Principles of Inheritance and Variation Test - 5

A single abnormal gene on one of the first 22 autosomal chromosomes from either parent can cause an autosomal disorder.
In case of myotonic dystrophy, the trait is present in mother due to presence of heterozygous (one dominant and one recessive) genes and trait can appear in the progeny which are heterozygous.

Point mutation involves substitution, deletion or insertion of a single nitrogenous base.
The heterozygous condition is called sickle-cell trait that produces both normal and abnormal beta chains. In affected individuals, the amino acid glutamic acid (Glu) is substituted by amino acid valine (Val) at the sixth position.
This substitution occurs due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG. This results a change in the shape of RBC to elongated sickle like structure.

Alkaptonuria is a rare genetic metabolic disorder characterised by the accumulation of homogentisic acid in the body. Urine of alkaptonuria person contains homogentisic acid, which spontaneously combines with oxygen to form a black pigment.
This happens because of the absence or inactivity of the enzyme homogentisate oxidase (catalyses the breakdown of homogentisic acid).

A normal man and a carrier woman produce 50% colour-blind sons and 50% carrier daughters. Let  refer to the X-linked allele that causes colourblindness. 'c' is recessive.

This pattern of heredity is often called criss-cross inheritance and shows that the gene from the father has been transferred to the grandsons through the daughters, which is typical for all recessive sex-linked traits; males show the traits, females only transmit them.

Klinefelter's syndrome is caused by XXY genotype. This genotype results from the union of a nondisjunct XX egg and a normal Y sperm or a normal X egg and an abnormal XY sperm. The individual has 47 (2n + 1) chromosomes. They are sterile males.

This is the correct option as both assertion and reason are true. Human skin colour has continuous variations. It is a polygenic trait with additive effect. Skin colour is primarily due to the presence of a pigment called melanin, which is controlled by at least 6 genes.

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. This shows incomplete dominance.

Recessive epistasis is when the recessive allele of one gene in a homozygous state masks the phenotypic expression of the dominant allele of another gene. Mendelian dihybrid ratio is 9:3:3:1.The modified F₂ phenotypic ratio of recessive epistasis is 9 : 3 : 4.

When both of the genes are needed to produce a wild type phenotype, it is known as complementary gene action. In sweet pea, cross between two white parents produces purple progeny due to complementary gene action. This purple colour of the flower is due to the anthocyanin pigment.

This is the correct option as statement 'a' is incorrect. According to this theory, chromosomes are the carriers of genetic information and hence, act as physical units of heredity.