Class 12, Zoology PPT - 4. Principles of Inheritance and Variation
Class 12, Zoology PPT - 4. Principles of Inheritance and Variation
Chapter outline
4.1 Multiple alleles
4.2 Human blood groups
4.3 Genetic control of Rh factor
4.4 Sex determination
4.5 Sex linked inheritance
4.6. Karyotyping
4.7. Pedigree analysis
4.8. Mendelian disorders
4.9. Chromosomal abnormalities
4.10.Extra chromosomal inheritance
4.11.Eugenics, euphenics and euthenics
Learning objectives
➢ Learns the inheritance of multiple alleles
with reference to human blood groups.
➢ Understands the mechanism of sex
determination in human beings, insects and birds.
➢ Learns about sex linked (X and Y) inherited diseases in human beings.
➢ Understands the Mendelian disorders and diseases associated with chromosomal abnormalities.
➢ Gains knowledge on extra chromosomal
inheritance.
➢ Realises the significance of the applications of genetics in the improvement of human race.
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Principles of Inheritance – Quik Hints
1. Sex Determination
Humans: XX means female, XY means male ( it is clear)
Female: homogametic so every egg carries an X chromosome.
Male: heterogametic so each sperm carries either an X or a Y chromosome.
X + X = XX, which's female
X + Y = XY, which is male
Therefore the sperm decides whether the child will be female or male (this is the point)
Birds: ZZ means male ZW means female (just like humans but different letters)
Honeybees: haplodiploidy – a male comes from an egg, a female from a fertilized egg (interesting fact)
2. Blood Group Inheritance
The ABO system uses three alleles: Iᴬ, Iᴮ and Iᴼ or i.
Iᴬ and Iᴮ are codominant (both show up)
Iᴼ or i is recessive (it requires two copies to appear)
Blood group A: IᴬIᴬ or Iᴬ Iᴼ (these are the two ways)
Blood group B: IᴮIᴮ or IᴮIᴼ(likewise)
Blood group AB: IᴬIᴮ (both alleles present)
Blood group O: IᴼIᴼ or ii (two copies)
AB is a recipient for ABO red-cell transfusions with compatibility rules (this is useful)
O is a universal donor for ABO red-cell transfusions but Rh and cross-matching matter too (be careful)
Rh⁺ means the Rh antigen is present (keep this in mind)
Rh⁻ means the Rh antigen is absent (note this)
Rh incompatibility may cause erythroblastosis foetalis in some pregnancies.
3. Genetic Disorders
A. Mendelian Disorders:
Haemophilia is X-linked recessive (common in males)
Colour blindness is X-linked recessive (often seen in males)
Sickle-cell anaemia is autosomal recessive (requires two copies)
Phenylketonuria is autosomal recessive (two copies needed)
Thalassemia is autosomal recessive (two copies required)
B. Chromosomal Disorders
Down syndrome is Trisomy 21 ( chromosome)
Klinefelter syndrome is XXY in a male (extra X)
Turner syndrome is XO, in a female (missing X)
Edward syndrome is Trisomy 18 (extra chromosome)
Patau syndrome is Trisomy 13 (extra chromosome)
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