Maternal effect genes:
Maternal effect genes are a specific class of genes that play a crucial role in early embryonic development. These genes are unique in that their expression is exclusively contributed by the mother, meaning that they are present in the mother's genome and are deposited into the egg during oogenesis.

Contribution exclusively by the mother:
Maternal effect genes are exclusively contributed by the mother. This means that the genetic information for these genes is present in the mother's genome and is transmitted to the developing embryo through the egg. The embryo relies on the gene products encoded by these genes for proper development during the early stages.

Defining polarity of the egg:
One of the important roles of maternal effect genes is to define the polarity of the egg. Polarity refers to the asymmetrical distribution of molecules within the egg, which is essential for proper embryonic development. Maternal effect genes regulate the expression of various molecules that establish this polarity, including proteins and RNAs.

Mutation in such genes are lethal:
Mutation in maternal effect genes can have severe consequences for the development of the embryo. Since these genes play a crucial role in early embryonic development, any disruption in their function can result in developmental defects or embryonic lethality. Mutations in maternal effect genes can lead to abnormalities in cell division, patterning, and differentiation, which are vital processes for proper embryonic development.

Gene products as transcription factors:
The gene products encoded by maternal effect genes can act as transcription factors that regulate the expression of genes involved in further development. These transcription factors bind to specific DNA sequences and control the transcription of downstream target genes, thereby influencing the developmental processes in the embryo.

In summary, maternal effect genes are exclusively contributed by the mother and play a crucial role in early embryonic development. They define the polarity of the egg, and mutation in these genes can be lethal. The gene products encoded by maternal effect genes can act as transcription factors to regulate the expression of genes involved in further development.