ICSE Class 10 Biology • Chapter 02
CONCEPT CHECK Two plants have the genotype TT and Tt. Will their phenotype be the same?
Ans: Yes. Both will be Tall because 'T' is dominant.
Gregor Mendel is known as the Father of Genetics. He worked on Garden Pea (Pisum sativum).
Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The one that expresses is Dominant, and the one suppressed is Recessive.
The two members of a pair of factors separate during the formation of gametes. Each gamete receives only one factor. Gametes are always pure for a character.
(Applicable to Dihybrid Cross) The distribution of one pair of factors is independent of the distribution of other pairs.
Cross involving only one pair of contrasting characters (e.g., Height: Tall vs Dwarf).
| Type | Ratio | Explanation |
|---|---|---|
| Phenotypic Ratio | 3 : 1 | 3 Tall : 1 Dwarf |
| Genotypic Ratio | 1 : 2 : 1 | 1 Homozygous Tall (TT) : 2 Heterozygous Tall (Tt) : 1 Homozygous Dwarf (tt) |
Cross involving two pairs of contrasting characters (e.g., Seed Shape & Color: Round Yellow vs Wrinkled Green).
Mechanism in Humans (XX-XY type):
BOARD QUESTION "Mothers are responsible for the sex of the child." Is this statement true? Justify.
Ans: False. The mother produces only one type of egg (X). The father produces two types of sperms (X and Y). It is the sperm that fertilizes the egg which determines the sex.
Traits controlled by genes located on the Sex Chromosomes (mostly X-chromosome). Examples: Colour Blindness & Haemophilia.
"Carrier" Concept: Females have two X chromosomes ($XX$). If one carries the defect ($X^c$) and the other is normal ($X$), the normal gene masks the defect. She is a Carrier (Phenotypically normal but carries the gene). Males ($XY$) have no backup X, so a single $X^c$ makes them Affected.
Mutation: A sudden, spontaneous change in the genetic make-up (gene or chromosome structure) of an organism. Example: Sickle Cell Anemia.
BOARD QUESTION Give the biological term for: The specific position of a gene on a chromosome.
Ans: Locus.
THINKING In a certain species of animals, black fur (B) is dominant over brown fur (b). Predict the genotype of the parents if the offspring are 50% Black and 50% Brown.
Ans: Heterozygous Black (Bb) x Homozygous Brown (bb). (Test Cross ratio 1:1).
CONCEPT CHECK Why is colour blindness more common in males than in females?
Ans: It is an X-linked recessive disorder. Males ($XY$) need only one defective gene ($X^c$) to express the disease, whereas females ($XX$) need two ($X^cX^c$).