7 Genetics, populations, evolution and ecosystems (A-level only)

Question 1

What is a gene?

Answer 1

A gene is the length of DNA, a sequence of nucleotides that normally code for a particular polypeptide.

Polypeptides can function as enzymes necessary for the production of specific characteristics.

Question 2

What are alleles?

Answer 2

Alleles are different forms of a gene that occupy the same locus.

An individual can have two or more different alleles for a gene.

Question 3

Can more than one allele occur at the locus of any chromosome?

Answer 3

No, only one allele of a gene can occur at the locus of any chromosome.

Question 4

What is the characteristic of chromosomes in diploid organisms?

Answer 4

In diploid organisms, chromosomes occur in homologous pairs, with each pair having 2 loci carrying one allele of a gene each.

Question 5

What is heterozygous?

Answer 5

Heterozygous refers to having different alleles on each chromosome.

Question 6

What happens when an organism is heterozygous?

Answer 6

Usually, only one allele is expressed in the phenotype.

Question 7

What is the dominant allele?

Answer 7

The dominant allele is expressed when heterozygous.

Question 8

What is the recessive allele?

Answer 8

The recessive allele is not expressed when heterozygous.

Question 9

What does homozygous mean?

Answer 9

Homozygous means having the same alleles on each chromosome.

Question 10

What is homozygous dominant?

Answer 10

Homozygous dominant occurs when both alleles are dominant, leading to the expression of the dominant trait.

Question 11

What is homozygous recessive?

Answer 11

Homozygous recessive occurs when both alleles are recessive, leading to the expression of the recessive trait.

Question 12

Recessive alleles need to be _______ to be expressed.

Answer 12

homozygous recessive.

Question 13

What is the order of producing a genetic cross diagram ?

Answer 13

1- Alleles
2- Parent Phenotypes
3- Parent genotypes
4- Gametes
5- Punnett square
6- Offspring genotypes
7- Offspring phenotypes

Question 14

What is monohybrid inheritance?

Answer 14

Monohybrid inheritance is the inheritance of characteristic controlled by a single gene.

Question 15

Who studied the color of the pods of pea plants? What did he discover?

Answer 15

Gregor Mendel
He found that if pea plants repeatedly breed with each other and give rise to green pods - pure breeding for green pods - they are homozygous
- if pure breeding green plants are bred with pure breeding yellow plants, all offspring ( in filial generation) produce green pods

Green pods = dominant
Yellow pods = recessive

Question 16

What is dihybrid inheritance?

Answer 16

The inheritance of 2 characteristics from 2 different genes located on different chromosomes at the same time.

F1= 4 types of gametes because the gene for one characteristic and gene for another are on separate chromosomes.
F2= 2 F1’s bred together

Question 17

What is the typical ratio for a dihybrid cross in F2 generation?

Answer 17

9:3:3:1

Question 18

What’s the typical ratio for a monohybrid cross in F2 generation?

Answer 18

3:1

Question 19

What is the genotypes of an organism?

Answer 19

The genetic constitution of an organism

Question 20

What is the phenotype of an organism?

Answer 20

The expression of the genetic constitution of an organism and its interaction with the environment.

Question 21

What does the Law of Segregation state?

Answer 21

In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete.

Question 22

According to the Law of Segregation, can both alleles from a pair be present in one gamete?

Answer 22

No, only one of each pair of alleles can be present in a single gamete.

Question 23

What does the Law of Independent Assortment describe?

Answer 23

Each member of a pair of alleles may combine randomly with either of another pair.

Question 24

In the genotype RrGg, how can the alleles combine according to the Law of Independent Assortment?

Answer 24

Any R can combine with any of the G’s, it is completely random which one pairs.

Question 25

True or False: The allele a gamete receives for one gene influences the allele received for another gene.

Answer 25

False.

Question 26

What happens during meiosis regarding homologous chromosomes?

Answer 26

Homologous chromosomes are randomly distributed to daughter cells.

Question 27

What is the result of the independent segregation of different chromosomes during meiosis?

Answer 27

Gametes that have unique combinations of chromosomes.

Question 28

What is codominance?

Answer 28

When both alleles are expressed in the phenotype, both equally dominant, no recessive.

Question 29

What does an allele code for ?

Answer 29

A protein or enzyme which produces the characteristic.

Question 30

Define the term multiple alleles.

Answer 30

When there are more than two alleles of which two may be present at the loci of an individual’s homologous chromosomes.

