Ear type Number of F2 piglets Erect ears 34 Floppy ears 14 Total 48 (a) In a breed of pig, the allele for erect ears (E) is dominant over the allele for flo...

(a)(i) The genotypes of the parents are: boar = EE (homozygous dominant) [1]; sow = ee (homozygous recessive). [1] The boar is stated to be homozygous erect-eared, and the sow has floppy ears which can only occur with the genotype ee.

(a)(ii) The genotype of the F1 piglets is Ee (heterozygous). [1] Each F1 piglet inherits one E allele from the EE father and one e allele from the ee mother. All F1 piglets have erect ears because the dominant E allele masks the recessive e.

(a)(iii) Crossing two F1 piglets (Ee x Ee):

Parents: Ee x Ee

Gametes from each parent: E and e

Offspring genotypes: EE, Ee, Ee, ee [1]

Expected genotype ratio: 1 EE : 2 Ee : 1 ee [1]

Expected phenotype ratio: 3 erect ears : 1 floppy ears [1] (EE and Ee both show the dominant erect-ear phenotype; only ee shows floppy ears.) [1]

(b)(i) Calculating the ratio of erect-eared to floppy-eared piglets from the data:

Erect ears = 34, Floppy ears = 14

\[ \text{Ratio} = \frac{34}{14} : \frac{14}{14} = 2.4 : 1 \] [1] for method (dividing both by the smaller number), [1] for arriving at approximately 2.4 : 1.

(b)(ii) The ratio is not exactly 3:1 because fertilisation is a random event, and the sample size (48 piglets) is relatively small. [1] Chance variation means the actual numbers will fluctuate around the expected ratio; a perfect 3:1 would require 36 erect : 12 floppy, but random sampling produced 34:14 instead.

(c) Heterozygous means having two different alleles for a particular gene - one dominant and one recessive (e.g. Ee). [1] The dominant allele determines the phenotype in a heterozygous individual.

(d) How to use a test cross to determine whether an erect-eared pig is homozygous or heterozygous:

1. Cross (mate) the erect-eared pig with a floppy-eared pig (ee), which is known to be homozygous recessive. [1]
2. If the erect-eared pig is homozygous (EE): all offspring will be Ee, and every piglet will have erect ears. No floppy-eared offspring will appear. [1]
3. If the erect-eared pig is heterozygous (Ee): the expected offspring are Ee and ee in a 1:1 ratio, so approximately half will have erect ears and half will have floppy ears. [1]
4. Therefore, any floppy-eared offspring proves the tested pig is heterozygous (Ee), since a floppy-eared piglet must have inherited a recessive allele from each parent. [1]