Genetics

Question 1

What is a gene?

Answer 1

A gene is a small section of DNA (a sequence of bases) that codes for a specific polypeptide or functional RNA.

Question 2

What are alleles?

Answer 2

Alleles are alternate versions of a gene that are found at the same locus in a pair of chromosomes.

Question 3

What are homologous chromosomes?

Answer 3

Homologous chromosomes are chromosomes which are similar in size and shape and they carry genes controlling the same features at the same loci on each.

Question 4

What are dominant alleles?

Answer 4

Versions of a gene which are always expressed when present. They stop recessive alleles from presenting. They are shown in genetic diagrams with a capital letter.

Question 5

What are recessive alleles?

Answer 5

Versions of a gene which are only expressed when there is no dominant allele of the gene present (must have 2 recessive alleles for that characteristic to show). Represented in diagrams with a lower case letter.

Question 6

What is polygenic inheritance and what are some examples of polygenic traits?

Answer 6

Polygenic inheritance is when a characteristic is caused by the interaction of many different genes. Most characteristics are polygenic (such as height, natural hair colour, eye colour, intelligence, risk of disease, bipolar disorder).

Question 7

What is a genotype?

Answer 7

The combination of alleles present (e.g. BB)

Question 8

What is the phenotype?

Answer 8

The actual characteristic expressed caused by genetic constitution and the environment. E.g. brown hair

Question 9

What does homozygous mean?

Answer 9

An individual has two identical alleles of a particular gene. They are either homozygous dominant or homozygous recessive. Two identical homozygous individuals that breed together are pure-breeding.

Question 10

What does heterozygous mean?

Answer 10

An individual has two different alleles of a particular gene (not pure-breeding).

Question 11

What is monohybrid inheritance?

Answer 11

The inheritance of a characteristic controlled by a single gene.

Question 12

How do you format a punnet square?

Answer 12

Write out the parents genotypes and the possible gametes (define what each of the letters you are using means first). Place the gametes of one individual on the left of the punnet square and the gametes of the other on the top. Complete the cross, then write out the possible genotypes of the offering and the ratio of the phenotypes that would create.

Question 13

What is the ratio you would expect in the F2 (second filial) generation from the monohybrid cross of a homozygous recessive individual and a homozygous dominant individual?

Answer 13

This cross produces only offspring which are heterozygous (the F1 generation). Breeding members of the F1 generation produces a 3:1 ratio of dominant:recessive traits.

Question 14

What kind of individual would you use in a test cross (to work out the genotype of another individual)?

Answer 14

Homozygous recessive. If any of the offspring show the recessive trait, then the original individual must have been heterozygous. If none on the offspring show the recessive trait, the individual is homozygous dominant.

Question 15

What was Mendel’s law of segregation?

Answer 15

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

Question 16

What is dihybrid inheritance?

Answer 16

The inheritance of two distinct traits, controlled by two separate genes, located on different chromosomes.

Question 17

What was Mendel’s law of independent assortment?

Answer 17

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

Question 18

How do you do a punnet square for a dihybrid cross?

Answer 18

The same as for monohybrid, but the square will be 4x4 instead of 2x2.

Question 19

What is the expected phenotypic ratio for the F2 generation in a cross between a homozygous recessive individual and a homozygous dominant individual?

Answer 19

The F1 generation will all be heterozygous for both traits, and when crossed, they will produce a 9:3:3:1 ratio assuming no autosomal linkage or epistasis. 9 - both dominant traits 3- one dominant one recessive 3 - the other dominant and the other recessive 1 - both recessive

Question 20

Why might the ratio of the offspring not match the expected ratio?

Answer 20

The expected ratios are just probabilities (the most likely outcome - not what will happen). If the ratios are drastically off, it is probably due to autosomal linkage or epistasis.

Question 21

What are sex-linked characteristics?

Answer 21

Where the gene responsible for a characteristic is located on one of the sex chromosomes. This means that recessive sex-linked diseases carried on the x-chromosomes are more likely to be inherited by men.

Question 22

What does hemizygous mean?

Answer 22

Biological males are hemizygous for characteristics on the sex chromosomes (they only have one allele for each characteristic).

Question 23

What is codominance?

Answer 23

If multiple alleles in a heterozygous organism are expressed ‘equally and independently’ in the phenotype, then they are codominant.

Question 24

What are some examples of codominance?

Answer 24

The alleles for flower colour in snapdragon flowers are codominant. The alleles for white and for red coats are codominant in cattle, producing a roan. Blood group alleles I^A and I^B are codominant, producing the AB blood group when both are present.

Question 25

What are the autosomal chromosomes?

Answer 25

All of the chromosomes which are not the sex chromosomes (numbers 1-22)

Question 26

How do pedigree charts indicate an autosomal dominant condition?

Answer 26

If both parents are affected and an offspring is unaffected, the trait must be dominant (both parents are heterozygous). All affected individuals must have at least one affected parent. If both parents are unaffected, all offspring must be unaffected (homozygous recessive).

