Question

X ray Mutagen treatment of spermatogonia Gametes + + + Possible sibling matings මට ඖෂම ඌට ෙම | ON 00 Black embryos are homozygous for an induced mutation.

Genetic screens

Geneticists use the F₃ screen (see below) to isolate and characterize new mutations in a wide variety of animal and plants. Use your knowledge of mutagenesis and Mendelian inheritance to answer the following questions.

A. Why is the F₃ screen necessary, i.e. why is it that most mutants cannot be identified in the F₁ generation?

B. Would it work to mate the F₁ fish carrying a mutation to one of its mutant siblings? Why or why not?

C. A single zebrafish mating produces around one hundred offspring. In practical terms, a geneticist who performs the F₃screen will separate out each individual F₁ offspring, and carry its descendants through the F₂ and F₃ generations independently of the others. Why is this necessary?

Answer 1

crossing over between t, progeny o- ® + + + to Parents gametes Normal (N) Phenotypic ratio = 301, Genotypic ratio=182) Mutant Mutant Normal o entant (3) . (1) N (u i - Heter N: zygous (1) 30 (2) © (a) + + + (6) + + x (c) + + x + + + + + ( ++ genotype of all) (6) et to X + + (a) + + x +t ++ tt ++ tttt . (Black) type = 1020) etype = 38) Go

A). According to the question, mutants are recessive over the dominant alleles. In the F1 generation, all offsprings are in a heterozygous situation in which one is a normal allele and another is a mutant allele. Dominant allele inhibits or suppresses the expression of a recessive allele that is why in F1 condition mutants do not express.

All offsprings of F1 have genotype: + +* (normal)

B). All normal offspring with 1:1 genotype.

C). One black offspring with 1:2:1 genotype and 3:1 phenotype.