A level Biology 16.1 Passage of information from parents to offspring – Exam style question – Paper 4

(a) (i)

The results support the mutation conclusion because:

1. The transparent phenotype reappeared in the F₂ generation but was not present in the F₁ generation, showing it’s an inherited trait rather than an environmental effect.
2. The pattern of inheritance follows Mendelian genetics, suggesting the transparent phenotype is caused by a new recessive allele that arose through mutation.
3. The transparent phenotype wasn’t present in either parent population before 2012, indicating it’s a new genetic variant.

(a) (ii)

The approximate whole-number ratio is 9:3:3:1 (wild-type:albino:transparent with black stripes:transparent with white stripes).

This can be seen by dividing all numbers by the smallest count (19):
173 ÷ 19 ≈ 9, 57 ÷ 19 ≈ 3, 58 ÷ 19 ≈ 3, 19 ÷ 19 = 1

(a) (iii)

The results show that:

1. There are two genes involved in determining these phenotypes.
2. The genes are on separate chromosomes (not linked) as shown by the 9:3:3:1 ratio.
3. Both genes have two alleles each – one dominant and one recessive.
4. The albino (white stripes) phenotype is caused by a recessive allele at one gene.
5. The transparent body phenotype is caused by a recessive allele at the other gene.
6. The wild-type phenotype is dominant for both genes (gold with black stripes).
7. The phenotypes are not sex-linked as they appear in both sexes.

(b)

The variation in F₂ offspring occurs through these meiotic processes:

1. Independent assortment of chromosomes during metaphase I, where homologous chromosome pairs align randomly at the cell equator.
2. Crossing over during prophase I, where homologous chromosomes exchange genetic material, creating new allele combinations.
3. Random segregation of chromosomes during anaphase I, where each homologous chromosome moves randomly to opposite poles.
4. Separation of sister chromatids during anaphase II, further increasing genetic diversity.
5. Each F₁ fish produces gametes with different combinations of the alleles for color and transparency.
6. When these gametes randomly combine during fertilization, they produce the four different phenotypes seen in the F₂ generation.

This meiotic variation explains why the F₂ generation shows all possible combinations of the parental traits in the classic 9:3:3:1 dihybrid ratio.