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Flashcards · J247 · Topic 5
Genes, inheritance and selection flashcards
22 cards on Topic 5, subtopics B5.1 to B5.2. Pick a subtopic or study them all.
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B5.1 Inheritance
- Gene vs allele
- A gene is a section of DNA that codes for a protein (the instruction); an allele is one version of it.
- Genotype vs phenotype
- Genotype: the alleles an organism has, e.g. Gg. Phenotype: the observable characteristic, e.g. green.
- Homozygous vs heterozygous
- GG and gg are homozygous (two identical alleles); Gg is heterozygous (two different alleles).
- Genome vs gene
- The genome is all of the genetic material of an organism, including non-coding DNA; genes are only part of it.
- Continuous vs discontinuous variation
- Discontinuous: separate categories (eye colour, ABO blood group); mostly one or a few genes; bar chart. Continuous: any value in a range (height, weight); many genes plus the environment; histogram.
- Mutation, and its usual effect on phenotype
- A change in the DNA base sequence. Most have no effect on phenotype, some influence it, very few determine it. More likely with ionising radiation, UV light and some chemicals.
- How can a variant in coding DNA, or in non-coding DNA, change the phenotype?Higher
- Coding DNA: changes the order of amino acids, so the protein shape (e.g. an active site) and its activity. Non-coding DNA: changes how the gene is expressed (switched on or off), so more or less protein; the amino acid sequence is unchanged.
- Advantages of asexual and of sexual reproduction
- Asexual: one parent, quick, many offspring; good in a stable environment. Sexual: variation, so some offspring are likely to survive a new disease or a changing environment.
- Haploid and diploid, and why gametes must be haploid
- Diploid (2n): two sets of chromosomes, in body cells and the zygote (human 46). Haploid (n): one set, in gametes (human 23). Meiosis halves the number so fertilisation restores the diploid number.
- Two heterozygous plants are crossed and 200 seeds grow. Expected numbers?
- Tt × Tt gives 1 TT : 2 Tt : 1 tt, a 3 : 1 phenotype ratio. 200 × 3/4 = 150 dominant and 200 × 1/4 = 50 recessive.
- How is sex determined in humans?
- Females XX, males XY. All eggs carry X; sperm carry X or Y in equal numbers, so the sperm decides. Probability of a boy = 1/2 for every pregnancy.
- What did Mendel do, and why was his work not accepted at first?
- In the 1860s he bred pea plants and counted offspring, finding separate inherited units and 3 : 1 ratios. Chromosomes and genes were not yet known, so there was no physical explanation, and his work was not widely read.
B5.2 Natural selection and evolution
- Species
- Organisms that can breed together to produce fertile offspring.
- Sources of genetic variation
- Mutation, meiosis (a random mix of alleles in gametes) and random fertilisation.
- Natural selection: the chain
- Variation exists; more offspring are born than survive; individuals with the better-suited phenotype are more likely to survive and reproduce; they pass on the allele; the allele becomes more common over generations.
- How does a new species form?
- A population is separated into groups that do not interbreed. Each faces different selection pressures and mutations. Over many generations they become so different they cannot produce fertile offspring together.
- Fossils as evidence for evolution, and a limitation
- Fossils show past organisms differed from today's, with features changing through rock layers and intermediate forms. Limitation: the record is incomplete (soft bodies decay, few remains fossilise, many are undiscovered).
- How does antibiotic resistance evolve?
- A random mutation gives one bacterium a resistance allele; the antibiotic kills non-resistant cells; the resistant cell survives and divides; the allele spreads and the antibiotic stops working.
- Artificial vs natural classification
- Artificial: based on a few chosen, convenient features; does not show relationships. Natural: based on many features and shared ancestry; shows relationships.
- How has DNA sequencing changed classification?
- Molecular phylogenetics compares sequences: fewer differences mean a more recent common ancestor. It can regroup look-alike organisms. Ribosomal RNA sequencing led to three domains: Archaea, Bacteria, Eukaryota.
- Darwin and Wallace
- Both developed natural selection independently; their ideas were presented together in 1858, and Darwin published On the Origin of Species in 1859. The ideas were challenged partly because the mechanism of inheritance was unknown.
- Why are seedbanks kept?
- Seeds are stored cold and dry so that genetic variation survives if wild populations are lost.