Plant Genetics and Genetic Screening - AS Biology

Inheritance Edexcel International AS Biology

Contents

  • Mutations

  • Patterns of Inheritance & Sex Linkage

  • Cystic Fibrosis

  • Genetic Screening

  • Ethical & Social Issues of Genetic Screening

Mutations

Nature of Mutations

  • Definition: A gene mutation is a change in the sequence of bases in a DNA molecule.

  • Effects: Mutations may result in an altered polypeptide because the DNA base sequence of a gene determines the sequence of amino acids that make up a polypeptide.

  • Occurrence: Mutations occur spontaneously during DNA replication.

Types of Mutations

Point Mutations

  • Substitution: A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base.

    • Only changes the amino acid for the triplet in which the mutation occurs, affecting only that triplet.

    • Types of Substitution Mutations:

    • Silent Mutations: Do not alter the amino acid sequence of the polypeptide due to the degenerate nature of the genetic code.

    • Missense Mutations: Alter a single amino acid in the polypeptide chain.

      • Example: Sickle cell anaemia, caused by a single substitution mutation changing one amino acid in hemoglobin protein.

    • Nonsense Mutations: Create a premature stop codon, resulting in an incomplete polypeptide chain.

      • Example: Cystic fibrosis can be caused by a nonsense mutation.

Insertion

  • Definition: A mutation that occurs when a nucleotide is randomly inserted into the DNA sequence.

  • Impact: Changes the triplet coded by the original base and all subsequent triplets, leading to a frameshift mutation.

  • Result: Can dramatically change the amino acid sequence produced, impacting the polypeptide's function.

Deletion

  • Definition: A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence.

  • Impact: Like insertion mutations, deletion mutations also cause frameshift mutations, altering every downstream triplet in the sequence.

Effects of Mutations

  • Most mutations do not significantly alter the polypeptide or do so slightly due to the degeneracy of the genetic code.

  • A small number of mutations can significantly alter polypeptides, affecting their structure and function, which can lead to advantages or disadvantages for the organism.

  • Beneficial Mutations: Rarely, mutations can provide an advantage such as antibiotic resistance or the ability to digest new foods, driving evolutionary processes through natural selection.

  • Harmful Mutations: More frequently, mutations negatively impact the ability of proteins to function.

    • Example: In cystic fibrosis, a mutation in the CFTR gene leads to non-functional chloride channels causing thick, sticky mucus.

    • Example: In sickle-cell disease, a mutation causes hemoglobin proteins to clump, leading to misshapen red blood cells.

Mutagens

  • Definition: Agents that increase the likelihood of mutation.

  • Examples include ionizing radiation and certain chemicals.

Patterns of Inheritance & Sex Linkage

Key Terms

Genetics

  • Chromosome: A long DNA molecule that contains many genes.

  • Gene: A length of DNA that codes for a single polypeptide.

  • Locus: The position of a gene on a chromosome.

  • Alleles: Different forms of a gene at the same locus on a chromosome, arising from mutations.

Homozygous & Heterozygous

  • Individuals have two copies of each allele; one from each parent in a homologous pair.

  • Homozygous: Two identical alleles at a locus.

  • Heterozygous: Two different alleles at a locus.

Genotype & Phenotype

  • Genotype: The alleles for a specific gene of an organism.

  • Phenotype: The observable characteristics of an organism.

    • Examples:

    • Genotype AA leads to phenotype black coat.

    • Genotype aa leads to phenotype chestnut coat.

Dominant & Recessive Alleles

  • Dominant Alleles: Always expressed in the phenotype (e.g., AA or Aa is expressed).

  • Recessive Alleles: Only expressed in homozygous recessive individuals (e.g., aa).

Codominance

  • Definition: Both alleles are expressed in the phenotype of a heterozygous individual.

    • Example: In cattle, the coat color gene can show codominance, where a heterozygous cow will have a roan coat.

Genetic Pedigree Diagrams

  • Definition: Diagrams used to trace the inheritance of traits through generations.

  • Males are squares, females are circles; affected individuals are shaded.

