Genes, alleles and inheritance A2

Question 1

How do multiple alleles of a gene arise?

Answer 1

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

Question 2

What is meant by a recessive allele?

Answer 2

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

Question 3

What does Hardy Weinberg’s equation
predict?

Answer 3

• 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 4

Define gene linkage

Answer 4

• (Genes/loci) on same chromosome;

Question 5

Define epistasis

Answer 5

• The allele of one gene affects or masks the
  expression of another in the phenotype;
• when two genes interact

Question 6

Describe why observed phenotypes
don’t match expected values?

Answer 6

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

Question 7

Define codominance

Answer 7

Both alleles expressed in the phenotype;

Question 8

What is meant by the term phenotype?

Answer 8

● The physical expression of the genotype;
● And the physical expression of environmental
factors;

Question 9

Define dihybrid inheritance.

Answer 9

Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time.

Question 10

What is meant by sex linkage?

Answer 10

Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual.

Question 11

Why are males more likely to express a recessive sex-linked allele?

Answer 11

Most sex linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if its recessive. Since females get two alleles, they’re less likely.

Question 12

Which parent do males inherit sex-linked characteristics from?

Answer 12

The mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass the trait on.

Question 13

What is meant by autosomal linkage?

Answer 13

Where there are two or more genes located on the same (non sex) chromosome. In this case, only 1 homologous pair is needed for all four alleles to be present. For genes that aren’t linked, two homologous pairs are needed.

Question 14

Define two types of epistatis.

Answer 14

Recessive epistasis - where two homozygous recessive alleles mask expression of another allele.
Dominant epistasis - where one dominant allele masks the expression of multiple other alleles

Question 15

What is the chai squared test?

Answer 15

A statistical test to find out whether the difference between observed and expected is due to chance or a real life effect.

Question 16

What are the criteria for the chai squared test?

Answer 16

• data placed in discrete categories
• large sample size
• only raw count data allowed (not %)
• no data values equal zero