Cue Column | Note-Taking Column
What determines sex in humans? | In the XX-XY system, the father is the heterogametic parent (XY). He produces X-bearing sperm → daughter (XX) and Y-bearing sperm → son (XY). The mother (XX) is homogametic — all eggs carry one X. Therefore the father's gamete determines offspring sex.
What is hemizygosity? | Males have only ONE X chromosome. There is no corresponding allele on the Y chromosome to mask recessive alleles. Any allele on the X chromosome of a male — dominant or recessive — is expressed. This is why X-linked recessive conditions are more common in males.
How do chromosomal disorders arise? | Non-disjunction during meiosis — failure of chromosomes to separate properly. Results in n+1 gametes (trisomy when fertilized) or n-1 gametes (monosomy when fertilized). Maternal non-disjunction is the more common source (~95% of trisomy 21 cases).
What are the four NEET chromosomal disorders? | Down (47, +21) = trisomy 21; Turner (45, XO) = monosomy X, female; Klinefelter (47, XXY) = male (Y present, SRY gene); Super Female (47, XXX) = female, often normal.
What is a Mendelian disorder? | Caused by mutation in a single gene. Follows Mendelian ratios. Detected by DNA/biochemical testing. Examples: sickle cell anaemia, PKU, thalassemia, haemophilia A, colour blindness.
What is the sickle cell mutation? | Point mutation in HBB gene (chromosome 11): codon 6 GAG→GUG in mRNA (Glu→Val). HbS polymerizes under low → sickle-shaped RBCs → vaso-occlusion + haemolytic anaemia.
What distinguishes X-linked recessive pedigrees? | More affected males; no male-to-male transmission (fathers pass Y to sons); carrier females; affected female needs carrier/affected mother + affected father.
Summary (Bottom Section)
Sex determination varies across organisms: XX-XY (human/Drosophila — male heterogametic), XX-XO (grasshopper — male heterogametic, no Y), ZW-ZZ (birds/butterflies — female heterogametic). X-linked recessive disorders (haemophilia A, colour blindness) are rare in females due to hemizygosity in males. The four chromosomal disorders tested in NEET are Down (47,+21), Turner (45,XO), Klinefelter (47,XXY), and Super Female (47,XXX). Mendelian disorders (sickle cell, PKU, thalassemia) arise from single-gene mutations. Pedigree analysis identifies inheritance pattern from family trees using sex ratio, generation-skipping, and transmission routes.
