genetics and inheritance

Question 1

State three causes of genetic variation.

Answer 1

• Mutation
• Crossing over
• Independent segregation / assortment (of homologous chromosomes)
• Random fusion of gametes / fertilisation / mating

Question 2

What is meant by a genome?

Answer 2

• (All) the DNA in a cell/organism;
• ‘(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’
• ‘the total number of DNA bases in a cell/organism’

Question 3

What is a gene pool?

Answer 3

• All the alleles in a population;

Question 4

How do multiple alleles of a gene arise?

Answer 4

• mutations;
• which are different / at different positions in the gene;

Question 5

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.

Suggest two reasons why.

Answer 5

• Small sample size;
• Fusion/fertilisation of gametes is random;
• Linked Genes; Sex-linkage / crossing over;
• Epistasis;
• Lethal genotypes;

Question 6

What is meant by a recessive allele?

Answer 6

• Only expressed in the homozygote / not expressed in the heterozygote / not expressed if dominant present;

Question 7

What does Hardy Weinberg’s equation predict

Answer 7

• The frequency/proportion of alleles (of a particular gene);
• Will stay constant from one generation to the next/over generations / no genetic change over time;
• Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration;

Question 8

Define gene linkage

Answer 8

• (Genes/loci) on same chromosome;

Question 9

Define epistasis

Answer 9

• The allele of one gene affects or masks the expression of another in the phenotype;

Question 10

Describe why observed phenotypes don’t match expected values.

Answer 10

• Fertilisation is random
  OR
• Fusion of gametes is random;
• Small/not-large population/sample;
• Selection advantage/disadvantage/lethal alleles;

Question 11

Define codominance

Answer 11

• Both alleles expressed in the phenotype;

Question 12

Rules for Dominant alleles

Answer 12

• Affected offspring MUST have at least one affected parent.
• Unaffected parents ONLY have unaffected offspring.
• If both parents are affected and have an unaffected offspring, both parents must be Heterozygous

Question 13

Rules for recessive alleles

Answer 13

• Unaffected parents can have an affected offspring (if they are Heterozygous)

Question 14

Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why.

Answer 14

• (Recessive) allele is always expressed in males / males have one (recessive) allele;
• Females need two recessive alleles / females need to be homozygous recessive / females could have dominant and recessive alleles / be heterozygous;

Question 15

Expected offspring phenotype ratios from heterozygous parents:
1. Monohybrid
2. Dihybrid
3. Epistasis
4. Autosomal linkage

Answer 15

Dominant : recessive

1. 3:1
2. 9:3:3:1
3. 9:4:3 or 15:1 or 9:7
4. 3:1 (no x over) (no other pattern other than four phenotypes with recombination of alleles)

Question 16

What is meant by the term phenotype.

Answer 16

• (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
• (Expression / appearance / characteristic due to) interaction with environment;

Question 17

Explain how a single base substitution causes a change in the structure of a polypeptide.

Answer 17

• Change in (sequence of) amino acid(s)/primary structure;
• Change in hydrogen/ionic/disulfide bonds;
• Alters tertiary/30 structure;