IB DP Biology Topic 3: 3.4 Inheritance Question Bank HL Paper 2


The spider Dolomedes plantarius usually has white bands down the left and right sides of its body, but some individuals lack these bands. The photograph shows the banded form of D. plantarius with a ball of spiderlings.

Crosses were performed to investigate the inheritance of this trait, by allowing specific males and females to mate. Numbers of banded and unbanded spiderlings that hatched out from all the eggs laid by the female were recorded. Results are shown in the table.

(a) Explain the conclusion that can be drawn from Cross 1.

(b) Deduce reasons for the difference between the results of Cross 2 and Cross 3.

(c) There were 79 progeny in Cross 4. Predict the expected results by completing the table.

(d) Identify, using one recognition feature visible in the photo, the phylum in which
D. plantarius is classified.



a. allele/trait/gene for banded is dominant / for unbanded is recessive;
b. because there is a ratio of 3 banded:1 unbanded
because two banded spiders produced some unbanded offspring;
c. both parents are heterozygous;

a. (1:1 ratio) in cross 2 as banded parent is heterozygous/has one copy of each allele;
b. (no unbanded offspring) in cross 3 as banded parent is homozygous/has two alleles for banded;
c. (in crosses 2 and 3) banded parental phenotypes are the same but their genotypes are different;


(d)  arthropods (as spider has) segmentation/exoskeleton/jointed limbs/jointed
appendages/bilateral symmetry;


Thomas Hunt Morgan established that genes for body colour and wing size in Drosophila are autosomally linked. The allele for grey body (b+) is       dominant over that for black body (b) and the allele for normal wing size (vg+) is dominant over that for vestigial wing (vg).

      1. A fly that is homozygous dominant for both body colour and wing size mates with a fly that is recessive for both characteristics. In the table, draw the arrangement of alleles for the offspring of this mating and for the homozygous recessive parent. [2]

        Heterozygous offspring

        (grey body, normal wings)

        Homozygous recessive parent (black body, vestigial wings)



      2. The offspring, which were all heterozygous for grey body and normal wings, were crossed with flies that were homozygous recessive for both genes. The table shows the percentages of offspring produced.

          grey body, normal wings

          48 %

          grey body, vestigial wings

          3 %

          black body, normal wings

          2 %

          black body, vestigial wings

          47 %

          Explain these results, based on the knowledge that the genes for body colour and wing size are autosomally linked. [2]





    a.not a 1:1:1:1 ratio «because of linkage»
    not independent assortment
    grey normal and black vestigial types/parental combinations/double dominant and doublerecessive were commoner than 25 %/commoner than expected ✔
    Accept any of these points from an annotated diagram.
    2 max
    b.«linked genes» so were on the same chromosome ✔
    c.grey body vestigial wing and black body normal wing are recombinants
    2 % plus 3 % of the offspring are recombinants ✔
    d.recombinants due to crossing over/exchange of genes between «non-sister» chromatids
    2 % and 3 % of offspring were due to crossing over
    genes inherited together unless separated by crossing over ✔
    e.crossing over between the two loci/between the two genes on the chromosomes ✔
    f.few recombinants/not much crossing over because genes/gene loci close together ✔


    Outline how two parents could have a child with any of the four ABO blood groups [4]



    a IA, i for one set of gametes/parental genotype  Answers can be given in a Punnett grid or in prose. 

    b IB, i for the other set of gametes/parental genotype 

    c «genotypes of offspring are» IAIB, IAi, IBi, ii ✔

    d «phenotypes of offspring are» AB, A, B, O


    Distinguish between autosomes and sex chromosomes in humans.


    Describe the inheritance of hemophilia including an example using a Punnett grid.


    Explain how meiosis results in an effectively infinite genetic variety of gametes.



    X and Y chromosomes determine sex;
    females XX and males XY;
    X chromosome is larger than / carries more genes than the Y chromosome;
    22 types/pairs of autosomes;
    males and females have same types of autosomes;


    sex-linked / due to gene on the X chromosome;
    more common in males who only receive one X chromosome;
    female is hemophilic if homozygous recessive / homozygous recessive normally fatal;
    XH for dominant/normal allele and Xh for recessive/ hemophilia allele; (accept in Punnett grid/square)
    example in Punnett grid/square with correct parental genotype and gametes;
    correct genotypes of offspring;
    correct phenotype ratio or percentage;

    half the males are hemophilic and half of the females are carriers / OWTTE;

    Allow marks for correct genotypes if the alleles are not shown superscript on an X, as long as the Y chromosome is indicated.
    Do allow marking point d. if the letters for the dominant and recessive allele are not upper and lower case versions of the same letter.


    one (homologous) chromosome is from the mother and one from the father;
    homologous chromosomes pair (in prophase I);
    crossing over/chiasma formation in prophase I;
    recombination of linked genes / alleles/genes swapped;
    many possible points of crossing over;
    crossing over occurs at random positions;
    due to crossing over the two chromatids of metaphase I chromosomes are not identical;
    random orientation (of bivalents) in metaphase I;
    in anaphase/at end of metaphase I chromosomes move to opposite poles;
    independent assortment of chromosomes/genes;
    2n/223 combinations (without considering crossing over);
    four genetically different nuclei/gametes from each meiosis;
    Accept any of the above points in a clearly annotated diagram.



    When performing dihybrid crosses with fruit flies (Drosophila), Morgan discovered that his results did not correspond to the expected Mendelian ratios. He explained this by suggesting that there is an exchange of genetic material between chromosomes. The image shows his diagram for three gene loci on a pair of homologous chromosomes during meiosis.

    (a) Identify the stage of meiosis shown where exchange of genetic material occurs. [1]

    (b) Explain the reason that Morgan’s results did not agree with expected Mendelian ratios in a dihybrid cross. [2]


    a prophase 1/ first prophase; 
    b a. gene linkage/genes located on the same chromosome;

    b. independent assortment does not occur;
    c. no recombination unless there is crossing over (between the genes);

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