Genetics flashcards

Genetics is the study of heredity and variation. Students must understand how genetic information is stored in DNA as sequences of bases organised into genes, which code for proteins via transcription and translation. Genes occupy specific loci on chromosomes, and organisms carry two alleles per gene — which may be dominant, recessive, or codominant. Mendel's laws describe how alleles segregate during gamete formation and assort independently for genes on different chromosomes. Students must be able to construct and interpret Punnett squares, pedigree charts, and understand sex linkage. More advanced concepts include polygenic inheritance, gene linkage, crossing over, and how the chi-squared test is used to assess whether observed ratios differ significantly from expected ratios. Understanding how mutations and recombination generate genetic variation is also essential.

1. Card 1

   What is a gene?

   A gene is a sequence of DNA bases at a specific locus on a chromosome that codes for a particular polypeptide or functional RNA molecule.

2. Card 2

   What is the difference between a genotype and a phenotype?

   Genotype is the allele combination an organism carries; phenotype is the observable characteristic that results from genotype and environmental influences.

3. Card 3

   Define codominance and give an example.

   Codominance occurs when both alleles are expressed equally in the heterozygote, for example the ABO blood group where I^A and I^B alleles both produce antigens.

4. Card 4

   State Mendel's Law of Segregation.

   Segregation states that the two alleles of a gene separate during gamete formation so each gamete carries only one allele for that gene.

5. Card 5

   What is meant by a test cross?

   A test cross involves crossing an organism of unknown genotype with a homozygous recessive individual to determine whether the unknown is homozygous dominant or heterozygous.

6. Card 6

   What is sex linkage?

   Sex linkage describes genes located on a sex chromosome, usually the X chromosome; males (XY) are hemizygous and express recessive alleles more frequently than females.

7. Card 7

   What is polygenic inheritance?

   Polygenic inheritance occurs when a single characteristic is controlled by two or more genes, producing a continuous range of phenotypes, for example human height and skin colour.

8. Card 8

   What does it mean for two genes to be linked?

   Linked genes are located on the same chromosome and tend to be inherited together, deviating from the expected 9:3:3:1 ratio in a dihybrid cross.

9. Card 9

   How does crossing over affect linked genes?

   Crossing over during meiosis I exchanges segments between homologous chromosomes, producing recombinant gametes with new allele combinations from linked genes.

10. Card 10

    What is the purpose of the chi-squared test in genetics?

    The chi-squared test determines whether the difference between observed and expected offspring ratios is due to chance or is statistically significant.

11. Card 11

    What is a mutation?

    A mutation is a heritable change in the base sequence of DNA; point mutations alter a single base pair and may change the amino acid sequence of the resulting protein.

12. Card 12

    What is the difference between homozygous and heterozygous?

    Homozygous individuals carry two identical alleles at a locus; heterozygous individuals carry two different alleles, one of which may be dominant over the other.

• Students often confuse 'codominance' with 'incomplete dominance', writing that codominant alleles blend together rather than both being fully and separately expressed in the heterozygote.
• A common error is assuming that all sex-linked traits are X-linked and recessive, when in reality traits can be X-linked dominant or even Y-linked, and students must read the question carefully.
• Many students incorrectly apply Mendel's Law of Independent Assortment to linked genes, expecting a 9:3:3:1 ratio in a dihybrid cross even when the two genes are on the same chromosome.

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