Learning Objectives
- Understand Mendel's laws of inheritance
- Learn about incomplete dominance, codominance, and multiple allelism
- Study sex determination and sex-linked inheritance
- Understand chromosomal disorders and pedigree analysis
Key Concepts
Mendel's Laws
Gregor Mendel — Father of Genetics. Worked with garden pea (Pisum sativum), studied 7 contrasting traits. Law of Dominance: In a heterozygote, one allele (dominant) is expressed over the other (recessive). Law of Segregation (Purity of Gametes): During gamete formation, alleles of a gene segregate so each gamete gets only one allele. Monohybrid ratio: 3:1 (phenotypic), 1:2:1 (genotypic). Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes). Dihybrid ratio: 9:3:3:1.
Deviations from Mendelian Ratios
Incomplete Dominance: Neither allele is completely dominant. Heterozygote shows intermediate phenotype. Example: Snapdragon (Antirrhinum) — Red (RR) x White (rr) → Pink (Rr). F2 ratio: 1:2:1 (phenotypic = genotypic).
Codominance: Both alleles express equally in heterozygote. Example: ABO blood groups — IAIB genotype expresses both A and B antigens (blood group AB). Gene I has 3 alleles: IA, IB (codominant), i (recessive).
Pleiotropy: Single gene affects multiple phenotypic traits. Example: Sickle cell anemia gene (HbS) — affects RBC shape, causes anemia, organ damage.
Epistasis: One gene masks expression of another. Complementary genes (9:7 in sweet pea flower colour).
Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri. Genes are located on chromosomes. Linkage: Genes on the same chromosome tend to be inherited together — discovered by T.H. Morgan (experiments on Drosophila). Recombination frequency is used to map gene positions (genetic mapping). centiMorgan (cM) = unit of genetic distance.
Sex Determination
XX-XY type: Humans, Drosophila — females XX, males XY. Father determines sex of offspring. ZW-ZZ type: Birds, some reptiles — females ZW, males ZZ. XX-XO type: Grasshoppers — females XX, males XO. Haplodiploid: Honeybees — females diploid (2n=32), males haploid (n=16, from unfertilized eggs).
Sex-Linked Inheritance
Genes on sex chromosomes. X-linked recessive: Colour blindness (8% males, 0.4% females), Haemophilia (Royal disease — Queen Victoria's lineage). Affected father cannot pass to son (no X-X transmission); carrier mother can pass to sons. Y-linked: Hypertrichosis (ear hair in males).
Chromosomal Disorders
Aneuploidy: Abnormal chromosome number due to non-disjunction. Down syndrome (Trisomy 21): 47 chromosomes (2n+1); mental retardation, short stature, broad face. Klinefelter syndrome (XXY): 47 chromosomes; male with feminine features, sterile. Turner syndrome (XO): 45 chromosomes; female, short stature, sterile, underdeveloped features.
Summary
Mendel's laws of dominance, segregation, and independent assortment form the foundation of genetics. Deviations include incomplete dominance, codominance, and pleiotropy. Sex determination varies among organisms. Sex-linked disorders like colour blindness and haemophilia follow X-linked recessive inheritance. Chromosomal abnormalities result from non-disjunction during meiosis.
Important Terms
- Allele: Alternative forms of a gene at the same locus
- Linkage: Tendency of genes on same chromosome to be inherited together
- Pleiotropy: Single gene controlling multiple phenotypic effects
- Non-disjunction: Failure of chromosomes to separate during meiosis
- Pedigree analysis: Study of inheritance patterns in families using symbols
- Codominance: Both alleles equally expressed in heterozygote
- Aneuploidy: Having extra or missing chromosomes
- Carrier: Heterozygous individual for a recessive trait who doesn't show it
Quick Revision
- Mendel's ratios: Monohybrid 3:1; Dihybrid 9:3:3:1
- Incomplete dominance: 1:2:1 (snapdragon); Codominance: ABO blood groups
- Blood group AB = universal recipient (codominance of IA and IB)
- Colour blindness and Haemophilia: X-linked recessive
- Down syndrome: Trisomy 21; Klinefelter: XXY; Turner: XO
- Father determines sex (X or Y sperm)
- Linkage discovered by Morgan in Drosophila
- Sickle cell anemia: pleiotropic, autosomal recessive, heterozygote advantage (malaria resistance)