Genetics: Some Basic Fundamentals — ICSE Class 10 Biology Important Questions

Answer: (b) Phenotype.

Phenotype is the set of visible or expressed traits; genotype refers to the underlying genetic constitution.

Answer: (b) Garden pea (Pisum sativum).

Mendel chose the garden pea for its contrasting characters, ease of cultivation and self-pollinating nature.

Answer: (c) $9:3:3:1$9:3:3:1.

A dihybrid cross between two heterozygotes ($RrYy \times RrYy$RrYy × RrYy) gives an F2 phenotypic ratio of $9:3:3:1$9:3:3:1.

Answer: (b) $\dfrac{1}{4}$1/4.

The F1 are all $Tt$Tt. Selfing $Tt \times Tt$Tt × Tt gives $1\,TT : 2\,Tt : 1\,tt$1\,TT : 2\,Tt : 1\,tt, so dwarf ($tt$tt) plants are $\dfrac{1}{4}$1/4 of the F2.

Answer: (c) A is true but R is false. The father is heterogametic ($XY$XY) and produces X- and Y-bearing sperms, so he determines the sex; the mother ($XX$XX) produces only X-bearing eggs, so R is incorrect.

(i) Allele: One of the two or more alternative forms of a gene that occupy the same position (locus) on homologous chromosomes and control contrasting forms of a character, e.g. $T$T for tallness and $t$t for dwarfness.

(ii) Heterozygous: An organism having two different (unlike) alleles of a gene for a character, e.g. $Tt$Tt.

A dominant trait is one that expresses itself in the F1 hybrid even when only one allele for it is present, e.g. tallness ($T$T) in pea plants. A recessive trait is one that is masked in the hybrid and appears only when both alleles for it are present (homozygous condition), e.g. dwarfness ($t$t) in pea plants.

P generation: $TT$TT (tall) $\times$× $tt$tt (dwarf).

F1: All plants are $Tt$Tt — tall (tallness is dominant).

F1 selfed: $Tt \times Tt$Tt × Tt. Gametes: $T$T and $t$t from each parent.

F2 genotypes: $1\,TT : 2\,Tt : 1\,tt$1\,TT : 2\,Tt : 1\,tt.

F2 phenotypes: $3$3 tall $: 1$: 1 dwarf, i.e. the phenotypic ratio is $\mathbf{3:1}$3:1.

In humans, of the 23 pairs of chromosomes, one pair is the sex chromosomes. Females are $XX$XX (homogametic) and males are $XY$XY (heterogametic).

The mother produces only X-bearing eggs, while the father produces two kinds of sperm — X-bearing and Y-bearing.

• If an X sperm fertilises the egg $\rightarrow XX \rightarrow$→ XX → girl.
• If a Y sperm fertilises the egg $\rightarrow XY \rightarrow$→ XY → boy.

$XX\ (\text{mother}) \times XY\ (\text{father}) \rightarrow XX : XY = 1:1$XX (mother) × XY (father) → XX : XY = 1:1

Thus there is a $50\%$50\% chance of a boy and $50\%$50\% of a girl, and the father determines the sex of the child.

Law of dominance: When two organisms differing in a pair of contrasting characters (alleles) are crossed, only one character — the dominant one — appears in the F1 generation, while the other (recessive) remains hidden.

Law of segregation (law of purity of gametes): The two alleles of a character present in an organism separate during the formation of gametes, so that each gamete carries only one allele of the pair. The alleles remain pure and reunite at random during fertilisation. This is why the recessive character can reappear in the F2 generation.

P: $RRYY$RRYY (round, yellow) $\times$× $rryy$rryy (wrinkled, green).

F1: All $RrYy$RrYy — round and yellow seeds (both dominants expressed).

Gametes of F1 ($RrYy$RrYy): $RY,\ Ry,\ rY,\ ry$RY, Ry, rY, ry.

Punnett square (F1 $\times$× F1):

F2 phenotypes:

• $9$9 round, yellow
• $3$3 round, green
• $3$3 wrinkled, yellow
• $1$1 wrinkled, green

F2 phenotypic ratio $= 9:3:3:1$= 9:3:3:1. This illustrates Mendel's law of independent assortment — the two characters (seed shape and colour) are inherited independently of each other.

(a) Reasons for choosing the garden pea:
(i) It shows a number of clear, contrasting characters (e.g. tall/dwarf, round/wrinkled seeds).
(ii) It is naturally self-pollinating, giving pure lines, yet can be easily cross-pollinated by the experimenter.
(iii) It has a short life cycle and produces many offspring in one generation, allowing quick results over many generations.

(b) Definitions:

• Genotype: The genetic constitution (allelic make-up) of an organism for a character, e.g. $Tt$Tt.
• Phenotype: The observable or expressed characteristic of an organism, e.g. tall.
• Homozygous: An organism having two identical alleles of a gene for a character, e.g. $TT$TT or $tt$tt (also called pure).

(c) Test cross: A cross between an organism showing the dominant phenotype (whose genotype is unknown) and a homozygous recessive individual, used to find out whether the dominant organism is homozygous or heterozygous. If any recessive offspring appear, the tested parent is heterozygous.

(i) Man (heterozygous roller) $= Rr$= Rr; woman (non-roller) $= rr$= rr.

(ii) Cross $Rr \times rr$Rr × rr. Gametes: man $R, r$R, r; woman $r, r$r, r.

Children: $2\,Rr$2\,Rr (rollers) $: 2\,rr$: 2\,rr (non-rollers), i.e. genotypes $Rr$Rr and $rr$rr in a $1:1$1:1 ratio.

(iii) Tongue-rollers ($Rr$Rr) are $\dfrac{2}{4} = \mathbf{50\%}$2/4 = 50\% of the children.

(iv) Tongue-rolling is a dominant character (it is expressed in the heterozygous $Rr$Rr children).