Genes and inheritance

Question 1

What is a gene?

Answer 1

A length of DNA at a particular location on a chromosome that codes for a particular polypeptide/protein and determines a particular feature.

Question 2

What is an allele?

Answer 2

A particular version of a gene.

Question 3

What is a genotype?

Answer 3

The combination of of alleles present in genome of organism.

Question 4

What is a phenotype?

Answer 4

Characteristic of an organism which result both from the genes it possesses and the environment in which it has developed. An observable or measurable characteristic.

Question 5

What is a dominant allele?

Answer 5

An allele that, if present, will always be expressed in a phenotype, even if only one copy is present.

Question 6

What is a recessive allele?

Answer 6

A characteristic in which the allele responsible is only expressed in the phenotype if 2 copies are present (i.e. no dominant alleles).

Question 7

What is codominance?

Answer 7

A characteristic where both alleles are expressed in a phenotype of a heterozygote.

Question 8

What is a locus?

Answer 8

The specific location of a gene on a chromosome.

Question 9

What is a homozygote?

Answer 9

An organism that carries 2 copies of the same allele in genome.

Question 10

What is a heterozygote?

Answer 10

An organism that carries 2 different alleles.

Question 11

What is a carrier?

Answer 11

A person carrying an allele that is not expressed (recessive allele in heterozygotes) but is capable of passing on allele to offspring.

Question 12

What is linkage?

Answer 12

When 2 or more genes are located on the same chromosome and are always inherited together, unless crossing over occurs in meiosis.

Question 13

What is sex-linkage?

Answer 13

A characteristic which is controlled by a gene which is located on a sex chromosome (either X or Y).

Question 14

How is a genetic diagram constructed?

Answer 14

A genetic diagram usually has the following categories:

• P1 Phenotype: Phenotypes of parents.
• P1 Genotype: Genotype of parents.
• P1 Gametes: All possible combinations of alleles in gametes produced by parents (with circle around each)/
• Fertilisation: Can use punnet square or genetic cross diagram to determine all possible genotypes of offspring.
• F1 Genotype: All possible genotypes of offspring.
• F1 Phenotype: All possible phenotypes of offspring.
• Phenotype/genotype ratio: Ratio of different phenotypes/genotypes theoretically produced by parents.

Question 15

What are examples of codominance?

Answer 15

• Sickle-cell anaemia.
• Roan cattle.
• Human blood groups.

Question 16

What causes sickle-cell anaemia?

Answer 16

• β-strands in haemoglobin abnormal due to slightly different primary structure.
• Deoxygenated haemoglobin becomes insoluble and crystallises.
• Red blood cells become sickle-shaped.
• Sickle-shaped red blood cells are unable to squeeze through capillaries so get stuck and block capillaries, reducing blood-flow to vital organs.
• Organs suffer from oxygen deprivation and are damaged.

Question 17

What are the genetics behind sickle-cell anaemia?

Answer 17

• Sickle-cell anaemia is codominant.
• H^A (normal haemoglobin) and H^S (sickle-cell haemoglobin) both code for expressed genes.
• An individual with an H^A H^A genotype are people with normal haemoglobin.
• An individual with an H^A H^S genotype are carriers. Some of their haemoglobin is abnormal while the rest is normal, causing no symptoms.
• An individual with an H^S H^S genotype are sufferers of the disease. All their haemoglobin are abnormal, so all red-blood cells are sickle-shaped, causing sickle-cell anaemia.

Question 18

What are the genetics behind roan cattle?

Answer 18

• C^W codes for white hair.
• C^R codes for red hair.
• Individuals with C^W C^W genotype only produce proteins for white hair, so are white.
• Individuals with C^W C^R genotype produce proteins for both red hair and white hair, so are roan.
• Individuals with C^R C^R genotype only produce proteins for red hair, so are red.

Question 19

What evidence is there in a family pedigree diagram to suggest that there is a sex-linked disease in the family?

Answer 19

When a higher majority of one sex has possessed the disease than the other throughout the family history.

Question 20

What is the nature of sex-linked inheritance?

Answer 20

• If condition is Y-linked, only males will inherit the disease.
• If the condition is X-linked, both females and males can inherit the disease, but females need to inherit 2 faulty X chromosomes in order to acquire disease whilst males only need 1.
• Males cannot be carriers of the disease as they only have 1 X chromosome, so they either have the disease or they don’t.

Question 21

What evidence is there in a family pedigree to indicate the condition is recessive?

Answer 21

When 2 unaffected individuals produce an affected offspring. This indicates that both parents were carriers of the disease.

Question 22

What is monohybrid inheritance?

Answer 22

Inheritance of a characteristic controlled by a single gene.

Question 23

What is dihybrid inheritance?

Answer 23

Inheritance of 2 characteristics controlled by 2 different genes at 2 different loci.

Question 24

What are the 2 types of dihybrid inheritance?

Answer 24

1. Unlinked: Two genes are on 2 different chromosomes and are inherited separately, independent of one-another.
2. Linked: Two genes are on the same chromosome and are always inherited together. The same alleles are also always inherited together unless crossing over occurs (no random assortment).

Question 25

What are the genotype ratios concerned with unlinked dihybrid inheritance?

Answer 25

For the 2 genes A/a, B/b, when the 2 heterozygous individuals with genotype AaBb are crossed, genotype ratios:
A-B- 9
A-bb 3
aa-B- 3
aabb 1

are produced.

Question 26

What is epistasis?

Answer 26

When one gene is capable of masking the expression of a different gene at a different loci.

Question 27

What is recessive epistasis?

Answer 27

When a homozygous recessive gene masks the expression of another gene and a characteristic.

Question 28

What is the mechanism of recessive epistasis?

Answer 28

When a protein coding for a certain phenotype has the synthesis pathway:

Precursor A –(Gene 1: enzyme 1)–> Precursor B –(Gene 2: enzyme 2)–> Protein

• If there are 2 alleles for gene 1 coding for enzyme 1, A/a. But only A produced enzyme 1, then individual with genotype aa would not produce enzyme 1 and precursor B, preventing the expression of gene 2.

Question 29

What is epistasis by complementary action?

Answer 29

When both genes require at least one dominant allele in order for the characteristic determined by both genes to be expressed.

Question 30

What is the mechanism for epistasis by complementary action?

Answer 30

When a protein coding for a certain phenotype has the synthesis pathway:

Precursor A –(Gene 1: enzyme 1)–> Precursor B –(Gene 2: enzyme 2)–> Protein

• If there are 2 alleles for gene 1 coding for enzyme 1, A/a and gene 2, B/b; but only A/B produced enzyme 1/2, then individual with genotype aa or bb would not produce enzyme 1 or enzyme 2, preventing the characteristic from being expressed.

Question 31

What is dominant epistasis?

Answer 31

When the presence of one dominant allele in genotype of one gene masks the expression of the other.

Question 32

How does dominant epistasis work in terms of genetics?

Answer 32

• Summer squash.
• 2 genes are responsible for colour; gene 1 and gene 2.
• Gene 1 has alleles A/a.
• Gene 2 has alleles B/b.
• Presence of allele A in genotype of gene 1 gives white squash no matter Genotype of gene 2.
• Presence of allele B in gene 2 genotype gives yellow squash, and bb genotype gives green squash, only of genotype of gene 1 is aa.