Genetics

Question 1

Define genotype.

Answer 1

The genetic constitution of an organism.

Question 2

Define phenotype.

Answer 2

The expression of the genotype and its interaction with the environment.

Question 3

Alleles may be __________, ____________ or _______________.

Answer 3

Dominant, recessive or codominant.

Question 4

What is a gene?

Answer 4

A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic eg eye colour.

Question 5

Define allele.

Answer 5

A different form of a gene. Only one occurs at the locus of a chromosome.

Question 6

Define dominant.

Answer 6

The allele of the heterozygote that expresses itself in the phenotype, even if there’s only one copy.

Question 7

Define recessive.

Answer 7

An allele who’s characteristic only appears in the phenotype of two copies are present.

Question 8

Define codominant

Answer 8

Alleles that are both expressed in the phenotype - neither one is recessive.

Question 9

What is a locus?

Answer 9

The fixed position of a gene on a chromosome.

Question 10

Define homozygote.

Answer 10

An organism that carries two of the same alleles of genes at the same locus on homologous chromosomes. Eg BB.

Question 11

Define heterozygote.

Answer 11

An organism that carries two different alleles of genes at the same locus on a homologous chromosome. Eg Bb.

Question 12

What is a carrier?

Answer 12

A person carrying an allele which is not expressed in the phenotype but that can be passed onto offspring.

Question 13

In what kind of organisms are homologous chromosomes found?

Answer 13

Diploid organisms.

Question 14

What is monohybrid inheritance?

Answer 14

The inheritance of a single gene.

Question 15

What are genetic diagrams used for?

Answer 15

Genetic diagrams are used to predict the genotype and phenotypes of the offspring produced if two parents crossed (bred).

Question 16

What is a ratio?

Answer 16

A measure of the relative size of two groups that is expressed as a proportion.

Question 17

What is dihybrid inheritance?

Answer 17

How 2 characters, determined by 2 different genes located on different chromosomes are inherited.

Question 18

What is the theoretical phenotypic ratio at F2 in monohybrid inheritance?

Answer 18

3:1 (dominant:recessive)

Question 19

What is the theoretical phenotypic ratio in F2 in dihybrid inheritance?

Answer 19

9:3:3:1

Question 20

What is the theoretical phenotypic ratio in F2 of codominant inheritance?

Answer 20

1:2:1

Question 21

Why is it that often you don’t get the expected phenotypic ratio?

Answer 21

Due to sex linkage, autosomal linkage or epistatis.

Question 22

What does ‘multiple alleles mean?

Answer 22

There are more than two alleles, of which only two may be present at the loci of the homologous chromosome.

Question 23

When does codominance occur?

Answer 23

When both alleles are equally dominant. Eg strawberry roan.

Question 24

Give an example of ‘multiple alleles’.

Answer 24

The human ABO blood groups. (Allele IA, IB, IO).

Question 25

Humans have 23 pairs of chromosomes. How many of these have homologous partners that are identical in appearance?

And why this number?

Answer 25

22. The remaining pair are the sex chromosomes. In the male, there is XY, and female is XX.

Question 26

What is a sex-linked gene?

Answer 26

Any gene that’s carried on either the X or Y chromosome.

Question 27

Why are males more likely to show recessive phenotypes?

Answer 27

Because they only have one X chromosome, so often only have one allele for sex linked genes.
So, as they only have one copy, the express the characteristic of this aloe even if it’s recessive.

Question 28

Give an example of an x-linked disorder.

Answer 28

Colour blindness and haemophilia.

Question 29

Why is colour blindness much rarer in women than men?

Answer 29

Because females would need 2 copies of the recessive allele to be colour blind, whereas male sonny need one colour.

Question 30

Why do males obtain haemophilia from their mothers?

Answer 30

Because males can only obtain a Y chromosome from their father, so it follows that their X chromosome comes from the mother.

Because the defective allel that does not code for the clotting protein is linked to the X chromosome, males always inherit the gene from their mother.

Question 31

How can fathers pass on haemophilia to their offspring?

Answer 31

Fathers can pass their X chromosome onto their daughters (but only Y to sons), meaning the daughters can become carriers.

Question 32

What are autosomes?

Answer 32

The remaining 22 chromosomes (excluding the sex chromosomes)

Question 33

What is autosomal linkage?

Answer 33

The situation when two or more genes are carried on the same autosome.

Question 34

What is epistasis?

Answer 34

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

Question 35

How does epistasis work?

Answer 35

The allele of one gene masks (blocks) the expression of the alleles of other genes.