Genetics

Crazy Chick Disease, technically known as nutritional encephalomalacia, is a neurological condition in young chicks characterized by ataxia, tremors, and paralysis. It is caused by a deficiency of Vitamin E, which is a crucial antioxidant that protects the brain's cellular membranes from oxidative damage.

In the genetic code, the mRNA codon AUG serves two purposes. It codes for the amino acid Methionine and also acts as the primary start codon, signaling the initiation of protein synthesis.

In the standard genetic code, there are three specific codons that signal the termination of translation (protein synthesis). These are known as stop codons or nonsense codons. They are UAA (ochre), UAG (amber), and UGA (opal). The codon CUG codes for the amino acid Leucine.

The number of Barr bodies in a somatic cell is calculated using the formula: Number of Barr bodies = (Number of X chromosomes) - 1. In this case, the individual has a 47,XXXX karyotype, meaning there are 4 X chromosomes. Number of Barr bodies = 4 - 1 = 3.

Selfing (self-fertilization) is the most extreme form of inbreeding. In each generation of selfing, the proportion of heterozygotes is halved. For example, if you start with a population that is 100% heterozygous (Aa), after one generation of selfing, the offspring will be 25% AA, 50% Aa, and 25% aa. The heterozygosity has dropped from 100% to 50%. Therefore, heterozygosity is lost by 1/2 in each generation.

The phenomenon described is epistasis, which is an interaction between genes at different loci (non-allelic genes). In epistasis, the expression of one gene (the epistatic gene) masks or modifies the expression of another gene (the hypostatic gene). Dominance refers to the interaction between alleles of the same gene (intra-allelic).

Prophase-I of meiosis is divided into five stages. The key event of the zygotene stage is synapsis, where homologous chromosomes pair up and attract each other to form bivalents. Tetrad formation is visible in the pachytene stage, crossing over (exchange of genetic material) occurs during pachytene, and chiasmata (the physical manifestation of crossing over) become visible during the diplotene stage.

Organelles like mitochondria and chloroplasts contain their own ribosomes (70S type), which are essential for synthesizing proteins within these organelles. Since ribosomes are themselves complex structures (often considered non-membranous organelles) and are found inside other organelles, the ribosome is the correct answer. The other options are distinct, membrane-bound organelles within the cytoplasm.

The ZW sex-determination system is found in birds, some reptiles, some insects (like butterflies and moths), and some fish. In this system, the female is the heterogametic sex (ZW), and the male is the homogametic sex (ZZ). This is the reverse of the XY system found in humans and many insects.

The domestic chicken (\textit{Gallus gallus domesticus}) has a diploid chromosome number of 78 (2n = 78). This consists of 38 pairs of autosomes and one pair of sex chromosomes (ZW for females and ZZ for males).