Patterns Of Inheritance

Question 1

Define homozygote?

Answer 1

An organism that carries two copies of the same allele, e.g. BB or bb

Question 2

Define heterozygote?

Answer 2

An organism that carries two different alleles, e.g. Bb

Question 3

What is continuous variation?

Answer 3

When the individuals in a population vary within a range- there are no distinct categories e.g. humans can be any height within a range (139cm, 175cm, 185.9cm etc), not just tall or short.

Question 4

Examples of continuous variation?

Answer 4

Waist circumference- e.g. humans can have any waist within a range
Fur length- e.g. dogs can have any length of fur within a range

Question 5

What is discontinuous variation?

Answer 5

When there are two or more distinct categories- each individual falls not only one of these categories, there are no intermediates.

Question 6

Examples of discontinuous variation?

Answer 6

Blood group- e.g. humans can be group A, B, AB or O
Violet flower colour- e.g. violets can either be coloured or white

Question 7

Define codominant?

Answer 7

Alleles that are both expressed in the phenotype- neither one is recessive, e.g. the alleles for haemoglobin

Question 8

Example or codominant alleles in humans?

Answer 8

The allele for sickle-cell anaemia.
People who are homozygous for sickle haemoglobin (SS) have a sickle-cell anaemia- all their blood cells are sickle-shaped
People who are heterozygous (NS) have an in-between phenotype, called sickle-cell trait- they have some normal haemoglobin and some sickle haemoglobin. The two alleles are codominant because they’re both expressed in the phenotype.

Question 9

What are the parents in a monogenic cross?

Answer 9

Homozygous dominant x homozygous recessive (e.g. RR x rr)

Question 10

What are the parents in a dihybrid cross?

Answer 10

Homozygous dominant x homozygous recessive ( RRYY x rryy)

Question 11

What is dihybrid inheritance?

Answer 11

The inheritance of two characteristics which are controlled by different genes

Question 12

What are the parents in a codominant cross?

Answer 12

Homozygous for one allele x homozygous for the other allele (NN x SS)

Question 13

What is the phenotype ratio in F2 in a monogenic cross?

Answer 13

3:1
Dominant: Recessive

Question 14

What is the phenotype ratio in F2 in a dihybrid cross?

Answer 14

9:3:3:1

Question 15

What is the phenotype ratio in F2 in a codominant cross?

Answer 15

1:2:1
Homozygous for one allele: heterozygous: homozygous for the other allele

Question 16

What is epistasis?

Answer 16

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

Question 17

What is the predicted ratio for epistasis crosses- involving a recessive epistasis allele?

Answer 17

9:3:4

Question 18

What is the predicted ratio for epistasis crosses- involving a dominant epistasis allele?

Answer 18

12:3:1