Question 31

Why is there a maximum of two alleles that can be present at the loci of an individual at any one time?

Answer 31

There are only two homologous chromosomes so only two gene loci, one for each allele.

Question 32

Give an example of multiple alleles.

Answer 32

Blood Groups - There are three alleles with the gene I (immunoglobulin gene), which leads the the presence of different antigens on the cell surface membrane of red blood cells.

Question 33

Name the different alleles within the immunoglobulin gene and the different antigens they lead to.

Answer 33

Allele I^A - leads to the production of antigen A Allele I^B - leads to the production of antigen B Allele I^O - which does not lead to the production of an antigen

Question 34

Which alleles are co-dominant in blood groups?

Answer 34

Alleles I^A and I^B whereas allele I^O is recessive to both

Question 35

Which blood group is the universal donor and why?

Answer 35

Blood group O as it has no antigens on their surface so cannot be rejected

Question 36

Which blood group are universal recipients?

Answer 36

Blood group AB, as they can receive A and B antigens as well as O blood as that has no antigens.

Question 37

How any pairs of chromosomes do humans have?

Answer 37

23 pairs, of which 22 pairs have homologous partners that are identical in appearance, whether in a male or female.

Question 38

Why are the sex chromosomes the pair that doesn’t have an equivalent homologous portion?

Answer 38

The X chromosomes is much longer than the Y and differs in shape n size so for most of the length of the X chromosome their is no equivalent homologous portion of the Y.

Question 39

How many typed of gametes can be produced in a male/female?

Answer 39

Male - (X or Y) Female - 1 (X)

Question 40

Why do the characteristics controlled by recessive alleles on the non homologous portion of the X chromosome appear more frequently in the male?

Answer 40

There is no homologous portion on the Y chromosome that may have the dominant allele to prevent the expression of the recessive allele.

Question 41

What is the probability of sex inheritance?

Answer 41

50/50

Question 42

How is an x-linked disorder caused?

Answer 42

A defective gene on the x-chromosome.

Question 43

Give an example of an x-linked disorder?

Answer 43

Haemophilia - causes blood to clot more slowly, can be lethal if not treated. The condition is almost entirely confined to males but haemophiliac females do exist.

Question 44

What’s one of the causes of haemophilia?

Answer 44

A recessive allele with an altered sequence of DNA nucleotide bases on the X chromosome , coding for a faulty protein which is non-functioning.

Question 45

How is haemophilia inherited?

Answer 45

Males - inherit it from their mothers as Y chromosome from father doesn’t contain the faulty allele Females - inherit gene from either parent (X,X) and will be carrier if faulty gene is only inherited from on parent.

Question 46

What is autosomal linkage?

Answer 46

Where two or more genes are carried on the same autosome, assuming there is no crossing over these genes remain together during meiosis and so pass into gametes and hence the offspring, together.

Question 47

What is a gene?

Answer 47

A small section of DNA coding for a particular sequence of amino acids, coding for a specific protein.

Question 48

What is an autosome?

Answer 48

The remaining 22 chromosomes that aren’t sex chromosomes.

Question 49

What is epistasis?

Answer 49

When the allele of one gene affects one masks the expression of another in the phenotype.

Question 50

Give the reason why genetic crosses are rarely the same as predicted results

Answer 50

- fertilisation is random - small/not large population/sample - selection advantage/disadvantage

Question 51

What is meant by the term recessive epistasis and what’s the typical ratio?

Answer 51

When the presence of two recessive alleles at the first locus prevents the expression of the allele at the end of the locus. Typical ratio in F2 generation = 9:4:3

Question 52

What is meant by dominant epistasis and what’s the typical ratio?

Answer 52

Having at least one allele of the gene on the first locus blocks the expression of the gene at the 2nd locus. Typical ratio in F2 = 12:3:1

Question 53

What is the Chi-squared test?

Answer 53

A statistical test used to find out whether the difference between observed vs expected data is due to chance (accepting or rejecting the null hypothesis)

Question 54

What is the null hypothesis?

Answer 54

No significant difference between observed/expected data/frequencies (opposite to alternate hypothesis)

Question 55

How do you calculate degrees of freedom?

Answer 55

No. of categories - 1

Question 56

How do you know when to accept/reject the null hypothesis?

Answer 56

If the critical value at p= 0.05 ( 5% probability ) you can reject the null hypothesis and accept the alternative hypothesis