6.1.2: Patterns of inheritance

Question 1

Types of variation

Answer 1

• Discontinuous

* Continuous

Question 2

Discontinuous variation

Answer 2

• Qualitative differences between phenotypes
• Clear phenotypic categories (blood group, sex)
• Generally monogenic; if polygenic, genes interact in an epistatic way
• Different alleles at single locus have large effects on phenotype
• Unaffected by environment

Question 3

Continuous variation

Answer 3

• Quantitative differences between phenotypes
• No distinct categories (height, weight)
• Usually polygenic; each gene provides an additive effect to the phenotype
• Strongly influenced by the environment

Question 4

Polygenic (of a characteristic)

Answer 4

A characteristic is controlled by two or more genes

Question 5

Both _______ and ________ contribute to phenotype

Answer 5

Both genotype and environment contribute to phenotype

Question 6

Example of how genotype and environment contribute to phenotype

Answer 6

Height
⟶ Have genetic potential for certain height
⟶ Malnutrition can mean this potential is not reached

Question 7

Variation

Answer 7

differences between members of the same species, arising as a result of mutations and essential in natural selection and therefore evolution.

Question 8

Allele

Answer 8

version of a gene

Question 9

Dominant allele

Answer 9

version of a gene that will always be expressed if present

Question 10

Recessive allele

Answer 10

version of a gene that will only be expressed if two copies of the allele are present

Question 11

Genotype

Answer 11

The genetic information of an organism

Question 12

Phenotype

Answer 12

The observable characteristics of an organism

Question 13

Homozygous

Answer 13

Two identical alleles for a characteristic

Question 14

Heterozygous

Answer 14

Two different alleles for a characteristic

Question 15

How variation plays a role in selection

Answer 15

• When environment changes, organisms well adapted will survive and reproduce
• They pass on ALLELES to offspring
• This forms the basis of evolution by natural selection

Question 16

How is genetic variation produced?

Answer 16

• Prophase 1: crossing over of non-sister chromatids –> formation of recombinants
• Metaphase 1: independent assortment –> it is random which chromosomes end up at each pole of the cell
• Metaphase 2: Random orientation –> centromere splits, different alleles to each pole of cells –> large number of possible allele combinations for gametes
• Mutations (random) in DNA replication in Interphase
• Fusion of gametes in fertilisation is random

Question 17

What is an example of codominant alleles in context of disease

Answer 17

• Sickle cell anaemia
• Missense mutation
• When haemoglobin is deoxygenated, becomes crystalline and deforms RBC, eventually impeding blood flow and leading to tissue damage
• Codominant heterozygotes: typically phenotypically symptomless ∵ presence of normal Hb in RBC prevents sickling

Question 18

What is sex linkage?

Answer 18

When genes are carried on (either sex chromosome)

⟶ Tend to be carried on the X chromosome because Y chromosome v. short

Question 19

In a classic Mendelian dihybrid cross

Answer 19

• The 2 genes don’t affect each other
• The 2 genes are not linked on the same chromosome
• Independently assorted at M1 of meiosis

Question 20

Why do actual ratios differ from expected ratios?

Answer 20

• Random fertilisation
• Genes may be linked
⟶ Larger sample = closer

Question 21

Recessive epistasis

Answer 21

A homozygous recessive genotype at 1 locus overrides the genotype at the other locus, even if there is a dominant allele present there.

Question 22

Difference between epistasis and dominance?

Answer 22

Epistasis occurs between 2 genes coding for different characteristics
Dominance occurs between 2 alleles of the same gene

Question 23

Epistasis

Answer 23

Interaction between gene loci; occurs between two genes coding for different characteristics.

Question 24

Dominant epistasis

Answer 24

A single dominant allele at 1 locus overrides all alleles at the other locus.

Question 25

The alleles that are masking the effect are called the...

Answer 25

epistatic alleles

Question 26

The alleles whose effect is being masked is called the...

Answer 26

hypostatic alleles

Question 27

Ratio that suggests dihybrid inheritance of 2 unlinked genes

Answer 27

9:3:3:1

Question 28

Ratio that suggests recessive epistasis

Answer 28

9:3:4

Question 29

Ratio that suggests dominant epistasis

Answer 29

12:3:1 or 13:3

Question 30

Ratio that suggests epistasis by complementary action

Answer 30

9:7

Question 31

9:7 suggests

Answer 31

Epistasis by complementary action

Question 32

12:3:1 or 13:3 suggests

Answer 32

Dominant epistasis

Question 33

9:3:4 suggests

Answer 33

Recessive episasis

Question 34

9:3:3:1 suggests

Answer 34

Dihybrid inheritance of 2 unlinked genes

Question 35

Hardy Weinberg equation describes populations that

Answer 35

are not evolving

Question 36

Genotype frequencies stay the same if what conditions are met:

Answer 36

1) Very large population: no genetic drift 2) No gene flow: no emigration or immigrant 3) No mutations: no new alleles added to the gene pool 4) Random mating: no sexual selection 5) No natural selection: all traits aid equally in survival

Question 37

Hardy Weinberg equilibrium: equations

Answer 37

p + q = 1.00 p² + 2pq + q² = 1.00

Question 38

In the Hardy-Weinberg equilibrium equations, what does p represent?

Answer 38

Frequency of the dominant allele

Question 39

In the Hardy-Weinberg equilibrium equations, what does q represent?

Answer 39

Frequency of the recessive allele

Question 40

In the Hardy-Weinberg equilibrium equations, what does p² represent?

Answer 40

Frequency of homozygous dominant genotype

Question 41

In the Hardy-Weinberg equilibrium equations, what does q² represent?

Answer 41

Frequency of homozygous recessive genotype

Question 42

In the Hardy-Weinberg equilibrium equations, what does 2pq represent?

Answer 42

Frequency of the heterozygous genotype

Question 43

Factors affecting evolution

Answer 43

* Mutation * Sexual selection * Gene flow * Genetic drift * Natural selection

Question 44

How do mutations affect evolution?

Answer 44

Mutations lead to new alleles

Question 45

How does sexual selection affect evolution?

Answer 45

Sexual selection leads to alleles coding for mating success.

Question 46

Epistasis by complementary action

Answer 46

At least 1 dominant allele at each different loci needed to produce an effect

Question 47

Why is genetic drift more of a concern in small populations?

Answer 47

* 1 individual or allele has a higher effect proportionally in a smaller population * Less variation so more vulnerable to environmental change e.g. disease

Question 48

Why is maintaining genetic diversity good?

Answer 48

* Useful in changing climate * Prevention of inbreeding depression * Promotion of hybrid vigour * Prevent dwindling gene pool