Post-Mendelian Genetics Flashcards

Post-Mendelian Genetics

10.1 Introduction

• Post-Mendelian genetics studies phenomena not observed by Mendel, including:
  • Gene linkage and recombination
  • Multiple allelic genes
  • Polygenes
  • Lethal genes
  • Gene interactions

10.2 Objectives

• Identify when Mendel’s laws don't apply.
• Explain gene linkage and recombination and their effects on Mendelian phenotypic ratios.
• Define multiple alleles and explain their effect on Mendelian phenotypic ratios.
• Define lethal genes and explain their effects on Mendelian phenotypic ratios.
• Define polygenes and explain their effect on Mendelian genetics.

10.3 Autosomal Linkage and Gene Recombination

• Linked genes:
  • Located on the same chromosome.
  • Inherited together; do not behave independently.
  • Alleles far apart have a higher chance of crossing over due to the need for chiasma formation.
• Unlinked genes:
  • Located on different chromosomes.
  • Assort freely.
  • Segregate alleles into gametes randomly due to segregation and independent assortment.
• Gene linkage controls recombination, keeping certain character combinations together.
• Crossing over:
  • Chiasma formation between homologous chromosomes during meiosis.

10.4 Genetic Recombination and Variation

• F1 offspring:

  • From parents with different pure genotypes of the same species.
  • Show genotypic and phenotypic variability due to allele reshuffling in parental gametes.

• Recombination of alleles is a key function of meiosis and sexual reproduction, promoting long-term survival and evolution.

• Recombinants:

  • F2 individuals with new combinations absent in parental types.
  • Result from crossing over and independent assortment during meiosis in F1 gamete formation.

• Monohybrid cross (n=1):

  • 3 genotypes (3^n)
  • 2 phenotypes (2^n)
  • 3:1 phenotypic ratio with independent segregation

• Dihybrid cross (n=2):

  • 9 genotypes (3^n)
  • 4 phenotypes (2^n)
  • 9:3:3:1 phenotypic ratio with independent segregation

• Trihybrid cross (n=3):

  • 27 genotypes (3^n)
  • 8 phenotypes (2^n)

10.5 Lethal Alleles

• Lethal genes: Essential genes with alleles causing death in the homozygous condition.
  • Completely dominant lethal allele: Kills carriers in both homozygous and heterozygous conditions, preventing reproduction.
  • Recessive lethal allele: Causes death in homozygotes; most lethal genes are recessive.
  • Sub-lethal or semi-lethal genes: Operate at sexual maturity, causing handicaps but not death.

10.5.1 Effects of Lethal Genes on Mendelian Ratios

• Lethal genes eliminate phenotypic classes, modifying ratios.
• Monohybrid cross ratio (3:1) becomes 2:1 if a recessive allele is lethal.
• Examples:
  • Yellow fur allele (Y) in mice: Dominant lethal, causes death in YY homozygotes.
    • Yy (yellow) x Yy (yellow) → 2 yellow : 1 grey (instead of 3:1)
    • F_2: \frac{1}{4} YY \text{ (aborted)}, \frac{1}{2} Yy \text{ (yellow)}, \frac{1}{4} yy \text{ (grey)}
  • Achondroplastic dwarfism allele (A) in humans: Dominant lethal.
    • Aa (dwarf) x Aa (dwarf) → 2 dwarf : 1 normal
    • F_2: \frac{1}{4} AA \text{ (aborted)}, \frac{1}{2} Aa \text{ (dwarf)}, \frac{1}{4} aa \text{ (normal)}
  • Chlorosis allele (C) in maize: Results in plants lacking chlorophyll.
    • Cc (chlorotic) x Cc (chlorotic) → 2 chlorotic : 1 normal
    • F_2: \frac{1}{4} CC \text{ (die early)}, \frac{1}{2} Cc \text{ (chlorotic plants)}, \frac{1}{4} cc \text{ (normal plants)}

10.6 Multiple Alleles

• Genes with more than two allelic forms (e.g., human ABO blood type).
• Individuals have two copies of each gene, but the population has multiple alleles.
• F2 generation has more than three phenotypes.

10.6.1 The ABO Blood Group System

• Controlled by a single gene (I) with three alleles: I^A, I^B, and I^O.
  • I^A co-dominant to I^B
  • I^A dominant to I^O
  • I^B dominant to I^O
  • (I^A = I^B > I^O).
• Six possible allele combinations give six genotypes.

10.6.1.1 Inheritance of Blood Groups

• Inherited in Mendelian fashion, with individuals carrying two out of three alleles.
• Examples of crosses:
  • Group O x Group O (I^O I^O x I^O I^O) → All children are group O.
  • Group AB x Group O (I^A I^B x I^O I^O) → 1/2 Group A (I^A I^O), 1/2 Group B (I^B I^O).
  • Group AB x Group AB (I^A I^B x I^A I^B) → 1/4 Group A (I^A I^A), 1/2 Group AB (I^A I^B), 1/4 Group B (I^B I^B).

