Ratios (for example 3:1 or 9:3:3:1)?


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Problem 7 – The Value of Sample Size
You notice something strange as you continue to count offspring from (the extremely prolific) cross four. You still see the three phenotypes that you recorded initially, BUT the ratio shifts a bit and you note 575 dark bodied, long haired offspring.
Cross Number
Phenotype of parent 1
Phenotype of parent 2
Phenotype of offspring
Exact # of offspring shown in bold
4
A33 (matures to A34)
Brown short hair, F
A4
Brown, long hair, M
1925 (A90 and A92)
Black, F + M, short hair
4425 (A2 and A4)
Brown F + M, long hair
3075 (A34 and A36)
Brown, F + M, short hair
Do you need to revisit the hypothesis you generated in problem 6 in light of this larger sample size? If so, what do you now hypothesize about the arrangement for these genes and alleles on the chromosomes? Explain your thoughts if you do not think you need to revise your hypothesis.
Yes. Crossing over is happening here, because there are black long hair offspring but the ratio is not proportional, due to the random nature of crossing over (it doesn’t happen consistently.)
What percent of the gametes created by the female parent contain both recessive alleles?
lb
lB
LB
Lb
lb (50%)
lb/lb
lB/lb
LB/lb
Lb/lb
lB (50%)
lb/lB
lB/lB
LB/lB
Lb/lB
Calculation:
# of black long offspring/ total # of offspring = ?
575/10000 = 5.75%
5.75%/50% = 11.5%
What percent of the gametes created by the female contain both dominant alleles?
lb
lB
LB
Lb
lb (50%)
lb/lb
lB/lb
LB/lb
Lb/lb
lB (50%)
lb/lB
lB/lB
LB/lB
Lb/lB
Calculation:
Both dominant alleles means that offspring has both dominant traits.
Therefore black short hair, which is 1925.
1925/10000 = 19.25%
To double check your numerical answers for the previous two questions you decide to do another cross. You want to keep the female genotype the same. What male could you use in the cross to simplify the analysis of the data? Explain your thinking…
Problem 8 – Summary
The work for these problems should be done neatly – punnett squares can be typed or handwritten but all explanations should be typed. Please use complete sentences to explain your ideas. You can select your allele abbreviates but please follow the conventions for the notation for sex-linked traits (use an X or Y to show the sex chromosome and a supersсrіpt to show the allele) and use one letter to represent the alleles for a single gene (for the eye color allele for example you could use R for red and r for grey or E for red and e for grey, but do not use R for red and g for grey).
You may talk with classmates about this assignment but all of the writing should be done on your own.
1. Revise and correct your work for problems 2-7.
2. What did this problem set teach you about…
a. …the relationship between dominant and recessive alleles?
Dominant alleles are dominant because they show up in the phenotype if there is just one of them. Recessive alleles only show up in the phenotype if they are homologous.
b. …how to determine if a gene is on a sex chromosome or an autosome?
If a dominant father creates a recessive daughter, then the gene is not sex linked. Another way to tell is to do a reciprocal cross, when the genders of the parents are swapped. If the results of the offspring are unaffected, then the gene is autosomal.
c. …situations which can shift offspring ratios away from predicted simple dominant/recessive
ratios (for example 3:1 or 9:3:3:1)?
Crossing over can shift offspring ratios by ___.
d. …the importance of carefully selecting parental genotypes (think specifically about 2 and 7)?
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