Principles of Inheritance and Variation
1. Mendel - The Gardener Who Counted Peas
Imagine a monk in a garden who loves counting. That was Gregor Mendel. He took pea plants because they have clear opposite characters like tall vs dwarf, yellow seed vs green, round vs wrinkled, and they self pollinate easily.
First he crossed tall TT with dwarf tt. All children F1 were tall Tt. So tall hid dwarf, tall dominant. Then he allowed F1 to self marry Tt x Tt. In F2 he got 3 tall 1 dwarf. Look inside genes: 1 TT pure tall, 2 Tt mixed tall, 1 tt dwarf. Ratio 3:1 looks, 1:2:1 genes. This gave two rules: law of dominance one character hides other, law of segregation two copies of gene separate during gamete formation each gamete gets one, then random fusion.
Then he took two characters together: yellow round YYRR with green wrinkled yyrr. F1 all yellow round YyRr. F2 selfing gave 9 yellow round 3 yellow wrinkled 3 green round 1 green wrinkled 9:3:3:1. Means colour and shape go independently, law of independent assortment, works when genes far or different chromosomes.
Mendel pea 7 traits TT tt. Monohybrid F1 all tall Tt F2 3:1 1:2:1 dominance segregation alleles separate gamete one. Dihybrid YYRR x yyrr F1 YyRr F2 9:3:3:1 9 Y_R_ 3 Y_rr 3 yyR_ 1 yyrr independent assortment far genes different chromosome.
Mendel garden pea 7 traits. TT x tt → F1 all Tt tall, F2 3 tall 1 dwarf 1:2:1 genes dominance segregation. YYRR x yyrr → 9:3:3:1 independent assortment. Count large = key. No date, clean.
2. When Mendel Rules Bend - Pink, Both Together and Many Forms
Mendel said one allele fully hides other, but nature sometimes does middle path.
Snapdragon flower red RR x white rr gives pink Rr in F1, not red. F2 1 red 2 pink 1 white. Ratio 1:2:1 same as genes because pink intermediate, called incomplete dominance, like mixing red and white paint gives pink.
Co-dominance both show together. Best example ABO blood group. Gene I has three forms IA makes A antigen, IB makes B antigen, i makes none. IAIA or IAi = A blood, IBIB or IBi = B, ii = O, IAIB = AB both A and B antigens together like both colours shining. So IA and IB co-dominant to each other both dominant over i.
Multiple alleles means more than two forms of same gene in population, like ABO has three IA IB i, but one person has only two.
Pleiotropy one gene many effects. Example phenylketonuria PKU one enzyme missing causes mental retardation plus light hair skin because phenylalanine not converted, one defect many problems like one broken tap floods many rooms.
Incomplete dominance RR red x rr white → Rr pink F2 1:2:1 phenotype=genotype intermediate. Co-dominance IAIB AB both antigens IA IB co-dominant i recessive IAIA IAi A IBIB IBi B ii O IAIB AB. Multiple alleles >2 forms same locus population ABO 3. Pleiotropy one gene many effects PKU enzyme missing mental hair skin.
Red x white = pink → incomplete 1:2:1. IAIB = AB both antigens → co-dominance. ABO IAIA IAi A, IBIB IBi B, ii O, IAIB AB. Multiple alleles >2 in population. Pleiotropy one gene many effects PKU one enzyme many symptoms.
3. Genes on Chromosomes, Linkage and Many Genes One Trait
Sutton and Boveri said chromosomes are like buses carrying genes as passengers, they behave like Mendel factors during meiosis.
Morgan worked on tiny fruit fly Drosophila with red eyes and found white eye male, he saw genes on same chromosome tend to stay together, called linkage. Parental types more than 98%, new recombinants less than 2% when genes close together on same chromosome. If genes far apart, recombination more up to 50% like independent assortment. Recombination happens due to crossing over in meiosis exchange of parts.
Distance between genes measured by recombination percent 1% = 1 map unit centimorgan cM, used to make chromosome maps.
Polygenic inheritance many genes control one trait like human skin colour controlled by three genes, height also. Each gene adds small amount, continuous variation bell curve, not just tall dwarf two types but many shades intermediate. Environmental effect also adds like sun tanning darkens skin.
