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How to solve genetic problems without pedigrees

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How to solve genetic problems without pedigrees.
These problems will either tell you explicitly, or provide information that allows you to infer the pattern of inheritance of the characteristic in the problem. Implicit hints include: * if 2 pure breeding individuals with different phenotypes produce offspring of one phenotype, this means that the offspring phenotype represents a dominant allele
* if 2 individuals with the same phenotype produce offspring with a different phenotype, this means that the offspring phenotype represents a recessive allele
* if 3 phenotypes are present, this means that it is codominance, with the heterozygote showing the intermediate phenotype
Once the pattern of inheritance is established, the following steps should be followed:
1. Write symbols representing the alleles present. Use the following conventions:
* DOMINANT alleles are represented by UPPER CASE letters
* recessive alleles are represented by lower case letters
* CODOMINANT alleles are represented by UPPER CASE letters
* use the same letter for each allele of a gene (use superscripts for codominant alleles eg IA, IB)
* avoid use of letters where upper and lower cases are not easily distinguished (eg Ww, Ss, Cc)
* DO NOT use X or Y for alleles, they are reserved for the sex chromosomes
* sex linked genes are displayed as superscripts on the X chromosomes ( eg XH, Xh)
2. Write genotypes of the first generation (the parent or P generation).
3. Write the gametes produced by each parent.
4. Draw up a Punnett square and work out the genotype combinations possible in the offspring.
5. Repeat steps 2 - 4 for the next generation(s) (the filial or F generations) as necessary.
6. Check you have answered the question - eg probability of a particular phenotype, number of offspring expected to have the phenotype, etc
Sample question: A black haired true-breeding male guinea pig is mated with a white haired true-breeding female guinea pig. The allele for black hair is dominant over the allele for white hair, which is recessive. What are the chances of a white haired individual appearing in:
a) the first generation of offspring?b) the second generation of offspring?
Answer:
Let the symbols B = black hair alleleb = white hair allele
Parent generation (P) :male BBblackfemale bbwhite
Gametes: Bb
First generation (F1) :Bbie all black
F1 parents: male Bbfemale Bb
Gametes: B or bB or b
Second generation (F2) :Bb
BBBBb
bBbbbie 3 black : 1 white
Answer : (a) no chance of white haired individual (b) 1 in 4 chance of white haired individual
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