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Ch. 6 - Chromosome Mutations: Variation in Number and Arrangement
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 6 - Chromosome Mutations: Variation in Number and ArrangementProblem 16
Chapter 6, Problem 16

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the F₁ and F₂ results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

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Step 1: Understand the genetic setup. We have an autotetraploid organism with four copies of each gene. The green seed trait is dominant (W), and white seed is recessive (w). The homozygous green parent genotype is WWWW, and the homozygous white parent genotype is wwww.
Step 2: Determine the genotype of the F₁ generation. Since the green parent (WWWW) is crossed with the white parent (wwww), all F₁ offspring will inherit two W alleles from the green parent and two w alleles from the white parent, resulting in the genotype WWww.
Step 3: Understand allele segregation in autotetraploids. During meiosis, chromosome pairing (synapsis) is random, so gametes can have different combinations of W and w alleles. The possible gametes from a WWww individual can be calculated using the binomial distribution for allele segregation.
Step 4: Calculate the gamete genotype frequencies from the F₁ (WWww). The possible gametes are WW, Ww, and ww, with probabilities given by the formula for combinations of alleles in autotetraploid meiosis: \(P(k) = \frac{\binom{2}{k} \binom{2}{2-k}}{\binom{4}{2}}\), where \(k\) is the number of W alleles in the gamete.
Step 5: Predict the F₂ generation genotypes and phenotypes by crossing the gametes from the F₁ generation. Use the gamete frequencies to set up a Punnett square or probability matrix, then determine the genotype frequencies of the F₂. Since only one dominant allele is needed for green seeds, calculate the proportion of green (at least one W) and white (all w) phenotypes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Autotetraploidy and Chromosome Pairing

Autotetraploids have four copies of each chromosome, originating from genome duplication within a species. During meiosis, chromosomes can pair randomly as bivalents or multivalents, affecting segregation patterns. This random synapsis alters expected Mendelian ratios compared to diploids, influencing genotype and phenotype frequencies in offspring.
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Dominance and Allelic Interaction in Polyploids

In polyploids, a single dominant allele can mask the effect of multiple recessive alleles, producing the dominant phenotype. For a trait controlled by one dominant allele (e.g., green seeds), any genotype with at least one 'W' allele will show green seeds, while only the homozygous recessive (wwww) shows white seeds. This affects phenotypic ratios in progeny.
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Segregation and Phenotypic Ratios in Polyploid Crosses

Segregation in autotetraploids follows more complex patterns due to multiple chromosome copies and random pairing. The F₁ generation from a homozygous dominant (WWWW) crossed with homozygous recessive (wwww) will be all heterozygous (Wwww) and green. The F₂ generation shows modified ratios, with a higher proportion of dominant phenotypes than diploid Mendelian ratios due to multiple allele combinations.
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Related Practice
Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, normal bristles x diploid, bent bristles.

Textbook Question

Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?

Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, bent bristles x diploid, normal bristles

Textbook Question

Having correctly established the F₂ ratio in Problem 18, predict the F₂ ratio of a 'dihybrid' cross involving two independently assorting characteristics (e.g., P₁ = WWWWAAAA x wwwwaaaa).

Textbook Question

A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.

How would you explain the high incidence of past stillbirths?