Variation

Question 1

Gene

Answer 1

A portion of a DNA molecule that serves as the basic unit of heredity is called gene.

Question 2

Allele

Answer 2

Partner of a gene pair occupying the same gene locus is called allele.

Question 3

Locus

Answer 3

The position of a gene on the chromosome is called its locus

Question 4

Homozygous

Answer 4

Condition in which the alleles of a gene pair in an organism are identical is called homozygous

Question 5

Heterozygous

Answer 5

Condition in which the alleles of a gene pair in an organism are not identical is called heterozygous.

Question 6

Dominant

Answer 6

A trait which expresses itself in F1 generation is called dominant trait.

Question 7

Recessive

Answer 7

A trait which hides over itself in F1 generation is called recessive trait

Question 8

Phenotype

Answer 8

The form of appearance of a trait is called phenotype.

Question 9

Genotype

Answer 9

The genetic constitution underlaying a single trait or set of traits is called genotype.

Question 10

F1 generation

Answer 10

The offspring resulting from a parental cross are referred to as the first filial generation (or F1 generation).

Question 11

F2 generation

Answer 11

The offspring resulting from the cross between two F1 individuals (from F1 generation) are referred to as the second filial generation (or F2 generation)

Question 12

Epistasis

Answer 12

When a gene or gene pair at one locus interferes with or hides the effect caused by another gene or gene pair at another locus, such a phenomenon of gene interaction is called epistasis. E.g. Bombay phenotype.

Question 13

Dominance

Answer 13

The physiological effect of an allele over its partner allele on the same gene locus is called dominance.
Following are the types of dominance:
i. Complete Dominance
ii. Incomplete Dominance
iii. Co-dominance
iv. Over Dominance

Question 14

Complete Dominance

Answer 14

The type of inheritance in which both heterozygotes and dominant homozygotes have the same phenotype is called complete dominance. E.g. both ‘RR’ and ‘Rr’ produce round pea seeds.

Question 15

Incomplete Dominance

Answer 15

The dominance in which the phenotype of the heterozygote is intermediate between phenotypes of the two homozygotes is called incomplete dominance

Question 16

Co-Dominance

Answer 16

The type of inheritance in which heterozygotes fully express both alleles is called co-dominance
E.g. MN blood group system.

Question 17

Over Dominance

Answer 17

The type of inheritance in which the phenotypic expression of the heterozygote is greater than that of either homozygote is called over dominance
E.g. in Drosophila, the heterozygote (w+/w) has more quantity of fluorescent pigments in eyes than wild (w+/w+) or white eye (w/w) homozygotes

Question 18

Law of Segregation (monohybrid)

Answer 18

The two coexisting alleles for each trait in an individual segregate (separate) from each other at meiosis, so that each gamete receives only one of the two alleles. Alleles unite again at random fertilization of gametes when zygote is formed
E.G: Mendel crossed a true breeding tall plant (TT) and true breeding short plant (tt). This cross ( F1 generation) yielded only tall plants having a genotype of Tt.

Question 19

Law of Independent Assortment (dihybrid)

Answer 19

Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.
E.g: Two-hybrid rabbits are crossed. Both the rabbits have a genotype BbGg. Before breeding each rabbit produced gametes. During this, the alleles are separated and the copy of each chromosome is assigned to different gamete.

Question 20

Polygenic Inheritance

Answer 20

A polygenic trait is a characteristic, such as height or skin color, that is influenced by two or more genes. Because multiple genes are involved, polygenic traits do not follow the patterns of Mendelian inheritance. Many polygenic traits are also influenced by the environment and are called multifactorial

E.g. three different gene pairs, i.e. Aa, Bb and Cc at three different loci contribute to the wheat grain colour.

Question 21

Linkage (Genes)

Answer 21

The phenomenon of staying together of all the genes located on the same chromosome that are linked to each other is called gene linkage. E.g. genes for colour blindness, haemophilia, gout etc form one linkage group on human X - chromosome.
- There are 23 linkage groups in humans.
- Genes for colour blindness, haemophilia, gout etc form linkage group on human X-chromosome.
- Genes for sickle cell anaemia, leukemia and albinism make linkage group on human chromosome 11.

Question 22

Multiple Alleles

Answer 22

Multiple alleles are the alternative forms of the same gene so they influence the same trait. The wild-type allele is mostly dominant over the mutant alleles. The wild type is considered the standard and all other alleles are considered variants.

• E.g. IA, IB and i are the three multiple alleles for ABO blood group system.
• Some genes may have as many as 300 alleles.
• Any two of the multiple alleles can be present in the genome of a diploid organism, but a haploid organism can have just one of them in its genome.

Question 23

RH Factor

Answer 23

Rhesus (Rh) factor is an inherited protein found on the surface of red blood cells.
* If a person has this blood protein, his is Rh positive.
* If a person lacks this blood protein, he is Rh negative.

Question 24

Sex Determination

Answer 24

1. XY-XX Sex determination System
   E.g: Humans, Drosophila
2. XO-XX Sex determination System
   E.g: Grasshopper, Protenor bug
3. XX-XY/ZZ-ZW Sex Determination System
   E.g: Birds, Butterfly, Moth

Question 25

Color Blindness
(Cone Cells in Eyes)

Answer 25

Normal trichromatic colour vision is based on three different kinds of cone cells in the retina, each sensitive to only one of the three primary colours, red, green or blue. Each type of cone cell has specific light absorbing proteins called opsins.

Question 26

Genes for Opsin

Answer 26

The genes for red and green opsins are on X chromosome, while the gene for blue opsin is present on autosome 7. Mutations in opsin genes cause three types of colour-blindness

Question 27

Dichromacy

Answer 27

A dichromat can perceive two primary colours but is unable to perceive the one whose opsins are missing due to mutation.
1. Protanopia is red blindness.
2. Deuteranopia is green blindness
3. Tritanopia is blue blindness

Question 28

Protanomalous and Deuteranomalous

Answer 28

Some people can detect red and green but with altered perception of the relative shades of these colours. They have abnormal but still partially functional opsins. They are protanomalous and deuteranomalous for red and green weakness respectively.

Question 29

Monochromacy

Answer 29

A monochromat can perceive one colour. Monochromacy is true colour-blindness. Blue cone monochromacy is an X-linked recessive trait in which both red and green cone cells are absent. That is why it is also called red-green colour-blindness