10.6.1.2 Red Blood Cell Antigens and Serum Antibodies

• Red blood cells have surface antigens; serum contains antibodies against foreign RBC antigens.
• Antigen-antibody combinations:
  • Group A: A antigens, Anti-B antibodies.
  • Group B: B antigens, Anti-A antibodies.
  • Group AB: A and B antigens, no antibodies.
  • Group O: No antigens, Anti-A and Anti-B antibodies.

10.6.1.3 Blood Typing

• Agglutination (clumping) occurs when a blood group carrying a given antigen is mixed with a blood group carrying an antibody for that antigen.
• Blood typing reactions:
  • Anti-A serum:
    • Agglutination with A and AB blood.
    • No agglutination with O and B blood.
  • Anti-B serum:
    • Agglutination with B and AB blood.
    • No agglutination with O and A blood.

10.6.1.4 Blood Transfusion

• Group O is the universal donor (no A or B antigens).
• Group AB is the universal recipient (no anti-A or anti-B antibodies).
• Safe transfusions occur when there is no agglutination.
• Unsafe transfusions result in agglutination.

10.6 Polygenic Inheritance

• Phenotypic variation in populations shows:
  • Discontinuous variation: Clear-cut differences with no intermediates (e.g., blood groups, albinism).
  • Continuous variation: Range of measurements from one extreme to another.

10.6.1 Discontinuous Variation

• Individuals show clear-cut differences with no intermediates.
• Controlled by one or two major genes with two or more allelic forms.
• Phenotypic expression relatively unaffected by environmental conditions.
• Also known as qualitative variation.

10.6.2 Continuous Variation - Polygenic Inheritance

• One character controlled by many genes (polygenes) working together.
• Characters show a range within a population.
• Combined effects of many genes and the environment.
• Frequency distribution follows a normal curve.
• Each gene has a small effect on the phenotype, but their combined effect is significant.
• Also called quantitative traits or continuously varying traits.
• Studied using statistical methods.
• Examples:
  • Height in humans, affected by genetics and nutrition.
  • Yield in plants, affected by germination rate, photosynthesis, root amount, drought tolerance, etc.

10.7 Gene Interactions

• Genes at different loci interact, leading to unexpected phenotypes.
• Non-allelic gene interactions: epistasis, hypostasis, complementary action, and pleiotropy.

10.7.1 Epistasis

• One gene hides the expression of another gene.
• Epistatic gene interferes with the phenotypic expression of another (hypostatic) gene.
• Can be dominant or recessive.

10.7.1.1 Recessive Epistasis

• Coat color in mice: agouti (A dominant to black a) requires a second gene (C dominant to c) for color expression.
• cc is epistatic to the color gene, resulting in albino mice regardless of the A allele.
• AaCc x AaCc → 9 agouti: 3 black: 4 albino.

10.7.1.2 Dominant Epistasis

• Plumage color in chickens: White Leghorns (IICC) are white because I inhibits color (C). Pure-breeding White Wyandottes (iicc) are white because of the recessive c allele.
• I is epistatic to C.
• IiCc x IiCc → 13 white: 3 colored.

10.7.2 Hypostasis

• A gene (hypostatic) is hidden by another (epistatic) gene.

10.7.3 Pleiotropy

• A single gene has multiple phenotypic effects.
• A gene produces a product involved in a branched biochemical pathway; mutation affects different branches.
• A gene determines an enzyme common to multiple metabolic pathways; mutation blocks these pathways.
• Examples:
  • Sickle cell anemia: Mutation in one gene leads to abnormal hemoglobin, causing sickle-shaped red blood cells, organ damage, and severe anemia.
  • Albinism.

10.7.4 Complementary Genes

• Genes interact to produce an effect different from their individual effects.
• Comb type in chickens: Bateson and Punnett observed that pea comb (P) crossed with rose comb (R) produces walnut comb (P and R).
• PpRr x PpRr → 9 walnut: 3 pea: 3 rose: 1 single.
• Key is that F2 is in multiples of 1/16ths.

10.8 Revision Exercise

• Define lethal alleles.
• State the modified Mendelian ratios caused by epistasis and lethal alleles.
• Explain how multiple allelic characters affect the Mendelian genetics.
• Distinguish between single genes and polygenes.
• Explain how gene linkage affects the Mendelian phenotypic ratios.
• Fruit fly wing experiment (Vv x Vv).
• Chicken creeper experiment.
• Dominant lethal alleles and population disappearance.
• Dominance vs. epistasis.
• Human anaemia allele.
• Oat seed hull colour experiment.
• Poultry comb shape experiment.
• Labrador retriever coat colours.

10.9 Summary

• Post-Mendelian genetics explains deviations from Mendelian ratios caused by:
  • Gene linkage and recombination
  • Multiple allelic genes
  • Polygenes
  • Lethal genes
  • Gene interactions