Question

In Drosophila, an individual female fly was observed to be of the \(\mathrm{XXY}\) chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's father had red eyes, and the mother had white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, whiteeyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

Short answer

Answer: The chromosomal event, called non-disjunction, occurred in the mother during the meiosis process while forming her eggs. This led to the production of an egg with two Xw chromosomes, which, when fertilized by a Y-bearing sperm from the father, resulted in the white-eyed female offspring with an XXY chromosome complement.

Step-by-step solution

Determine the genotypes of the parents

Since the father has red eyes and the mother has white eyes, we can determine their genotypes as follows: Father (XrY, where r indicates red eyes); Mother (XwXw, where w indicates white eyes).

Analyze the meiosis process in parents

During the formation of gametes, meiosis occurs in both parents, and the chromosomes are separated. In the case of the father, there will be two types of sperm: Xr (red eyes) and Y (no color allele). In the case of the mother, her eggs will only have the Xw chromosome (white eyes).

Identify the chromosomal event and the parent responsible

An abnormality must have happened during the meiosis process in either the mother or the father that led to the XXY chromosome complement in their offspring (the white-eyed female Drosophila). This abnormality is called non-disjunction, which is the failure of chromosomes to separate during meiosis. In this case, the non-disjunction most likely happened in the mother during the formation of her eggs.

Explain the chromosomal event in-detail

During meiosis in the mother, non-disjunction caused one of her eggs to have two Xw chromosomes instead of the normal single Xw chromosome. Consequently, when this egg was fertilized by a Y-bearing sperm from the father, the resulting offspring had an XXY chromosome complement and carried the genotype XwXwY. Since white eyes are X-linked and recessive, the white-eyed female Drosophila expressed the white eye phenotype.

Determine when the chromosomal event occurred

The non-disjunction that caused the genetic and cytogenetic abnormality in the white-eyed female Drosophila occurred during the formation of the mother's eggs, which happened during the meiosis process in the mother.

Key concepts

X-linked Inheritance

X-linked inheritance refers to genes that are located on the X chromosome, one of the two sex chromosomes that determine an organism's sex. In most species, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Traits linked to genes on the X chromosome exhibit unique patterns of inheritance because males have only one copy of the X chromosome while females have two.

For instance, in the case of the Drosophila fly, white eye color is an X-linked recessive trait, meaning it's a trait determined by a recessive gene on the X chromosome. Because females have two X chromosomes, they can be heterozygous for the trait (one dominant allele for red eyes and one recessive allele for white eyes) and not express it, or they can be homozygous for the recessive allele and express white eye color. Conversely, males, with only one X chromosome, will express the white eye color if they inherit the recessive allele, since they lack a second X chromosome that could potentially carry a dominant allele for red eyes.

Therefore, if a female has white eyes, she must have received the recessive allele on both of her X chromosomes (X_wX_w). A male with red eyes has the dominant allele on his X chromosome (X_rY), and a male with white eyes has a recessive allele (X_wY). Understanding X-linked inheritance is essential in explaining genetic abnormalities, such as the one seen in the XXY white-eyed female fly.

Chromosome Abnormalities

Chromosome abnormalities, such as changes in chromosome number or structure, can result from errors during cell division, specifically in the processes of meiosis or mitosis. In the situation of the Drosophila exercise, the chromosome abnormality manifested as a female fly with an XXY chromosome complement, distinguishing her from typical females with an XX complement and males with an XY complement.

Chromosome number abnormalities result from nondisjunction events, which can occur in both meiosis and mitosis, leading to gametes or cells with an abnormal number of chromosomes. When a nondisjunction event happens in meiosis in one of the parents, it can result in offspring with a trisomic condition (having an extra chromosome) like the XXY female fly, or monosomic condition (missing a chromosome).

In Drosophila and other animals, chromosome abnormalities can have various effects, including changes in phenotype and fertility issues. These abnormalities are also relevant in human genetic conditions, such as Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY), underlining the importance of understanding the consequences of such genetic events.

Meiosis and Non-Disjunction

Meiosis is the process by which gametes (sex cells) are formed, and it consists of two consecutive cell divisions (meiosis I and II) that reduce the chromosome number by half. It is crucial for maintaining chromosome number across generations and ensures genetic diversity through recombination. Non-disjunction is an error in meiosis where chromosomes fail to separate properly during one of the two divisions, leading to some gametes with an extra chromosome and others with a missing chromosome.

When non-disjunction occurs during the formation of egg cells in the case of Drosophila, it can result in an egg with two X chromosomes, such as the one that created the XXY white-eyed female when fertilized by a Y-bearing sperm. This error is likely to have occurred in the mother during meiosis I if both X chromosomes migrated to the same daughter cell, a crucial point that can explain the resulting chromosomal makeup of the offspring.

Understanding the stages at which non-disjunction can take place in meiosis and the resulting gametes helps explain the derivation of genetically abnormal progeny. Proper segregation of chromosomes during meiosis is pivotal for genetic normalcy, and errors during this process can have profound phenotypic effects, as observed in the unusual phenotype of the XXY female Drosophila.