Biology Test 329

The male human is heterozygous for autosomal gene A and B and also hemizygous for haemophilic gene h, then his genotype will be AaBbX^hY because haemophilia is a sex linked trait that is present on X -chromosome. So, the total number of gametes will be abX^h, abY, ABX^h, ABY, AbX^h, AbY aB X^h, aBY. So the proportion of abXh sperm will be 1/8

Mendel worked on seven characters. These characters showed complete independent assortment despite the seven characters chosen by him were present on four chromosomes -1, 4, 5 and 7.

Down's syndrome is the example of autosomal aneuploidy. Here, an extra copy of chromosome 21 occurs. As it is an autosomal disease, the offsprings produced from affected mother and normal father should be 50%

Pleiotropic gene is such a gene which has a wider effect on phenotype i.e., it controls several phenotypic traits. Sickle cell anaemia is considered to be caused by one such pleiotropic gene. It is caused due to mutation in β -globin gene of haemoglobin.

Selfing is the process of fertilisation with polar or male gametes of the same individual. AABbCC will produce two type of gametes ABC and AbC. Thus, in F₂ generation three genotypes will be obtained. These are AABBCC, AABbCC and AAbbCC in the ratio of 1:2:1. Phenotypically AABBCC and AABbCC are same. So the phenotypic ratio in F₂ generation will be 3: 1 .

In the inheritance pattern of sex chromosomes X-chromosome of father always passes to daughter and X -chromosome of mother passes to son. As the father is diseased and all the girls inherit it, it is obvious the disease is sex-linked. The mother is not a carrier (as evident from the fact that no son is diseased). Thus, the gene is dominant and expresses even in heterozygous condition.

Explanation of the correct option:

1. Generally, complementary genes are located on different gene loci, but when they are present together it leads to the expression of a particular phenotype.
2. In this case, both genes are needed for phenotypic expression.
3. In Lathyrus odoratus, also called sweet pea, Bateson and Punette observed that both C and P genes that are dominant alleles are important for the pigment production in the flower.
4. Instead of the 9:3:3:1 ratio, the ratio that is obtained in this dihybrid cross is 9:7.

Transcription or the formation of RNA on template DNA in prokaryotic and eukaryotic, has three main events.

• Initiation - Binding of RNA polymerase to double-stranded DNA; this step involves a transition to single strand in the region of binding; RNA polymerase binds at 5’ end on a sequence of DNA called the promoter. Initiation is the most important step in gene expression.
• Elongation - The covalent addition of nucleotides to the 3' end of the growing polynucleotide chain. This involves the development of a short stretch of DNA that is transiently single-stranded.
• Termination - The recognition of the transcription termination sequence and the release of RNA polymerase. The promoter and terminator flank the structural gene in a transcription unit.

DNA is made up of two polynucleotide chains, where the backbone is made up of sugar and phosphate groups and the nitrogenous base projecting to the centre. The diameter of the strand is always equal due to pairing of purine and pyrimidine i.e., adenine is complementary to thymine while guanine is complementary to cytosine.

Lactose operon in E. coli is a catabolic pathway in which the structural genes remain switched off unless the inducer (lactose) is present in the medium. The lac operon consists of a cluster of three structural genes and a length of DNA including operator and promoter regions.