Problem 5 - Campbell Biology

Question

Application/Analysis Some dioecious species have the XY genotype for male and XX for female. After double fertilization, what would be the genotypes of the embryos and endosperm nuclei? (A) embryo XY/endosperm XXX or embryo XX/endosperm XXY (B) embryo XX/endosperm XX or embryo XY/endosperm XY (C) embryo XX/endosperm XXX or embryo XY/endosperm XYY (D) embryo XX/endosperm XXX or embryo XY/endosperm XXY

Step-by-Step Solution

Answer

(D) embryo XX/endosperm XXX or embryo XY/endosperm XXY.

Step 1: Understand the Problem

Identify the given information: dioecious species with XY genotype for male and XX genotype for female. Determine the genotypes of embryos and endosperms after double fertilization.

Step 2: Define Double Fertilization

Double fertilization is a process in flowering plants where two sperm cells fertilize cells in the embryo sac. One sperm cell fertilizes the egg cell to form the embryo, while the other fertilizes the central cell to form the endosperm.

Step 3: Determine Embryo Genotype

The embryo forms by the fusion of a male sperm cell and the female egg cell. Since male can provide an X or Y chromosome, and female provides an X chromosome, the potential genotypes of the embryo are XX (female) or XY (male).

Step 4: Determine Endosperm Genotype

The endosperm forms by the fusion of a male sperm cell and two polar nuclei from the central cell. Male can provide an X or Y chromosome, and the female polar nuclei each provide an X chromosome (2X). Therefore, the potential genotypes for the endosperm are XXX (from polar nuclei + X) or XXY (from polar nuclei + Y).

Step 5: Match with Options

Compare the obtained genotypes with the provided options. The genotypes derived are: embryo XY/endosperm XXX or embryo XX/endosperm XXY.

Key Concepts

Double Fertilization

Double fertilization is a key process unique to flowering plants. During this process, two sperm cells participate in fertilizing two different cells within the female ovule. One sperm cell fertilizes the egg cell, which leads to the formation of the embryo. The other sperm cell fuses with the two polar nuclei, resulting in the formation of the endosperm. This is an essential step for the development of seeds, providing both genetic material and energy reserves necessary for growth. It's like a two-for-one deal where one sperm cell starts a new plant and the other creates a food source for its development!

Embryo Genotype

The genotype of an embryo is determined during fertilization. In dioecious species, males have an XY genotype, and females have an XX genotype. The embryo forms when a male sperm cell (either X or Y) fertilizes the female egg cell (which always provides an X chromosome). Therefore, the potential genotypes of the resulting embryo could be:

• XX (if the male provides an X chromosome)
• XY (if the male provides a Y chromosome)

This means that the embryo can either be female (XX) or male (XY), depending on the sperm cell that fertilizes the egg.

Endosperm Genotype

The endosperm is a crucial part of seed development, as it supplies nutrients to the growing embryo. It forms when a male sperm cell fuses with two polar nuclei from the central cell of the ovule. Each polar nucleus contains an X chromosome from the female plant. Therefore, the possible genotypes of the endosperm are:

• XXX (if the male sperm cell provides an X chromosome)
• XXY (if the male sperm cell provides a Y chromosome)

This triploid nature (three sets of chromosomes) of the endosperm is vital for its function in supporting seed development.

XY Genotype

In dioecious species, the XY genotype is crucial for determining the sex of the individual. Males have an XY genotype, meaning they possess one X and one Y chromosome. This contrasts with females, who have an XX genotype. In the context of fertilization:

• The X chromosome from the male sperm can lead to female offspring (XX)
• The Y chromosome from the male sperm can lead to male offspring (XY)

Understanding the XY genotype is essential for studying genetic inheritance and predicting the sex of the offspring in dioecious species.