Genetics & Inheritance

Question 1

State three causes of genetic variation

Answer 1

• Mutation
• Crossing over
• Independent segregation
• Random fusion of gametes / fertilisation

Question 2

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

Answer 2

• Change in sequence of amino acids
• Change in (position of) hydrogen, ionic, disulfide bonds
• Alters tertiary structure

Question 3

Describe the process of crossing over and explain how it increases genetic diversity.

Answer 3

• Homologous pairs of chromosomes associate
• bivalent forms
• Chiasmata form
• Equal lengths of (non-sister) chromatids exchanged
• Producing new combinations of alleles

THIS IS ‘RARE’ = Unequal chance of recombinant alleles forming.

Question 4

Give two differences between mitosis and meiosis.

Answer 4

Mitosis given first

1. One division, two divisions in meiosis;
2. (Daughter) cells genetically identical, daughter cells genetically different in meiosis;
3. Two cells produced, (usually) four cells produced in meiosis;
4. Diploid to diploid/haploid to haploid, diploid to haploid in meiosis;
5. Separation of homologous chromosomes only in meiosis;
6. Crossing over only in meiosis;
7. Independent segregation only in meiosis;

Question 5

What is meant by a genome?

Answer 5

all the ‘genes’ in a cell

Question 6

What is a ‘gene pool’?

Answer 6

All the alleles in a population.

Question 7

How do multiple alleles of a gene arise?

Answer 7

• mutations;
• at different positions within the same gene;

Question 8

What is meant by a recessive allele?

Answer 8

Only expressed (in the phenotype) when homozygous

Question 9

Define dominant allele

Answer 9

Always expressed within the phenotype

Question 10

Define codominance

Answer 10

Both alleles (equally) expressed in the phenotype

Question 11

What is meant by the term phenotype

Answer 11

expression of its genetic constitution and its interaction with the environment

Question 12

Hardy Weinberg Equations

Answer 12

1. P + q = 1.0
   (P=dominant, q=recessive)

2.P^2 + 2Pq + q^2 = 1.0
(Pq= heterozygous)

Question 13

What does Hardy Weinberg’s equation predict?

Answer 13

the allelic frequency from one generation to the next will remain constant if there is no
* migration
* gene mutation
* selection pressures

*should be large population and random mating

Question 14

Expected offspring phenotype ratio from heterozygous parents:

Monohybrid
Codominance

Answer 14

M= 3 : 1
C= 1:2:1

Question 15

Monohybrid

Answer 15

• inheritance of a single gene, determines single characteristic

Question 16

Monohybrid multiple alleles

Answer 16

Question 17

Codominance

Answer 17

Question 18

Rules for Dominant alleles

Answer 18

• 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 19

Rules for recessive alleles

Answer 19

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

Question 20

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 20

• Small sample size;
• fertilisation of gametes is random;
• crossing over;

Question 21

Define independent segregation

Answer 21

• homologous chromosomes attach to spindle fibres and pair up side by side
• maternal and paternal chromosomes reshuffle in any combination

Question 22

Sex linked characteristics

Answer 22

• unaffected mum (het) has affected son
  unaffected father has affected son
• affected father has unaffected daughter (if mum homo D)

Question 23

Not sex linked

Answer 23

dad and daughter:
-if daughter affected (Homo R)
- dad not
-recessive characteristic
-not SL

mum and son:
- if son affected
- mum not (homo r)
-dominant characteristic
-not SL

Question 24

Dihybrid inheritance

Answer 24

Phenotype inherited as a result of 2 different genes, so 2 characteristics

can be on different chromosomes or linked on same

Question 25

Dihybrid ratio

Answer 25

9:3:3:1

Question 26

Autosomal linkage (dihybrid)

Answer 26

-chromosomes not directly in determining sex of organism

independent segregation and crossing over can randomly produce recombinant offspring
offspring displaying new combo of alleles from either parents (4:4:1:1)

Question 27

Linkage group definition

Answer 27

set of genes on same chromosome which tend to be inherited together (dont indep seg)

Question 28

What happens when 2 genes are autosomal linked

Answer 28

-Don’t follow dihybrid ratio
-follow monohybrid ratio (3:1)
-offspring tend to produce parental phenotypes

-recombinant phenotypes only evident if crossing over occurs in meiosis 1 and would be expected to appear in low numbers

genes closer together- less likely to be separated by crossing over

Question 29

Epistasis

Answer 29

interaction between 2 genes where one allele affects expression of other, leading to graduation of phenotype

Question 30

Epistasis ratios

Answer 30

15:1
9:7
9:4:3

add to 16 but not 9:3:3:1