Explains monohybrid and dihybrid with their phenotypic and genotypic ratios

1. MENDELIAN GENETICS - Continued
Dr. Valli Syam

2. MONOHYBRID CROSS
(Cross involving Single pair of Contrasting Characters)

3. Yy
yy
YY
Y y
Y
y
YY Yy
Yy yy
y
Y y
Parental
Generation
Gametes
F1 Generation
Gametes
F2
Generation
Phenotypic ratio 3 : 1
Yellow : Green
Genotypic ratio 1 : 2 : 1
YY Yy yy
Homozygous Heterozygous Homozygous
Dominant Recessive
SEED COLOUR
Y

4. Rr
RR
R r
R
r
RR Rr
Rr
R r
R r
Parental
Generation
Gametes
F1 Generation
Gametes
F2
Generation
Phenotypic ratio 3 : 1
Round : Wrinkled
Genotypic ratio 1 : 2 : 1
RR Rr rr
Homozygous Heterozygous Homozygous
Dominant Recessive
rr
rr
SEED SHAPE

5. LAW OF SEGREGATION
When a two or more pairs of contrasting alleles are brought together
in a heterozygote (hybrid) the two members of the allelic pair remain
together without being contaminated and during gametogenesis the
two separate from each other and only one enters each gamete.

6. DIHYBRID CROSS
(Cross involving Two pairs of Contrasting Characters)

7. YYRR yyrr
YR yr
YyRr
YR Yr yR yr
YR Yr yR yr
yr
yR
Yr
YR YYRR YYRr YyRR
YyRr
YyRr
YyRr
YyRr
YYrr
YYRr Yyrr
YyRR
Yyrr
yyRR yyRr
yyRr yyrr
Parental
Generation
Gametes
F1 Generation
Gametes
F2 Generation
Phenotypic ratio
9 : 3: 3 : 1
Yellow Round : Yellow Wrinkled : Green Round : Green Wrinkled
Genotypic ratio
1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
SEED COLOUR & SEED SHAPE

8. LAW OF INDEPENDENT ASSORTMENT
When two (or more) characteristics are studied simultaneously,
the alleles of two (or more) different genes assort independently
during gamete production, giving different traits an equal
opportunity of occurring together.

9. REFERENCES
• Genetics – VB Rastogi – 4th Edition – Medtech Publishers -2019
• Cell Biology, Genetics, Molecular Biology, Evolution And
Ecology - P.S. Verma &V.K. Agarwal - S. Chand & Company Ltd.
2005
• Genetics A conceptual approach – Benjamin A Pierce – W.H.
Freeman & Company – 2012