IB Biology SL D3.2 Inheritance Questions

Practise IB Biology SL D3.2 by predicting inheritance from crosses and pedigrees while linking alleles, genotypes and phenotype.

Exam points

Identify haploid gametes and distinguish genotype, phenotype and allele combinations.
Use flowering-plant or autosomal Punnett grids to predict genotypes, phenotypes and ratios.
Infer dominance and unknown genotypes from test-cross results or offspring evidence.
Deduce ABO cross outcomes from IA, IB and i, including codominance and recessive alleles.
Distinguish codominance from incomplete dominance and predict or interpret offspring phenotype ratios.
Calculate XX/XY offspring probabilities; explain how Y-chromosome SRY/TDF initiates testis development.
Use X-linked haemophilia grids to calculate carrier and affected outcomes by sex.
Infer dominant, recessive or X-linked inheritance and genotype risks from pedigrees.
Explain recessive PKU from PAH deficiency to phenylalanine build-up and dietary treatment.
Explain how environment alters gene expression and phenotype without changing genotype.
Distinguish alleles by an SNP's single-base difference.
Read medians, quartiles, range and outliers from box plots to compare continuous data.

Covered syllabus points

D3.2.1—Haploid gametes + fusion = diploid zygote D3.2.2—Genetic crosses in flowering plants D3.2.3—Genotype D3.2.4—Phenotype D3.2.5—Dominant and recessive alleles D3.2.6—Phenotypic plasticity D3.2.7—Phenylketonuria (PKU)D3.2.8—SNPs and multiple alleles D3.2.9—ABO blood groups D3.2.10—Incomplete dominance and codominance D3.2.11—Sex determination D3.2.12—Haemophilia D3.2.13—Pedigree charts D3.2.14—Continuous variation D3.2.15—Box-and-whisker plots

[Maximum number: 1]

For what reason do gametes contain only one allele of each gene?

To prevent inbreeding in a population

Haploid cells contain only one set of chromosomes

The two alleles of a gene are separated during mitosis

Crossing over will always produce one allele of a gene

[Maximum number: 4]

The dolichos bean (Lablab purpureus) is cultivated as a food crop in tropical countries. Leaf cells in L. purpureus have 24 chromosomes.

(a)

L. purpureus can have purple or white flowers. Two pure-breeding varieties were crossed: HA 4 with white flowers and GL 424 with purple flowers. All of the $F_{1}$ plants had purple flowers. The $F_{1}$ plants were self-pollinated to produce an $F_{2}$ generation. There were 97 plants with purple flowers and 38 plants with white flowers in the $F_{2}$ generation.

Using a Punnett grid, explain the results of this cross.

[ 3 ]

Male and female gamete genotypes/alleles shown as P and p, or other letters following convention with a suitable key, in a Punnett grid.
$\mathrm{F}_{2}$ genotypes shown as $\mathrm{PP}$ , $\mathrm{Pp}$ , $\mathrm{pP}$ and $\mathrm{pp}$ .
$\mathrm{F}_{2}$ phenotypes indicated for each genotype on the Punnett grid / 3 purple to 1 white ratio indicated.

Official Punnett-grid answer image from the markscheme.

Accept other upper-case and lower-case letters for the alleles.
No punnett grid = 2 MAX.
Allow ECF if alleles incorrect.

(b)

Suggest a reason for the $F_{2}$ generation not corresponding exactly to the expected ratio of purple-flowered and white-flowered plants.

[ 1 ]

[Maximum number: 4]

(a)

Define the term genotype.

[ 1 ]

(b)

PKD (polycystic kidney disease) is an inherited disorder that causes the kidneys to become enlarged and lose function over time. PKD is caused by a dominant allele, D, which is not sex-linked.

The pedigree chart shows the inheritance of PKD in a family.

[ 3 ]

(i)

Calculate the probability of individuals 1 and 2 having a child with PKD.

[ 1 ]

0.5 / 50% / $\frac{1}{2}$ ;

(ii)

Explain how the pedigree chart shows that the dominant allele causing PKD is not on the X chromosome.

[ 2 ]

(If on the X chromosome)

ALTERNATIVE 1 Father evidenced route:

a. 1 has only one dominant allele on X (and not on the Y ) / would be $\mathrm{X}^{\mathrm{D}} \mathrm{Y}$ / OWTTE;

b. 1 passed his X chromosome/dominant allele to 3/7 I OWTTE OR

son/8 could not inherit the disease;

c. (so) all daughters would be affected / not possible for 3/7 to be healthy / OWTTE;

ALTERNATIVE 2 Mother evidenced route:

d. 2 does not have the dominant allele / is homozygous recessive / would be $\mathrm{X}^{\mathrm{d}} \mathrm{X}^{\mathrm{d}}$ IOWTTE;

e. 2 passed her X chromosome/ X ${ }^{d}$ /recessive allele(s) to 6/8;
OR

6/8/sons would receive the Y chromosome with no (dominant) allele / OWTTE;

f. (so) all sons/8 would be healthy / not possible for 8 to be affected / OWTTE;

Marking guidance:

Accept Punnett grids, providing they are
clearly annotated and identify specific
individuals.

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Covered syllabus points

Question 1

Question 2

Question (a)

Question (b)

Question 3

Question (a)

Question (b)

Question (i)

Question (ii)