Inheritance

Question 1

Mendel’s 2nd law (law of independent assortment)

Answer 1

background info:
- 2 characters being controlled by 2 genes
- which are located on 2 gene loci on 2 different chromosomes
(i.e. genes are not linked)
- independent assortment of homologous chromosomes occur during metaphase 1
- resulting in random distribution of paternal and maternal chromosomes in the gametes
- * the segregation of one pair of alleles is independent of the segregation of other pairs
- thus all four combinations will be formed with equal probabilities

Question 2

Mendel’s 1st law (Law of segregation)

Answer 2

background info:
- alleles occur in pairs
- and each allele is located on one of the pair of homologous chromosomes in a diploid organism
- when the pair of homologous chromosomes separate from each other during anaphase 1, they take their alleles with them
- * the paired alleles separate randomly
- * so that each gamete receives one or the other allele with equal likelihood

Question 3

why it is difficult to investigate the patterns of inheritance in man

Answer 3

• some human characters display continuous variation
• and hence do not follow Mendelian ratios
• humans have a longer gestation time
• and hence takes a longer time before results can be observed
• humans have a smaller no of offsprings
• and hence, results might not be representative
• restricted by bioethics, e.g. respect for human choice of partners

Question 4

how to draw genetic diagram (for Mendelian inheritance)

Answer 4

1. definition of alleles (if not already stated)
2. parental phenotype
3. parental genotype
4. parental gametes
   - note: must draw out all gametes of both parents even if they are the same
5. random fertilisation
   - table (with labels “female gametes” and “male gametes”)
   - note: just randomly choose which one to label female or male if qn did not specify
6. F1 genotypic ratio
7. F2 phenotypic ratio

Question 5

meaning of true/pure breeding

Answer 5

homozygous recessive at all gene loci

Question 6

why certain phenotypes breed true while others do not

Answer 6

• the ones that do only have 1 genotype (homozygous, double recessive, e.g. aabb)
• the ones that do not have variety of genotypes (heterozygous, e.g. A_bb which includes AAbb and Aabb)