Ch 5 covers Mendelian genetics — laws of inheritance, dominance patterns (complete, incomplete, co-dominance), linkage and crossing over, sex determination mechanisms, chromosomal abnormalities, and genetic disorders.
Law of Dominance: one allele masks the other. Law of Segregation: alleles separate during gamete formation (monohybrid ratio 3:1). Law of Independent Assortment: alleles of different genes assort independently (dihybrid ratio 9:3:3:1). Exceptions: incomplete dominance (1:2:1 phenotypic), co-dominance (ABO blood groups: Iᴬ, Iᴮ, i).
Linkage: genes on same chromosome tend to be inherited together (Morgan, Drosophila). Crossing over reduces linkage. Sex determination: XX-XY (humans), ZW-ZZ (birds). Sex-linked: X-linked recessive — colour blindness, haemophilia (more common in males). Chromosomal disorders: Down syndrome (trisomy 21), Turner (45, X0), Klinefelter (47, XXY).
Download: https://ncert.nic.in/textbook/pdf/lebo105.pdf | Complete book: https://ncert.nic.in/textbook/pdf/lebo1ps.zip
Males have only one X chromosome (XY). If that X carries a recessive allele (e.g., colour blindness), there is no second X to mask it with a dominant normal allele. Females (XX) need the recessive allele on both X chromosomes to express the disorder. One normal X is sufficient to mask the recessive allele, making females carriers instead of affected in most cases.
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