Chromosomal theory Sutton Boveri chromosomes carry genes meiosis explains Mendel. Linkage Morgan Drosophila white eye miniature wing same chromosome parental > recombinant close strong linkage low recombinant far high up to 50% crossing over recombination map 1% =1cM. Polygenic many genes one trait additive continuous bell curve skin 3 genes height quantitative environment.
Chromosomes = buses genes passengers Sutton Boveri. Linkage same chromosome stay together Morgan white eye fly. Close = less recombinant <2%, far = more up to 50%. 1% recombination = 1 map unit cM. Polygenic many genes one trait skin colour 3 genes bell curve continuous.
4. How Boy or Girl Decided and Reading Family Tree
Sex determination different in different animals.
Human XX female XY male, Y has SRY gene that makes testis. So male decides sex of baby because he gives X or Y, female always X.
Birds opposite ZZ male ZW female female decides. Honeybee haplodiploidy fertilized egg 2n becomes female queen or worker, unfertilized egg n becomes male drone by parthenogenesis without father. Some reptiles temperature decides hot female cold male.
Pedigree is family tree drawing with square male circle female shaded affected half shaded carrier. It helps find pattern. Autosomal dominant appears every generation males females equally affected no skipping affected child has affected parent. Autosomal recessive can skip generations unaffected parents can have affected child often in relatives marriage consanguineous. X-linked recessive more males affected, carrier mother passes to son, father cannot pass X to son, so no male to male transfer, example haemophilia colour blindness. X-linked dominant affected father passes to all daughters. Easy trick: every generation = dominant, skip = recessive, mostly males = X-linked recessive, father to all daughters = X-linked dominant.
Sex determination human XX female XY male SRY Y male decides, birds ZZ male ZW female female decides, bee fertilized 2n female queen worker unfertilized n drone parthenogenesis temperature reptile. Pedigree square male circle female shaded affected carrier autosomal dominant every gen equally, recessive skip consanguineous, X-linked recessive more males carrier mother→son no male-to-male, X-linked dominant father→all daughters.
Human XX female XY male SRY male decides, bird ZZ male ZW female female decides, bee fertilized female 2n unfertilized male n. Pedigree square male circle female shaded affected. Every gen = dominant skip = recessive mostly males = X-linked recessive no male-to-male = X-linked.
5. Diseases Due to Genes
Two types: single gene mutation Mendelian and chromosome number problem chromosomal.
Haemophilia X-linked recessive blood does not clot Factor VIII missing, more males, father passes carrier to daughter, prolonged bleeding. Colour blindness X-linked recessive red green not seen 8% males 0.4% females.
Sickle cell anemia autosomal recessive chromosome 11 HBB gene glutamic acid to valine, RBC becomes sickle shape like moon, causes pain, 25% affected if both parents carriers, carrier advantage resists malaria. Thalassemia also autosomal recessive globin chain less alpha or beta, small pale RBC anemia common in Mediterranean. Phenylketonuria PKU autosomal recessive enzyme phenylalanine hydroxylase missing phenylalanine becomes toxic brain damage treated by low phenylalanine diet early detection.
Chromosomal: Down syndrome extra chromosome 21 trisomy 47 total short stature flat face single palm crease mental retardation. Klinefelter 47 XXY male extra X tall less beard small testis gynecomastia breast growth infertile. Turner 45 XO female only one X short webbed neck no ovary shield chest sterile.
Mendelian single gene: Haemophilia X-linked recessive Factor VIII bleeding, Colour blindness X-linked recessive 8% M, Sickle autosomal recessive chr11 Glu→Val sickle 25% carrier malaria resistant, PKU autosomal recessive Phe hydroxylase neurotoxic diet, Thalassemia autosomal recessive globin microcytic Med. Chromosomal Down trisomy21 47 +21 flat palm crease, Klinefelter 47 XXY male gynecomastia infertile, Turner 45 XO female webbed neck no ovary.
Haemophilia + colour blindness X-linked recessive males more father→carrier daughters. Sickle PKU thalassemia autosomal recessive 25% if both carriers. Down 47 +21 trisomy, Klinefelter 47 XXY male tall infertile, Turner 45 XO female short sterile. No date clean.