Inheritance — IB Diploma Biology HL

Revision notes

10 short notes, in the order of the specification. Each one in short:

1. Haploid gametes, diploid zygotes and genetic crosses in plants

   Inheritance in all eukaryotes with a sexual life cycle works by parents producing haploid gametes that fuse to form a diploid zygote, which has two copies of each autosomal gene. Genetic crosses in flowering plants transfer pollen, which contains the male gametes, to the stigma of another plant, and follow the P, F1 and F2 generations using Punnett grids.

2. Genotype, phenotype, dominance and phenotypic plasticity

   Genotype is the combination of alleles inherited by an organism, and phenotype is the observable traits resulting from genotype and environmental factors. A dominant allele has the same effect in one or two copies, so homozygous-dominant and heterozygous individuals look alike. Phenotypic plasticity is the capacity to develop traits suited to the environment by varying gene expression.

3. Phenylketonuria, SNPs, multiple alleles and ABO blood groups

   Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in an autosomal gene for the enzyme that converts phenylalanine to tyrosine. Single-nucleotide polymorphisms create many alleles of a gene in a gene pool, but each individual inherits only two. ABO blood groups are an example of multiple alleles: Iᴬ, Iᴮ and i.

4. Incomplete dominance and codominance

   In incomplete dominance, heterozygotes have an intermediate phenotype, such as pink four o'clock flowers from red and white parents. In codominance, heterozygotes have a dual phenotype, with both alleles fully expressed, such as blood group AB from Iᴬ and Iᴮ. Both differ from complete dominance, where heterozygotes look like the homozygous dominant.

5. Sex determination, sex linkage and haemophilia

   In humans the sex chromosome in the sperm, X or Y, determines whether a zygote develops male-typical or female-typical characteristics. The X chromosome carries far more genes than the Y, so males have only one copy of most X-linked genes. Haemophilia is a sex-linked recessive disorder, so it is much more common in males.

6. Pedigree charts

   A pedigree chart shows the inheritance of a trait through several generations of a family, with squares for males, circles for females and shading for affected individuals. Patterns in the chart allow deduction of whether a disorder is dominant, recessive or sex-linked, and then the genotypes of particular individuals.

7. Continuous variation and box-and-whisker plots

   Continuous variation is variation in which a trait can take any value in a range, due to polygenic inheritance and/or environmental factors, such as skin colour or height. Discrete variables, such as ABO blood group, fall into separate classes. A box-and-whisker plot shows a continuous variable's minimum, quartiles, median, maximum and outliers.

8. Segregation, independent assortment and dihybrid crosses

   HL only

   Segregation is the separation of the two alleles of a gene into different gametes during meiosis. Independent assortment means the alleles of unlinked genes on different chromosomes go into gametes in all combinations equally, because bivalents orient randomly at metaphase I. Crossing two double heterozygotes gives a 9:3:3:1 ratio; a test cross gives 1:1:1:1.

9. Gene loci, autosomal linkage and recombinants

   PracticalHL only

   A gene's locus is its position on a particular chromosome, which databases record with its polypeptide product. Autosomal gene linkage occurs when genes are close together on the same autosome, so their alleles tend to be inherited together and fail to assort independently. Recombinants are new combinations of alleles produced by crossing over between linked genes.

10. Chi-squared test on dihybrid cross data

    HL only

    A chi-squared test on data from a dihybrid cross tests whether the difference between observed and expected numbers is statistically significant. The null hypothesis is that there is no significant difference, for example that the genes are unlinked. If the calculated value exceeds the critical value at p = 0.05, the null hypothesis is rejected.