Question 27

How do pedigree charts indicate an autosomal recessive condition?

Answer 27

If both parents are unaffected and the offspring is affected, the trait must be recessive (parents are heterozygous carriers). If both parents show a trait, all offspring must also exhibit the trait.

Question 28

How do pedigree charts indicate an X-linked recessive condition?

Answer 28

If a female shows a trait, so too must all her sons as well as her father. An unaffected mother can have affected sons if she is a carrier (heterozygous). X-linked recessive traits tend to be more common in males.

Question 29

How do pedigree charts indicate an X-linked dominant condition?

Answer 29

If a male shows a trait, so too must all daughters as well as his mother. An unaffected mother cannot have affected sons or an affected father. X-linked dominant traits tend to be more common in females.

Question 30

If a pedigree chart indicates a particular kind of condition (e.g. X-linked recessive) is this a guarantee that that condition is X-linked recessive?

Answer 30

No, pedigree charts do not provide conclusive proof of a particular type of inheritance. Genetic testing is the only way to prove this.

Question 31

What is autosomal linkage?

Answer 31

Alleles on the same chromosome are linked, and will not assort independently during metaphase I of meiosis. Linked alleles will stay together and the number of possible alleles decreases (recessive alleles stay together and dominant alleles stay together).

Question 32

How do less frequent gametes form in linked genes?

Answer 32

Crossing over results in the formation of new combinations of alleles in linked genes. This is when homologous chromosomes wrap around each other and exchange genetic material at chiasmata (where they touch).

Question 33

What is the expected ratio for the F2 generation of a homozygous dominant x homozygous recessive cross with autosomal linkage?

Answer 33

The only gametes are AB and ab, so the F1 phenotypes are all the dominant traits and all AaBb (heterozygous for both traits). The possible gametes are AB ab for both (assuming no crossing over), giving a ratio of 3 both dominant traits : 1 both recessive traits

Question 34

What is epistasis?

Answer 34

The interaction of different gene loci so that one gene locus masks or suppresses the expression of another (can reduce phenotypic variation).

Question 35

What is an epistatic gene?

Answer 35

A gene that interferes with or masks the expression of another gene (sometimes called inhibiting genes).

Question 36

What is a hypostatic gene?

Answer 36

A gene whose expression is affected by an epistatic gene.

Question 37

What is recessive epistasis?

Answer 37

The homozygous presence of a recessive allele may prevent the expression of another allele at a second locus.

Question 38

What is dominant epistasis?

Answer 38

A dominant allele at one gene masks the expression of the alleles at a second gene locus.

Question 39

What is complementary epistasis?

Answer 39

At least one dominant allele for both gene loci can result in them complementing one another.

Question 40

What are the expected ratios for a cross between individuals heterozygous for two epistatically linked traits?

Answer 40

9:3:4 = recessive epistasis
12:3:1 = dominant epistasis
9:7 = complementary epistasis

Question 41

What are examples of epistasis?

Answer 41

Fur colour in labradors (if they are homozygous recessive for one trait, this means they have no pigmentation, which masks another allele controlling if they have a black or brown coat). If you need two enzymes to get to a final product, but the first one is inhibited by epistasis, you won’t know if the second one would work or not.

Question 42

What is Chi-squared testing?

Answer 42

Chi-squared tests compare expected and observed values in order to accept or reject a null hypothesis. The formula is sum of (O-E)^2/E = chi-squared. The degrees of freedom is the number of categories - 1 and the critical value is given for each probability value and degrees of freedom value.

Question 43

What are some assumptions required to use the Hardy-Weinberg equation?

Answer 43

• there is a large population
• random mating occurs
• no selective advantage for any organism
• no mutations occur
• no immigration/emigration (populations are genetically isolated)
• no genetic drift (random changes in allele frequencies that occur over time)

Question 44

What are the Hardy-Weinberg equations?

Answer 44

p + q = 1
p^2 + 2pq + q^2 = 1
p=frequency of the dominant allele
q = frequency of the recessive allele
p^2 = frequency of homozygous dominant
q^2 = frequency of homozygous recessive
2pq = frequency of the heterozygous

Question 45

What is a population?

Answer 45

A group of organisms of the same species occupying a particular area at a particular time, that can potentially interbreed.

Question 46

What is a gene pool?

Answer 46

All the alleles in a population

Question 47

What is allele frequency?

Answer 47

The number of times an allele appears in a gene pool

Question 48

Why are the assumptions made for the Hardy-Weinberg principle not realistic?

Answer 48

Mutations lead to new alleles and alter the relative frequency of others for the same gene. Natural selection can increase or decrease allele frequencies. Small populations are at greater risk of genetic drift, which involves random changes in allele frequency. Individuals migrating in or out of a population could change allele frequency. Individuals deliberately choosing mates that do or don’t have certain characteristics will alter allele frequencies over generations.