Principles of Inheritance and Variation - Comprehensive Notes

PRINCIPLES OF INHERITANCE AND VARIATION

  • Heredity: Transmission of heritable traits from parents to offspring.
  • Genetics: Branch of biology dealing with the principles and mechanisms of inheritance and variation.
  • Inheritance: Basis of heredity where traits are passed from parents to offspring.
  • Continuity of the gene pool is maintained by inheritance.
  • Genes: Basic unit of inheritance located on chromosomes.
  • Variation: Exists among individuals of a species, due to:
    • Crossing over
    • Recombination
    • Mutation
    • Environmental effects on gene expression
  • Gregor Johann Mendel: "Father of Genetics", performed experiments on garden peas.
    • Used 14 true-breeding pea plants with seven distinguishable characters, each with two opposite traits.
    • Referred to genes as "factors" passed from parents to offspring.
    • Alleles: Genes that code for a pair of opposite traits.
  • Seven pairs of contrasting characters were studied by Mendel.

LAWS OF INHERITANCE

  • Law of Dominance (First Law of Inheritance):
    • Each character is controlled by distinct units called factors, occurring in pairs.
    • In heterozygous pairs, one allele dominates the other.
  • Law of Segregation:
    • Two alleles for each trait segregate (separate) during gamete formation.
    • During zygote formation, alleles combine randomly with other alleles.
  • Law of Independent Assortment:
    • Inheritance of two or more genes simultaneously shows their distribution in gametes and subsequent progeny is independent of each other.

DIHYBRID CROSS

  • Example using seed shape and color:
    • R = Dominant allele for round seed shape
    • r = Recessive allele for wrinkled seed shape
    • Y = Dominant allele for yellow seed color
    • y = Recessive allele for green seed color
  • Parental generation: RRYY x rryy
  • F1 generation: All RrYy
  • Punnett square for F2 generation (resulting from RrYy x RrYy cross):
    • Female gametes: 1/4 RY, 1/4 Ry, 1/4 rY, 1/4 ry
    • Male gametes: 1/4 RY, 1/4 Ry, 1/4 rY, 1/4 ry
    • Genotypic ratio: 9/16 R-Y-, 3/16 R-yy, 3/16 rrY-, 1/16 rryy
    • Phenotypic ratio: 9/16 round yellow, 3/16 round green, 3/16 wrinkled yellow, 1/16 wrinkled green

TEST CROSS

  • Used to determine if an individual with an unknown dominant phenotype is homozygous or heterozygous.
  • Cross the individual with a known homozygous recessive individual (bb).
  • If unknown is homozygous (BB):
    • All offspring will have the dominant phenotype (Bb).
    • Phenotypic Ratio: 100% Black
  • If unknown is heterozygous (Bb):
    • Offspring will have a 1:1 ratio of dominant to recessive phenotypes (Bb:bb).
    • Phenotypic Ratio: 50% Black, 50% White

CONCEPT OF DOMINANCE

  • Refer to text for detailed explanation.

CHROMOSOMAL THEORY OF INHERITANCE

  • Mendel's work (1865) was unrecognized until 1900.
  • 1900: de Vries, Correns, and von Tschermak rediscovered Mendel’s results independently.
  • 1902: Walter Sutton and Theodore Boveri studied chromosome movement during meiosis.
  • Theory postulates:
    • Genes are located at specific locations on chromosomes.
    • Chromosomes and genes occur in pairs.
    • Homologous chromosomes separate during meiosis.
    • Fertilization restores chromosome number to diploid condition.
    • Chromosomes segregate and assort independently.

LINKAGE AND RECOMBINATION

  • Morgan's experiments with Drosophila (fruit flies) studied sex-linked genes.
  • Dihybrid crosses with yellow-bodied, white-eyed females and brown-bodied, red-eyed males.
  • F2 ratio deviated from 9:3:3:1, indicating genes are linked.
  • Linkage: Physical association of genes on a chromosome.
  • If genes are tightly linked, parental combinations are more frequent than non-parental combinations (recombinants).
  • Recombination: Rearrangement of genetic material, generating non-parental gene combinations, responsible for variation.
  • Linkage and recombination depend on the distance between genes:
    • Greater distance = higher probability of recombination.
    • Genes on different chromosomes or far apart on the same chromosome assort independently (unlinked).
    • Genes close together on the same chromosome are linked.
  • Recombination frequency: Used to determine if genes are linked and how tightly.

NON-MENDELIAN INHERITANCE

  • Incomplete Dominance:
    • Neither allele is dominant; heterozygote phenotype is intermediate.
    • Example: Snapdragon flower color (red RR x white rr = pink Rr).
    • F2 generation ratio: 1:2:1 (red:pink:white).
  • Co-dominance:
    • Both alleles express themselves together in an individual.
    • Example: ABO blood group in humans, controlled by gene I with alleles IA, IB, and i.
    • IA and IB code for different sugar polymers on RBCs; i produces no sugar.
    • IA and IB are dominant over i but co-dominant with each other.

