula before (although, we have percentages, it’s much easier!): Distance between loci = 100x number of recombinant (6+6=12) = 12 map units Total number of offspring ( =100)

16 Detecting linkage problem
Parent 1 Parent 2 Parent 1 (all parental) Parent 2 Parental Recombinant AQ Aq AAQQ AAQq aq AaQq Aaqq AAqq aQ AaQQ a A A A q Q q Q Number of parental type genotypes:_______ Number of parental type offspring: 212 Number of recombinant genotypes: _________ Number of recombinant offspring: 156 Distance between loci:

17 Pedigrees Family tree style diagrams that show the presence/absence of particular disorders within a family. Has a distinct set of symbols

18 Pedigree Key Carrier (heterozygote, in recessive disorders)

19 Autosomal dominant pattern
Characteristics of pattern: Both males and female affected All affected individuals MUST have an affected parent Once the trait disappears from a branch of the pedigree, it does not reappear In a large sample, approx. the same number of each sex will be affected Autosome: any chromosome that is not a sex chromosome

20 Autosomal recessive pattern
Characteristics of pattern: Both males and females can be affected equally Two unaffected parents can have an affected child (why… and what symbol could be useful here?) All children of two people with the disorder MUST also be affected The trait may disappear from a branch of the pedigree, but reappear later on

21 X linked dominant pattern
Characteristics of pattern: Male with trait passes it on to all of his daughters, and none of his sons Female with trait may pass it to both daughters and sons Every affected person has at least one parent with the trait If the trait disappears from a branch, it does not reappear More affected females than males

22 X linked recessive pattern
Characteristics of pattern: All sons of an affected female are affected All daughters of a father with the trait are carriers None of the sons of an affected father and unaffected mother will show the trait UNLESS their mother is a carrier All children of two affected individuals will also be affected More males than females show the trait

23 In groups: You will be given one of the pedigree patterns.
Provide explanations for EACH of the characteristics of the pattern Find out about at least two disorders with this pattern of inheritance

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