Unit 7 - Inherited Change

Question 1

Gene

Answer 1

• A section of DNA that codes for a specific sequence of amino acids which makes up the primary structure of a protein
• Alternatively codes for functional RNA (rRNA or tRNA)

Question 2

Allele

Answer 2

• An alternative form of a gene
• Occupies the same locus as a gene

Question 3

Genotype

Answer 3

All of the alleles that an organism carries on a chromosome

Question 4

Phenotype

Answer 4

The observable characteristics of an organism as a result of its genotype and environment

Question 5

Dominant

Answer 5

Only a single allele has to be present for this characteristic to be expressed

Question 6

Recessive

Answer 6

The characteristic is only displayed if no dominant allele is present

Question 7

Locus

Answer 7

The position occupied by a gene on a chromosome

Question 8

Homozygous

Answer 8

The 2 alleles of a characteristic are identical

Question 9

Heterozygous

Answer 9

The 2 alleles of a characteristic are different

Question 10

Monohybrid Inheritance

Answer 10

Transmission of one gene from parents to their offspring

Question 11

F₁ Generation

Answer 11

• The offspring from a cross between individuals with homozygous dominant and homozygous recessive genotypes
• E.g. TT and tt
• All the F₁ generation in a monohybrid cross will be heterozygous and express the dominant trait

Question 12

F₂ Generation

Answer 12

• The offspring from a cross between 2 heterozygous individuals from the F₁ generation
• The offspring in the F₂ in a monohybrid cross will exhibit a 3:1 ratio of of dominant to recessive phenotypes
• 1 homozygous dominant, 2 heterozygous, 1 homozygous recessive

Question 13

Dihybrid Inheritance

Answer 13

• Simultaneous inheritance of two genes controlling separate characteristics
• Hair colour and eye colour

Question 14

F₁ Generation for a Dihybrid Cross

Answer 14

All have an identical genotypes and therefore identical phenotypes

Question 15

F₂ Generation for a Dihybrid Cross

Answer 15

9:3:3:1 ratio of

Question 16

Reasons for unexpected ratios in a Dihybrid Cross

Answer 16

Random Fertilisation:
- Gamete fusion is a chance process
- Small sample sizes can lead to unexpected ratios but large samples minimise random effects
Linked Genes
- Close together on the same chromosome so during crossing over of non-sister chromatids they are crossed together
- As a result they are inherited together which decreases variation

Question 17

Codominance

Answer 17

• 2 alleles are equally expressed in an organism
• Results in a blend of the 2 characteristics or both characteristics being seen together

Question 18

Multiple Alleles

Answer 18

• Genes that exist in more than 2 allelic forms
• An individual can only have two alleles of a specific gene at any one time
• E.g Blood type alleles exist as IA, IB and IO

Question 19

Blood Type Inheritance

Answer 19

• IA and IB are codominant and are both expressed when inherited together
• IO is recessive

Question 20

Sex Linkage

Answer 20

The inheritance of genes located on the X or Y chromosomes

Question 21

Why are some sex linked conditions more common in men?

Answer 21

• Men’s sex chromosomes are XY
• Most sex linked genes are X-linked
• The lack of a 2nd X chromosome means that if a man carries a faulty allele on their X chromosome is has to be expressed as it can’t be masked by another X chromosome

Question 22

Autosomes

Answer 22

Chromosomes that don’t determine the sex of an organism

Question 23

Autosomal Linkage

Answer 23

When genes on an autosome are linked

Question 24

How does Autosomal Linkage impact Variation?

Answer 24

Decreases variation because:
- Non random association of alleles at different loci
- Phenotypic ratios observed in dihybrid crosses are unexpected compared to those that arise from independent assortment
- Parental allele combinations are preserved across generations

Question 25

Epistasis

Answer 25

Interactions between alleles where the expression of genes at a different locus is masked or supressed by another allele

Question 26

Hypostatic Gene

Answer 26

The gene whose expression is blocked by another gene

Question 27

Epistatic Gene

Answer 27

The gene whose allele affects the expression of the hypostatic gene

Question 28

Dominant Epistasis

Answer 28

- A dominant allele at one gene locus masks the expression of alleles at a second gene locus - Results in a 12:3:1 phenotype ratio

Question 29

Recessive Epistasis

Answer 29

- 2 recessive alleles at one gene locus will mask the expression of alleles at a second gene locus - Results in a 9:3:4 phenotype ratio

Question 30

Chi Squared Equation

Answer 30

x² = ∑(O-E)²/E - O = Observed phenotype number - E = Expected phenotype number - ∑ = The sum of