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Difficulty: HardApplications of Genetics in Medicine and Agriculture

An agricultural breeder crosses two pure-breeding lines of oil palm: Line A, which produces high oil yield but is highly susceptible to fungal wilt, and Line B, which produces low oil yield but is completely resistant to fungal wilt. The resulting F1F_1 hybrids exhibit exceptional vigor, producing higher oil yields and stronger disease resistance than either parent. However, when these F1F_1 hybrid palms are self-pollinated, the F2F_2 generation displays wide phenotypic variation, with many individuals showing reduced yield and high disease susceptibility. Which of the following biological principles best explains the performance decline observed in the F2F_2 generation?

  1. The segregation and independent assortment of alleles during meiosis break down high heterozygosity, unmasking deleterious recessive genes in homozygous F2F_2 offspring.Answer
  2. B
    Pathogen exposure during the growth of the F1F_1 generation induced somatic mutations that were acquired and passed down to the F2F_2 offspring.
  3. C
    Hybridization permanently converts quantitative continuous variations into discontinuous genetic traits that cannot be inherited past the F1F_1 generation.
  4. D
    The dominant alleles conferring yield and resistance undergo codominant suppression, forcing all F2F_2 plants into a 3:1 phenotypic ratio of non-viable offspring.

Answer

The decline in performance (loss of heterosis) in the F2F_2 generation is caused by the segregation and independent assortment of alleles during meiosis, which reduces heterozygosity and leads to the expression of harmful homozygous recessive genotypes.
The superior performance of F1F_1 hybrids is due to heterosis (hybrid vigor), which relies on a high degree of heterozygosity masking harmful recessive alleles. When F1F_1 plants self-pollinate, meiotic segregation segregates these alleles into homozygous combinations (aaaa or bbbb). This unmasks undesirable recessive traits, causing a breakdown of hybrid vigor and wide phenotypic variation in the F2F_2 generation.

Step-by-Step Solution

1
Identify the genetic principle demonstrated by the F1F_1 generation
Crossing two genetically distinct inbred lines produces F1F_1 hybrids that exhibit superior traits (hybrid vigor or heterosis) due to optimal heterozygous allele combinations (AaBbAaBb).
Heterosis masks harmful recessive alleles present in the parental lines.
2
Analyze what happens genetically during F1×F1F_1 \times F_1 self-pollination
Meiosis in F1F_1 parents causes segregation and independent assortment of homologous chromosomes, producing gametes with different allele combinations.
Recombination leads to homozygous genotypes (AAAA, aaaa, BBBB, bbbb) in the F2F_2 generation.
3
Determine the phenotypic outcome in the F2F_2 generation
Homozygosity increases across the population, unmasking deleterious recessive alleles and leading to inbreeding depression and loss of hybrid vigor.
Commercial breeders must produce fresh F1F_1 seeds every season rather than saving F2F_2 seeds.

Key Concept

Hybrid Vigor (Heterosis) and Inbreeding Depression in Agricultural Crop Breeding
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Topics

Applications of Genetics in Medicine and Agriculture

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