T8 - Origins of Genetic Variation

Question 1

How do gene mutations increases genetic variation?

Answer 1

Gene mutations give rise to new alleles of a gene

Question 2

What is a mutation?

Answer 2

Change to base sequence in a gene

Question 3

Explain why a deletion or addition is more likely to be harmful than a substitution.

Answer 3

A substitution may result in one codon being different, resulting in only one different amino acid in the polypeptide chain.
However if bases are added/deleted it may result in a frame shift, which means all the codon and therefore all the amino acids will be different following the mutation.

Question 4

Why might a mutation not result in a change to the amino acid sequence in the protein the gene codes for?

Answer 4

Because the genetic code is degenerate: more than one codon codes for the same amino acids.

Question 5

Explain how meiosis increases genetic variation

Answer 5

Crossing over of non-sister chromatids (in Prophase I) gives rise to new combinations of alleles in gametes.
Independent assortment of chromosomes gives rise to different combinations of maternal and paternal chromosomes in gametes (due to random orientation of homologous pairs in metaphase I and of chromatids in metaphase II).

Question 6

Explain how random fertilisation during sexual reproduction brings about genetic variation.

Answer 6

Every gamete contains different DNA due to events during meiosis.
The combination of gametes that fuses to form the zygote is also random.

Question 7

What is discontinuous variation?

Answer 7

A type of variation within a population produced by monogenic inheritance which produces phenotypes that can be placed into distinct categories

Question 8

What is continuous variation?

Answer 8

A type of variation within a population produced by polygenic inheritance where the phenotypes are spread over a range of values.

Question 9

What is an allele?

Answer 9

An allele is an alternative form of the same gene.

Question 10

State what is meant by recessive allele

Answer 10

A recessive allele is only expressed in the phenotype if two copies are present.

Question 11

State what is meant by dominant allele

Answer 11

A dominant allele is always expressed in the phenotype, even if one copy is present.

Question 12

Define the term homozygote

Answer 12

An organism that carries two of the same alleles of a gene (i.e. is homozygous).

Question 13

Define the term heterozygote

Answer 13

An organism that carries two different alleles of a gene (i.e. is heterozygous).

Question 14

State what is meant by codominant allele.

Answer 14

Codominant alleles both get expressed in the phenotype of an organism without mixing e.g. blood type.

Question 15

What are multiple alleles?

Answer 15

More than two possible versions of the same gene.

Question 16

Give an example of a characeteristic controlled by multiple alleles.

Answer 16

Blood type

Question 17

Define the term genotype

Answer 17

The combination of alleles of a gene carried by an organism

Question 18

Define the term phenotype

Answer 18

The observable characteristic of an organism resulting from the expression of alleles of a gene and the environment.

Question 19

How would you determine the genotype of an individual with the dominant phenotype?

Answer 19

Carry out a test cross with a known homozygous recessive individual:
If all offspring have the dominant phenotype, the unknown parent must be homozygous dominant.
If recessive phenotype appears in offspring, the unknown parent must be heterozygous.

Question 20

What are lethal alleles?

Answer 20

Alleles that result in death. Recessive lethal alleles cause death of an organism if two are present, but one can be tolerated e.g. cystic fibrosis, sickle cell, haemophilia. Individuals that carry a recessive lethal allele are called ‘carriers’.

Question 21

What is the typical phenotypic ratio expected in a monohybrid cross of heterozygous individuals?

Answer 21

3:1

Question 22

What is the typical phenotypic ratio expected in a dihybrid cross of heterozygous individuals?

Answer 22

9:3:3:1

Question 23

Explain how a mutation in the beta globin gene results in sickled red blood cells.

Answer 23

The single base substitution in the 𝛃-globin gene results in a different codon in the mRNA produced via transcription. This codon codes for a different amino acid - valine (Val) instead of glutamic acid (Glu) to be added to the 𝛃-globin polypeptide. As a result, the 𝛃-globin polypeptide folds incorrectly, causing it to become ‘sticky’, resulting in red blood cells becoming sickle shaped.

Question 24

What are unlinked genes?

Answer 24

Genes which are on different chromosomes which may be subject to independent assortment and are often not inherited together.

Question 25

What is autosomal linkage?

Answer 25

Linked genes are pairs or groups of genes which are inherited together as they are carried on the same chromosome / autosome (non-sex chromosomes).

Question 26

What is the typical phenotypic ratio expected in a dihybrid cross of heterozygous individuals for linked genes?

Answer 26

3:1

Question 27

If the offspring from a dihybrid cross of heterozygous individuals for linked genes results in unexpected phenotypes, what has occurred?

Answer 27

Crossing over occurred between gene loci to produce a small number of gametes containing recombinant chromosomes.

Question 28

Explain what determines how linked genes are?

Answer 28

The more close together the genes are on the chromosome, the more closely linked they are. This is because a recombination event during meiosis is much less likely to occur between the loci of the two genes.

Question 29

What are sex-linked genes?

Answer 29

Genes which are usually inherited together as they are on the same sex chromosome and so expression is sex-dependant.

Question 30

What sex chromosomes do females carry?

Answer 30

XX

Question 31

What sex chromosomes do males carry?

Answer 31

XY

Question 32

Why do males only have one copy of some genes?

Answer 32

Males have an X and Y chromsome. The X chromosome is larger; part of it does not have a homologous section on the Y chromosome, therefore only one allele of a gene will be present.

Question 33

Most X-linked diseases are from recessive alleles, so why does this mean that they will be more common in males?

Answer 33

Males only have one X chromosome. As X is larger, part of it does not have a homologous section on the Y chromosome. If the gene is carried on this non-homologous section, they will only have one allele, which will always be expressed.

Question 34

Give two examples of X-linked genetic disorders

Answer 34

Haemophilia, red-green colour blindness

Question 35

Name the statistical test used to test whether there is a significant difference between observed and expected frequencies

Answer 35

Chi Squared

Question 36

When can you use the chi-squared test?

Answer 36

if the sample size is sufficiently large (>20). It can only be used for discontinuous variation data.

Question 37

State the null hypothesis for a chi-squared test.

Answer 37

There is no significant difference between the observed and the expected frequencies.

Question 38

How do you calculate the degrees of freedom for a chi-squared test?

Answer 38

Number of categories/phenotypes - 1

Question 39

What is the critical value?

Answer 39

The value that is compared to the chi-squared value to determine whether to accept or reject the null hypothesis

Question 40

What probability (significance) level do biologists use in statistical tests?

Answer 40

p = 0.05 (5%)

Question 41

What if the chi squared value is less than the critical value at p=0.05?

Answer 41

ACCEPT null hypothesis; there is no statistically significant difference between expected and observed results; there is more than 5% probability that any difference is due to chance.

Question 42

What if the chi squared value is equal to or greater than the critical value at p=0.05?

Answer 42

REJECT null hypothesis; there is a statistically significant difference between expected and observed results; there is less than 5% probability that the difference is due to chance.

Question 43

If the chi-squared test shows there is a significant difference between the expected and observed frequencies of phenotypes in the offspring, what can this indicate?

Answer 43

Autosomal gene linkage / recombination has occurred