  • Key uses: Determine inheritance patterns, genotypes, and probabilities of traits.

Worked Example:

  • Albinism pedigree can help to analyze genetic traits. Albinism is caused by a recessive allele affecting melanin production.

Sex Linkage

Definition

  • Sex-Linked Genes: Genes located on sex chromosomes; notably, many are located on the X chromosome leading to different inheritance for males and females.

  • Males have one X and one Y (XY) chromosome and therefore only one copy of X-linked genes; females have two X chromosomes (XX).

Notation

  • X or Y for sex chromosomes with superscripts for alleles (e.g., X^b for color blindness).

Genotypes for Sex-Linked Traits

  • Males: X^B Y = unaffected, X^b Y = affected.

  • Females: X^B X^B = unaffected, X^B X^b = carrier, X^b X^b = affected.

Worked Example: Red-Green Color Blindness

  • Cause: A gene on the X chromosome; dominant allele B for normal vision, recessive allele b for color blindness.

  • Punnett Square Analysis: Shows offspring probabilities based on parental genotypes. E.g., parental genotypes X^B Y (father) and X^B X^b (mother), leading to a 50% chance of affected daughters.

Cystic Fibrosis

Overview

  • Definition: A genetic disorder caused by a recessive allele of the CFTR gene located on chromosome 7, affecting chloride ion channels.

  • Impact: Faulty chloride channels lead to thick mucus production affecting the respiratory and digestive systems.

Mechanism

  • Reduced movement of water into secretions results in thick and sticky mucus affecting gas exchange in the lungs and obstructing pancreatic enzymes.

Inheritance

  • Recessive Nature: Individuals must be homozygous recessive (aa) to have cystic fibrosis; carriers (Aa) are unaffected.

  • Probability of Inheritance: If both parents are carriers, the chance of a child having cystic fibrosis is 25%.

Body Systems Affected

Respiratory System

  • Thick mucus inhibits cilia movement, leading to frequent lung infections.

  • Difficulty in breathing due to blocked airways.

Digestive System

  • Blocked pancreatic ducts leading to malabsorption of nutrients.

  • Cysts forming in the pancreas impacting enzyme production.

Reproductive System

  • Blocked tubes in males prevent sperm transport, thinned cervical mucus in females can inhibit fertilization.

Genetic Screening

Uses of Genetic Screening

  • Identifying Carriers: Helps individuals determine allele presence for hereditary conditions.

  • Preimplantation Genetic Diagnosis (PGD): Screens embryos during IVF for specific genetic disorders before implantation.

  • Prenatal Testing: Tests fetal DNA for genetic disorders through methods like chorionic villus sampling or amniocentesis.

Carrier Testing

  • Offered to individuals with family histories of genetic disorders.

  • Tests for the presence of recessive alleles such as cystic fibrosis.

PGD Benefits and Considerations

  • Advantages: Less risk overall, embryos screened before implantation, minimization of termination complications.

  • Ethical Concerns: Discarding embryos may pose moral dilemmas.

Prenatal Testing Implications

Chorionic Villus Sampling

  • Performed at around 11-14 weeks, rapid results but with a 1-2% miscarriage risk.

Amniocentesis

  • Conducted at 15-20 weeks, 1% miscarriage risk, takes longer for results.

Decision Making

  • Allows informed decisions regarding pregnancy and care for affected children.

Ethical & Social Issues of Genetic Screening

Considerations

  • Genetic counseling is essential for prospective parents to understand risks, implications, and decisions based on screening results.

Ethical Concerns

  • Religious or moral beliefs can strongly influence decisions regarding screening and termination.

  • The choice of what genetic conditions warrant action can differ culturally.

  • Potential for discriminatory practices in genetic selection (designer babies) can arise from available genetic information.

Balancing Perspectives

  • Arguments for and against genetic screening must be presented holistically, considering varied ethical viewpoints.

Conclusion: Understanding mutations, inheritance patterns, genetic disorders like cystic fibrosis, and the implications of genetic screening is crucial in the study of genetics and its applications in health and society.