Genetics & Inheritance 7.1

Question 1

State three causes of genetic variation (3)

Answer 1

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

Question 2

What is meant by a genome? (1)

Answer 2

All the DNA in an organism
/ The total number of DNA bases / genetic material

Question 3

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios. Suggest two reasons why (2)

Answer 3

• small sample size;
• fusion/fertilisation of gametes is random;
• Crossing over / linked genes / sex-linkage;
• Epistasis;
• Lethal genotypes

Question 4

What is a gene pool? (1)

Answer 4

All the alleles in a population

Question 5

What does Hardy Weinberg’s equation predict?

Answer 5

• The frequency of alleles (on one gene);
• will stay constant over (many) generations;
• as long as there is:
  no mutation, no selection, no genetic migration, random mating & large population

Question 6

How do multiple alleles of a gene arise? (1)

Answer 6

• mutations;
• which are different / at different positon/loci of a gene

Question 7

What is meant by a recessive allele? (1)

Answer 7

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

Question 8

In fruit flies, the genes for body colour and wing length are linked. Explain what this means (1)

Answer 8

1. Both genes appear on the same pair of homologous chromosomes / autosomes

Question 9

Define gene linkage (1)

Answer 9

Genes on the same chromosome

e.g A & B and a & b

Question 10

Define epistasis (1)

Answer 10

The allele of one gene affects the expression of another in the phenotype

Question 11

Describe why observed phenotypes don’t match expected values

Answer 11

• Fertilisation/fusion of gametes is random;
• small population/sample size;
• Selection advantage/disadvantage/Lethal alleles

Question 12

Define codominance (1)

Answer 12

Both alleles expressed (equally) in the phenotype

Question 13

Rules for dominant alleles (3):

Answer 13

• Affected offspring must have at least one affected parent
• Unaffected parents ONLY have unaffected offspring
• Both parents must be heterozygous if are affected and have unaffected offspring

Question 14

Rules for recessive alleles (1):

Answer 14

• Unaffected parents can have affected offspring if they are heterozygous

Question 15

Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why (2)

Answer 15

• Recessive allele is always expressed in males / males have 1 recessive allele;
• Females need two recessive alleles / need to be homozygous recessive

Question 16

What are the expected offspring phenotype ratios from heterozygous parents in:
1. Monohybrid
2. Dihybrid
3. Epistasis
4. Autosomal Linkage

Answer 16

Dominant : Recessive
1. 3:1
2. 9:3:3:1
3. 9:4:3 or 15:1 or 9:7
4. 3:1 (recombinant if no crossing over)

Question 17

What is meant by the term phenotype? (1)

Answer 17

• Expression of genetic constitution/genome
  due to interaction with environment

Question 18

Explain how a single base substitution causes a change in the structure of a polypeptide (3)

Answer 18

• change in sequence of amino acids/primary structure;
• change in hydrogen/ionic/disulphide bonds;
• alters tertiary structure

Question 19

What is an allele?

Answer 19

different forms of a gene at the same locus of a chromosome
(one gene can have many alleles)

Question 20

How many alleles per gene do diploid organisms have?

Answer 20

2

Question 21

What is meant by homozygous and heterozygous?

Answer 21

Homozygous = both alleles are either dominant or recessive
Heterozygous = one allele is dominant and one is recessive

Question 22

Define monohybrid inheritance (1)

Answer 22

One phenotypic characteristic is controlled by a single gene

Question 23

Define dihybrid inheritance (1)

Answer 23

Two different characteristics are controlled by two different genes on DIFFERENT CHROMOSOMES at the SAME TIME.

Question 24

What is meant by sex-linkage? (1)

Answer 24

Where an allele is located on one sex chromosome, so its expression depends on the sex of the individual

Question 25

Which parent do males inherit sex-linked characteristics from?!

Answer 25

Mother - Y chromosome can only be inherited from father, so the mother must be a heterozygous carrier for sex-linked alleles

Question 26

Dads + Daughters =
Mums + Sons =

Answer 26

1. Recessive
2. Dominant

Question 27

What gametes would be produced from sex-linked genotypes: (Mother) XH Xh and (Father) XH Y

Answer 27

XH XH - Normal female
XH Xh - Carrier female
XH Y - Normal male
Xh Y - Affected male

Question 28

Describe autosomal linkage (2)

Answer 28

Two or more genes are located on the same (NON-SEX) chromosome.
Only one homologous pair is needed for all 4 alleles to be present
(Non-linked genes need two homologous pairs)

Question 29

Describe epistasis (1)

Answer 29

One (non-linked) gene masks the expression of another (non-linked) gene

E.g Allele for baldness affects expression of allele for hair colour?

Question 30

Define the two types of epistasis

Answer 30

Recessive epistasis: two homozygous recessive alleles (aa) mask expression of another allele

Dominant epistasis: one dominant allele (B) affects expression of multiple alleles
e.g B for baldness affects hair colour, length, curls, etc.

Question 31

Define the Chi-squared test (1)

Answer 31

Statistical test that finds out whether a difference between observed and expected data is due to random chance or not.
DATA IS CATEGORIC!

Question 32

Chi-Squared test MUST HAVE (4):

Answer 32

• Data in categories
• Large sample size
• Only raw data (no %)
• No data values = 0

Question 33

How is a Chi-squared test used in inheritance? (1)

Answer 33

Compare expected and observed phenotypic ratios (shows what alleles are inherited)

Question 34

Formula for Chi-Squared test

Answer 34

χ2 = (O - E)2
∑ ———
E

O = Observed
E = Expected

∑ = Sum

Question 35

How is the Chi-Squared test carried out? (3)

Answer 35

• Formula from table of categoric data (Χ2 = ∑ (O – E)2 divided by E)
• Find appropriate degrees of freedom as N - 1 (N = number of outcomes, e.g head or tails)
• Compare formula number with critical value (P)
• If number is greater than/equal to critical value, there is a significant difference between O and E / NOT due to chance

Question 36

Use your knowledge of gene linkage to explain the offspring of GgNn and ggnn (4)

Answer 36

Work out gametes
1. GN and gn are linked;
2. GgNn parent produces mainly GN and gn gametes;
3. Crossing over produces few Gn and gN gametes (as rare);
4. (so) fewer Ggnn and ggNn individuals.

Question 37

What is the expected ratio of offspring phenotypes from a heterozygous parent (GgNn) crossed with a homozygous parent? (ggnn)

Answer 37

1:1:1:1

Heterozygous produces 4 different types of gametes, homozygous only produces 1 type, so 4 types of offspring produced.

Question 38

Which statistical test could the scientist use to determine whether his observed results were significantly different from the expected results? (2)

Answer 38

1. Chi-squared (test);
2. (Because) the data is categoric.

Question 39

Define a linkage group (1)

Answer 39

A linkage group is:
A set of genes on the same chromosome which tend to be inherited together