Prepared for B. Sc. III Botany students, based on the syllabus prescribed by Dr. Babasaheb Ambedkar Marathwada University, Aurangabad.

1. B.SC. III: SEMESTER-VI
PAPER XIX: GENETICS AND
BIOTECHNOLOGY
INHERITANCE OF LETHAL GENES
Dr. A.S. Naik,
Head, Dept. of Botany
SBES College of Science

2. Explanation of inheritance
of lethal genes.
This is a deviation from Mendel’s classic onohybrid inheritance.
◦ Definition: Genes which result in viability reduction of individual or
become cause of death of individuals that carry them.
◦ F2 shows genotypic ratio other than Mendel’s monohybrid ratio,
as 2:1 and two phenotypic classes also in same proportion.
◦ Some examples:
1. Genes controlling chlorophyll content in leaves.
2. Coat colour in mice: Yellow, black, agouti
3. Hutington disease in humans.

4. Types as per
time of
expression.

6. Types- contd:

7. Inheritance of lethal genes in Antirrhinum majus
(Snapdragon)
◦ E. Baur (1907) observed lethal gene in Snapdragon (Antirrhinum) and found that it is
characterized by variegated leaves.
◦ The “golden” variety on selfing gives rise to 2 types of offspring, golden and green in
the ratio of 2:1 instead of 3: 1.
◦ The golden ones are heterozygous and the green ones breed true being recessive
homozygous.
◦ The recessive allelic combination results in plants with no chlorophyll (albino) that die.

9. Inheritance
of lethal
genes in
Antirrhinum
• Genotype GG: Dark green leaves with
adequate chlorophyll
◦ Gg: golden-green leaves (auria)
◦ Genotype-gg: White (colourless) leaves.
◦ Genotype GG breeds true-only green leaved
plants
◦ If parents with aurea leaves crossed: Gg X Gg
◦ gametes of Gg: G and g
◦ Will produce F2 with changed genotypic and
phenotypic ratios.

10. Male
parent
Female parent
G g
G GG (Green) Gg (Aurea)
g Gg (Aurea) gg (albino, Lethal)
Genotypic Ratio of F2: GG ¼: Gg 2/4: gg ¼ (Lethal, dies)
Hence gets modified as 2:1
Phenotypic ratio: Aurea : Green-2:1
F2:

11. Other examples:
Coat colour in mice.
◦ Example of dominant lethal genes
◦ Dominant allele in homozygous
condition proves lethal.

12. Lethal genes
in humans.
◦ Examples of human diseases caused by recessive
lethal alleles include cystic fibrosis, sickle-cell anemia, and
achondroplasia. Achondroplasia is an autosomal dominant bone
disorder that causes dwarfism. While the inheritance of one
achondroplasia allele can cause the disease, the inheritance of
two recessive lethal alleles is fatal.
◦

13. Assignment on Lethal genes
1. In Snapdragon, Green plants are produced by gene ‘C’ that controls adequate production
of chlorophyll. It is dominant over its allele ‘c’, that produces no chlorophyll and plants are
albino. The homozygous condition of the ‘c’ is lethal due to total lack of chlorophyll whereas
heterozygous condition ‘Cc’ produces yellow-green/Auria plants.
◦ What genotypic and phenotypic proportion can be expected from the following crosses?
(a) CC X Cc (b) Cc X Cc (c) CC X cc (d) Cc X cc
◦ 2. In cattle, gene ‘A’ produces phenotypically normal legged animals and is dominant over
its allele ‘a’ producing animals with amputated legs. The gene ‘A” in homozygous recessive
condition-‘aa’ has a lethal effect on animals. Find out phenotypic proportion of the offspring
expected from the following crosses.
◦ (a) AA X Aa (b) Aa X Aa (c) AA X aa (d) Aa X aa