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Heredity

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Why did Mendel choose Garden Peas?
1. Short life cycle (can study many generations).
2. Distinct contrasting characters (Tall/Short, Round/Wrinkled).
3. Self-pollinating (easy to get pure lines) but can be cross-pollinated artificially.

Mendel's 7 Pairs of Contrasting Characters:

Character	Dominant Trait	Recessive Trait
Stem Height	Tall	Dwarf (Short)
Flower Colour	Violet	White
Flower Position	Axial	Terminal
Pod Shape	Inflated (Full)	Constricted
Pod Colour	Green	Yellow
Seed Shape	Round	Wrinkled
Seed Colour	Yellow	Green

1. Mendel�s Experiments

A. Terminologies (Must Know)

            Gene: Unit of heredity that controls a trait (segment of DNA).
Allele: Alternative forms of a gene (e.g., T for Tall, t for Short).
Genotype: Genetic makeup (TT, Tt, tt).
Phenotype: Physical appearance (Tall, Short).
Homozygous: Having two identical alleles for a trait (e.g., TT or tt).
                    (Pure).
                
Heterozygous: Having two different alleles for a trait (e.g., Tt).
                    (Hybrid).
                

        

B. Monohybrid Cross (One Trait: Height)

Parent: Pure Tall (TT) x Pure Short (tt)
Gametes: (T) and (t)
F1 Generation: All Tt (Tall) -> Law of Dominance.
F2 Generation (Selfing Tt x Tt):

Gametes	T	t
T	TT (Tall)	Tt (Tall)
t	Tt (Tall)	tt (Short)

Monohybrid Ratios:
� Phenotypic Ratio: 3 Tall : 1 Short (3:1)
� Genotypic Ratio: 1 TT : 2 Tt : 1 tt (1:2:1)

PYQ Trend (Animals):
In a cross between Black Fur (BB) and White Fur (bb) rabbits, F1 will be Black (Bb). In F2, 25% will be White (bb). Concept remains the same as plants.

C. Dihybrid Cross (Two Traits: Shape & Colour)

Parent: Round Yellow (RRYY) x Wrinkled Green (rryy)
F1 Generation: All Round Yellow (RrYy).
F2 Generation Ratio: 9:3:3:1

9 Round Yellow (Parental)
3 Round Green (New Combination)
3 Wrinkled Yellow (New Combination)
1 Wrinkled Green (Parental)

Conclusion: Law of Independent Assortment (Traits are inherited independently).

D. Mendel's Three Laws of Inheritance (Important Theory)

            
                    Law of Dominance:

                    In a cross between two parents with contrasting traits (e.g., TT x tt), only one characteristic
                    appears in the F1 generation. This expressed trait is called
                    Dominant (Tall), and the hidden one is Recessive (Dwarf).
                
                    Law of Segregation (Law of Purity of Gametes):

                    During gamete formation, the two alleles of a gene separate (segregate) from each other so that
                    each
                    gamete receives only one allele. This ensures gametes are pure for a trait.
                
                    Law of Independent Assortment:

                    In a Dihybrid cross (two pairs of traits), the inheritance of one pair of characters (e.g.,
                    Shape)
                    is independent of the other pair (e.g., Colour). (Example: Round can go with
                    Green
                    or Yellow).

2. Sex Determination in Humans

Figure 4.1: Sex Determination Flowchart

In humans, sex is determined by the sperm (Male Gamete).

Male: XY chromosomes. Produces two types of sperms (50% X and 50% Y).
Female: XX chromosomes. Produces only one type of egg (100% X).

Sperm (Father)	Egg (Mother)	Zygote	Child Sex
X	X	XX	Female (Girl)
Y	X	XY	Male (Boy)

Conclusion: There is a 50-50 chance of a boy or a girl. The sex of the child depends entirely on which sperm (X or Y) fertilizes the egg.

Humans have 23 pairs of chromosomes. 22 pairs are Autosomes. 1 pair is Sex Chromosome.

Mechanism:
� Mother produces eggs only with X chromosome.
� Father produces sperms with X (50%) or Y (50%).
� If Sperm (X) fertilizes Egg (X) ? Girl (XX).
� If Sperm (Y) fertilizes Egg (X) ? Boy (XY).

Who determines the sex of the child?
The Father. Because the mother provides only X chromosomes, while the father provides either X or Y, which decides the outcome. It is a matter of chance (50-50 probability).

3. Acquired vs Inherited Traits

Inherited Traits	Acquired Traits
Passed from parents to offspring.	Developed during lifetime of individual.
Cause change in DNA of Germ cells.	Involve changes in Somatic (Body) cells only. No DNA change.
Examples: Eye colour, Skin colour, Blood group.	Examples: Muscular body of a wrestler, Learning French, Cut tail of a mouse.

?? TRICKY PRACTICE ZONE (Heredity)

A violet flower (VV) is crossed with a white flower (vv). Why do we not see ANY white flowers in the F1 generation? Hint: Law of Dominance. White (recessive) is hidden by Violet (dominant) in F1 (Vv).
"A wrestler's son is not born with muscular muscles." Explain scientifically. Hint: Muscles are an Acquired Trait (change in Somatic cells), which does not change the DNA of Germ cells (sperm/egg).
In a population of mice, does the environment select the survival of specific traits? (Give an example). Hint: Yes. If bushes turn brown, brown mice survive better than green mice (Natural Selection).