MULTIPLE ALLELISM

  • When a trait is governed by more than two alleles.
  • Example: ABO blood-type system (IA, IB, i).
    • IA codes for A molecules.
    • IB codes for B molecules.
    • i codes for no molecules.
    • IA and IB are codominant and dominant over i.
  • Each individual inherits only two alleles.

PLEIOTROPY

  • A single gene controls many phenotypic traits.
  • Often due to effects on metabolic pathways.
  • Example: Phenylketonuria, caused by mutation in the gene coding for phenylalanine hydroxylase, leading to mental retardation and reduced pigmentation.

POLYGENIC INHERITANCE

  • Quantitative inheritance where multiple independent genes have additive or similar effect on a single quantitative trait.
  • Characteristics:
    • Polygene: A gene with a slight effect on a phenotype.
    • Single gene effect is small and difficult to detect.
    • Continuous variation of the phenotype.
  • Difference from multiple allelism:
    • Multiple allelism: 2 or more alleles at the same locus.
    • Polygenic inheritance: multiple genes at different loci.
  • Example: Human skin color, controlled by multiple genes (e.g., A, B, C for dark pigmentation; a, b, c for light pigmentation).
    • AABBCC: Maximum melanin, very dark skin.
    • aabbcc: Minimum melanin, very light skin.
    • AaBbCc: Intermediate melanin, medium skin color.
  • Cross between AaBbCc x AaBbCc yields seven skin colors, represented by the number of capital letters (0 to 6).

SEX DETERMINATION

  • Different species have different mechanisms for sex determination.
  • Henking first noticed the X chromosome.
  • Sex chromosomes determine sexual character formation; the rest are autosomes.
  • Male heterogamety: Males produce two distinct types of gametes (e.g., grasshoppers, humans, Drosophila).
  • Female heterogamety: Females produce two distinct types of gametes (e.g., birds).

SEX DETERMINATION IN THE HONEY BEE

  • Haplo-diploid system
  • Female (queen or worker): Diploid (32 chromosomes), formed by fusion of egg and sperm.
  • Male (drone): Haploid (16 chromosomes), formed from unfertilized egg by parthenogenesis.
  • Sperms are produced by mitosis.

GENETIC DISORDERS

  • Disorders due to defects in the genetic structure, transmitted from one generation to another.
  • Two main types:
    • Mendelian disorders
    • Chromosomal disorders

MENDELIAN DISORDERS

  • Due to alteration or mutation in a single gene, may be dominant or recessive.
  • Follow Mendel’s principles of heredity.
Examples:
  1. Phenylketonuria
    • Mode of inheritance: Autosomal recessive
    • Cause: Deficiency of the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine.
    • Effect: Accumulation of phenylalanine, leading to mental retardation and reduced pigmentation. Musty odor in breath, skin, or urine. May cause neurological problems, eczema, seizures and lighter skin/hair color then the family because phenylalanine can't transform into melanin.
  2. Thalassemia
    • Mode of inheritance: Autosomal recessive
    • Cause: Mutation or deletion resulting in reduced synthesis of globin chains (α and β).
    • Effect: Formation of abnormal hemoglobin. Symptoms include pale skin, dark urine, bone deformities, excessive tiredness, and fatigue.
    • Alpha thalassemia: Controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16. Caused by mutation controlling how much Alpha globin to make.
    • Beta thalassemia: Controlled by a single gene (HBB) on chromosome 11. Change in gene causes less beta gobin to be made than typical.
  3. Colour Blindness
    • Mode of inheritance: Sex-linked (X-linked) recessive
    • Cause: Affects red or green cones in the eye.
    • Effect: Inability to distinguish between red and green colors.
  4. Haemophilia
    • Mode of inheritance: Sex-linked (X-linked) recessive
    • Cause: Affects a single protein involved in blood clotting.
    • Effect: Continuous bleeding from wounds due to failure of blood clotting.

CHROMOSOMAL DISORDERS

  • Disorders due to excess, absence, or abnormal arrangement of chromosomes.
  • Two types:
    1. Aneuploidy:
      • Gain or loss of one or more chromosomes. Due to non-separation of homologous chromosomes or chromatids during cell division (non-disjunction).
    2. Polyploidy:
      • Increase in the full set of chromosomes. Often found in plants; caused by failure of cytokinesis.
Examples:

  1. Down Syndrome (discovered by Langdown)

    • Genotype: Trisomy 21
    • Type of aneuploidy: Trisomy
    • Number of chromosomes: 47
    • Symptoms: Physical, psychomotor, and mental retardation. Broad palm with characteristic palm crease. Broad flat face with slanting eyes and congenital heart diseases. Partially opened mouth with furrowed tongue. Short stature, small round head with flat back.

  2. Klinefelter’s Syndrome

    • Genotype: XXY
    • Type of aneuploidy: Trisomy
    • Number of chromosomes: 47
    • Symptoms: Sterility, overall masculine development, gynecomastia (breast development).

  3. Turner Syndrome

    • Genotype: XO
    • Type of aneuploidy: Monosomy
    • Number of chromosomes: 45
    • Symptoms: Sterility, rudimentary ovaries, lack of female secondary sexual characteristics.