Concept explainers
Complete the following summary of Morgan’s crosses involving the mutant white-eyed fly by filling in the Punnett square and indicating the genotypes and
Genotypes
_____ _____ _____ _____
Phenotypes
_____ _____ _____ _____
To complete: The summary of Morgan’s cross that involves the mutant white-eyed fly by filling in the Punnett square and by indicating the genotypes and phenotypes of the F2 generation.
Introduction: Sex chromosomes are those chromosomes that play a crucial role in determination of sex of the individual. They have important function in the development of sexual characteristics in the organisms.
Answer to Problem 1IQ
Pictorial representation: Fig.1 represents the summary of Morgan’s cross that involves the mutant white-eyed fly by filling in the Punnett square and by indicating the genotypes and phenotypes of the F2 generation.
Fig.1: Morgan’s cross of mutant white-eyed fly with the genotypes and phenotypes of the F2 generation
Explanation of Solution
The X and Y chromosomes act as sex chromosomes in most of the organisms. The genes that are present on the sex chromosome are called sex-linked genes. There are approximately 1,100 X-linked genes. On the other hand, Y chromosome contains relatively very few genes.
The sex-linked genes display a unique pattern of inheritance. The expression of recessive X-linked trait requires two copies of alleles in females (homozygous) and only one copy of the allele in males (hemizygous). The probability of a male to receive a single mutated X-linked gene from their mother is high as compared to a female receiving both the X-linked mutated genes from the mother (carrier) and father (carrier).
When Morgan crossed red-eyed female (wild-type) with white-eyed male (mutant), all the offspring were red-eyed in the first generation (F1). Then, he again crossed the F1red-eyed female to the red-eyed male of F1. The typical ratio of 3:1 (red-eyed flies: white eyed flies) was observed in the F2 generation, but all white-eyed flies were males.
Morgan concluded that this could be due to hemizygous condition (presence of single X-chromosome “XW+Y”) of the males. Females required both the mutated alleles to express the phenotype (white eye color), while males needed only one mutated allele to show the phenotype. The probability to inherit both mutated copies is less.
Hence, four genotypes of F2 generation are as follows:
- XW+XW+
- XW+XW
- XW+Y
- XWY
The four phenotypes of F2 generation are as follows:
- Red-eyed female
- Red-eyed female
- Red-eyed male
- White-eyed male.
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