Introduction
Human genetics is the study of inheritance and variation in humans, focusing on how traits are transmitted from parents to offspring. Since controlled breeding experiments cannot be performed in humans, pedigree analysis is a vital tool for understanding inheritance Patterns, identifying carriers of genetic disorders, and predicting the probability of traits in future generations.
A pedigree chart is a symbolic family tree that traces the inheritance of specific traits across multiple generations. It helps geneticists determine whether a trait is dominant, recessive, autosomal, sex-linked or mitochondrial.
1. Making a Pedigree Chart and Showing Dominant and Recessive Traits
A pedigree chart is a diagram that shows how a particular trait or disorder is passed down through generations in a family. Each member is represented by standard symbols such as squares (males), circles (females), and shaded shapes (affected individuals).
Dominant Traits: Appear in every generation; e.g., polydactyly.
Recessive Traits: May skip generations; e.g., albinism.
2. Study of Autosomal Traits in a Pedigree
Autosomal traits are controlled by genes on non-sex chromosomes (1–22). Both males and females are equally affected.
Autosomal Dominant: Appears in every generation; example – Huntington’s disease.
Autosomal Recessive: May skip generations; example – Sickle cell anemia.
3. Study of Sex-Linked Traits in a Pedigree
Sex-linked inheritance involves genes located on the sex chromosomes (X or Y).
X-linked disorders are more common due to the larger number of genes on the X chromosome.
Examples: Hemophilia and Color blindness (X-linked recessive), Hairy pinna (Y-linked).
4. Study of Mitochondrial Traits in a Pedigree
Mitochondrial inheritance (maternal inheritance) involves genes in mitochondrial DNA.
Only mothers pass these traits to their children since sperm contribute negligible cytoplasm during fertilization.
Examples: Leber’s Hereditary Optic Neuropathy, MELAS Syndrome.
5. Genetic Counselling
Genetic counselling helps individuals understand and adapt to the medical, psychological, and familial implications of genetic disorders. It provides information about inheritance risks, testing options, and prevention strategies.
Types: Prospective, Retrospective, and Prenatal counselling.
FAQs
Q: What is the purpose of pedigree analysis?
A: It helps determine the mode of inheritance and predict the occurrence of genetic traits in future generations.
Q: Why are X-linked disorders more common in males?
A: Because males have only one X chromosome, a single recessive mutation expresses the disorder.
Q: What is mitochondrial inheritance?
A: It is the inheritance of genes located in mitochondrial DNA, passed only from the mother.
MCQs
1.Which of the following represents an autosomal dominant disorder?
a) Albinism
b) Hemophilia
c) Huntington’s disease
d) Color blindness
Answer: c) Huntington’s disease
2.Which of the following traits shows maternal inheritance?
a) Color blindness
b) Hemophilia
c) Leber’s hereditary optic neuropathy
d) Sickle cell anemia
Answer: c) Leber’s hereditary optic neuropathy
3. In a pedigree, a carrier female and normal male for hemophilia can produce:
a) All normal children
b) All affected daughters
c) 50% affected sons
d) All affected sons
Answer: c) 50% affected sons
4. Which of the following is NOT a feature of autosomal recessive inheritance?
a) Trait skips generations
b) Males and females equally affected
c) Affected individuals always have affected parents
d) May occur in siblings
Answer: c) Affected individuals always have affected parents
Worksheet
1. Draw a pedigree chart for an autosomal recessive trait showing three generations.
2. Identify dominant and recessive inheritance from a given pedigree.
3. Explain the inheritance of color blindness using a pedigree.
References
1. Griffiths, A.J.F., et al. (2020). Introduction to Genetic Analysis. W.H. Freeman & Co.
2. Klug, W.S., Cummings, M.R., et al. (2021). Concepts of Genetics. Pearson Education.
3. Hartl, D.L., & Ruvolo, M. (2020). Genetics: Analysis of Genes and Genomes. Jones & Bartlett Learning.
4. Vogel, F., & Motulsky, A.G. (2010). Human Genetics: Problems and Approaches. Springer-Verlag.
5. https://www.zoologys.co.in
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