Principles of Inheritance & Variation Test 1

The innate tendency of offspring to resemble their parents is called heredity. The offspring resembles to parent due to same genetic combination inherited from parents.

• Monohybrid ratio is the ratio of different phenotypic traits obtained on hybridizing single pair of trait. It is 3:1.
• When tall pea plant is crossed with dwarf, the offspring obtained are in 3/4 tall and 1/4 dwarf.

Gregor Mendel is known as the Father of Genetics. He experimented on pea plants and discovered the basic inheritance rules. 

• Mendel studied the common garden pea plant, Pisum sativum because it was easy to cultivate and had a relatively short life cycle of 3 months. The plant exhibited discontinuous characteristics such as flower color and pea texture.
• Owing to its anatomy, it was easy to control the self-pollination of the plant and cross-fertilization between desired parents could be accomplished artificially.
• The presence of pure breeding varieties and easily visible contrasting characters and presence of F₁ fertile hybrids were the additional advantageous characters for which Mendel chose garden pea as experimental material. 

• The physical appearance of a character is called as phenotype.
• The genetic make of individual is called genotype. Tallness, round, wrinkled, yellow etc. are physical appearance.

Phenotype Example:

In monohybrid and dihybrid cross F2 generation is obtained by selfing of F1 hybrids as male and female.

• In a dihybrid cross, two new kinds of plants with new traits are formed.
• This is possible only when all traits assort independently and recombine to form a new combination.

• Since the yellow seeds are dominant to green, the allele for yellow seeds should be represented with upper case letter while the one for green seeds should be written in lower case letter.
• Let's suppose that the allele for yellow seed is Y and that for green seeds is y. Hence the genotype of the heterozygous yellow seeded plant will be "Yy" and that for green seeded plant will be "yy" (as the recessive allele is expressed only in homozygous conditions).
• The cross between heterozygous yellow seeded plant and green seeded plant will produce 50% yellow seeded plants and 50% green seeded plants.

Genotype of pure tall plant with green pod = TTGG
Genotype of pure dwarf plant with yellow pod = ttgg
P generation:  TTGG    X  ttgg
F1 generation:TtGg(tall plant, green pod)
Selfing: TtGg X  TtGg

As can be seen from the table total 4 short plants with genotypes ttGG, ttGg, ttGg and ttgg are produced.

In a test cross:

• The organism with the dominant trait is always crossed with an organism with the recessive trait. 
• If any offspring show the recessive trait, the unknown genotype is heterozygous. 
• If all the offspring have the dominant trait, the unknown genotype is homozygous dominant.

• Mendenlian Inheritance: The transfer of traits from one generation into another into the next generation in the form of a particulate structure is called Mendelian Inheritance.
  Example: Cross based on height or colour and shape of seeds.
• Monogenic inheritance: It is also called the Monohybrid cross. Cross between a homozygous dominant parent and a homozygous recessive parent on the basis of a single character.
  Example: Cross based on the height of the pea plants.
• Complementary genes: Two independent pairs of genes, which interact to produce a trait together, but each dominant gene alone does not show its effect, are called complementary genes.
  Example: Cross based on colour of flowers in Sweet potato.
• Polygenic/Quantitative inheritance: Controlling a character by two or more genes is called Multifactor inheritance.
  Example: Inheritance of skin colour of human beings.

So, the correct option is 'B'  ‘ Polygentc/Quantitative inheritance.

Co-dominance is the phenomenon that deviates from Mendel’s law of inheritance. Both the alleles appear in offspring instead of one as in Mendel’s experiment. ABO blood grouping in human being is example of co-dominance in which both I^A and I^B appear simultaneously to form AB blood type.

For ABO system of blood groups, allele I^A produces N-acetylgalactosamine transferase enzyme which recognises H- antigen present in RBC membrane and adds N-acetylgalactosamine to sugar parts of H antigens to form A antigen.

The allele I^B produces galactosyl transferase enzyme which recognized H antigen to form B antigens. Allele i does not produce any sugar or antigen.

I^A  and I^B are completely dominant over i, in other words antigens A and B are produced. This is because of co-dominance. These antigens determine the type of blood group. Blood group A has antigens B have antigen, AB has both antigens while blood group. Blood group A have antigen A, group B have antigen B, AB has both antigens while blood group O do not carry any antigens.

Thus, six genotypes and four phenotypes are possible.

If the genotype is AaBb the alleles that will be produced will be AB, Ab, aB, ab, since there are two diallelic characters in the genotypes the person must be heterozygous for both genes. AABB is homozygous. So, the correct answer is "AaBb".

F1 pink flowered plants are heterozygous. When selfing is done the genotype and phenotype develops as 1:2:1 as 40 red, 80 pink and 40 white flowering plants.

• In Snapdragon flower, cross between true-breeding white and red coloured flower produce pink coloured flower in the F1 generation.
• This happens due to the incomplete dominance of alleles over the other.

• Crossing over brings recombination or new combination of genes and hence variations.
• It occurs during cell division in prophase stage of meiosis-I.

• Plotting of specific genes on the chromosome is known as chromosome map or linkage map or genetic map.
• It shows the position of genes on chromosome in respect to others.

• When two genes are situated very close to one another on chromosome, hardly any cross-over are produced.
• Such genes are called linkage and do not separate from each other during gamete formation.

Sex chromosomes of human beings are heteromorphic as they are of different size. Y chromosome is smaller than X chromosome in size.

• Bridge between one generation and the next is through sperm and ovum.
• The sperm provides only the nuclear part to the zygote. As such hereditary characters must be carried by nuclear materials.
• There is fusion of the sperm and egg nuclei during fertilization.
• Both the sperm and egg contribute equally in the heredity of the offspring. The sperm provides only the nuclear part to the zygote.
• As such hereditary characters must be carried by nuclear materials. Therefore, chromosomes must carry the hereditary traits.

Hence , the correct answer is option B.

• Sex determination depends on the male gamete because it contains XY Chromosome while XX chromosome of female gamete i.e. after coitus if male gamete produces X and female produce X then female childbirth, while XY Produce male child.
• XXY chromosome is due to an additional copy of X chromosome results trisomy of sex chromosome, represent (2n+1) means (46+1=47) in this, individual have masculine, but feminine development occurs ( breast i.e. gynecomastia), in this male are sterile, chromosomal disorder study easily by karyotypes ( study about - shape, size, and number).
• But, the YY chromosome doesn't occur. So, the correct options will be A.

In grasshopper, sex determination is of XO type, in which the males have only one X-chromosome besides the autosomes whereas females have a pair of X-chromosomes.

Male produce two kinds of sperms, half carrying X and half carrying Y sex chromosome besides 22 autosomes. If ovum is fertilized by sperm carrying X chromosome the sex of child developed is female.

Henking trace a specific structure in 50% of sperms. He gave the name x-body to this structure but he was not able to explain the significance of this structure. Further, investigation revealed that x-body was a chromosome, called X-chromosome.