NEET Practice Questions (MCQs) with Answers & Solutions

Polygenic inheritance is the inheritance pattern of a trait controlled by two or more genes. Genes may be on the same or different chromosomes and each gene may have two or more alleles. The gene expression is influenced by a variety of factors including gender, nutrition, breed, rate of growth, and amount of exercise. These traits are quantitative traits - that is, there is a wide range within the population. Such traits include height, weight, character, working abilities, and some genetic defects.

One example is height in humans - there are a larger number of phenotypes, each differing slightly from the next and forming a graduated series. Some variation in height in humans is due to environmental factors such as diet, exercise and disease. However, if the environmental factors were constant, there would still be continuous variation in height due to such things as formation and hormone levels. Themore genes that control a characteristic, the more possible gene combinations exist and the more phenotypes.

Other examples of polygenic inheritance include : skin colour in humans, colour in wheat kemels, egg weight in poultry, fleece wieght in sheep. Kemal colour in wheat is determined by two gene pairs, so called polygenes that produce a range of colors from white to dark red depending on the combination of alleles. Dark red plants are homozygous AABB and white plants are homozygous aabb. When these homozygotes are crossed the F₁ offspring are all double heterozygotes AaBb. Thus crossing individuals with the phenotype extremes yeild offspring that are a 'blend' of the two parents.

Test cross is across made to identify hidden recessive alleles in an individual of unknown genotye. This individual is crossed with one that is homozygous for the allele being investigated (i.e., a homozygous recessive). The homozygous recessive individual may be the parent of the individual being investigated.

Turner's syndrome is characterised by the monosomy of XO type. It is characterized by a lack of ovaries an menstrual cycle. Affected women are sterile and lack secondary sexual characteristics, although the exrternal genitalia are present. The syndrome is named after the US endocrinologist H.H. Turner (1892-1970), who first described it. 

Down's Syndrome is a congential form of mental retardation due to a chromosome defect in which there are three copies of chromosome no. 21 instead of the usual two. The affected individual has a short broad face and slanted eyes (as in the MOngolian races), short fingers, and weak muscles. Down's syndrome can be detected before birth by aminocentesis. It is name after the British physician John Down (1828-96), who first studied the incidence of the disorder. Klinefelter's syndrome is characterised by trisomy (XXY). These are male individuals, who are phenotypically fairly normal but have a fairly low sperm count and therefore sterile. 

The physical distance between two genes determines both the strength of the linkage and the frequency of the crossing over between two genes.The strength of the linkage increases with the closeness of the two genes. On the other hand the frequency of crossing over increases with the increase in the physical distance between the two genes.

Turner's syndrome is caused due to the absence of one of the X chromosomes i.e., 45 chromosomes total with sex chromosomes as XO. Such females are sterile as ovaries are rudimentary. Besides this, other features include lack of other secondary sexual characters, etc. 

In normal females, number of Barr bodies is one less than the number of X chromosomes. One Barr body means the female has two X chromosomes. Turner's syndrome (44 + X0) is a genetic disorder that affects only females. In this condition, cells of the affected female possess only one X chromosome, instead of two.Due to the presence of only one X chromosome, no Barr body is present in females suffering from Turner's syndrome.

Barr body is also known as sex chromatin or X chromatin. Barr body is partially inactivated and development of this facultative heterochromatin occurs in one of the two X chromosomes in interphase nuclei of human female. Any of the two X-chromosomes can become heterochromatin.

When a heterozygous individual (Aa) is crossed with a homozygous recessive individual (aa), the F1 progeny will show a 1:1 ratio of dominant and recessive phenotypes. This follows the Mendelian law of segregation, where the two alleles separate during gamete formation.

If a pea plant with violet-colored flowers (heterozygous, Vv) is crossed with a plant having white-colored flowers (vv), the progeny will show a 1:1 ratio of violet and white flowers. Since 50% of the progeny had white flowers, the parent with violet flowers must be heterozygous (Vv).

E. coli contain single chromosome. So, the number of linkage groups in E. coli is 1.