IB BIO TOPIC 4.3 : THEORETICAL GENETICS

Question 1

Define a genotype

Answer 1

the alleles possessed by an organism

Question 2

Define a phenotype

Answer 2

the characteristics of an organism

Question 3

Define dominant allele:

Answer 3

an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state

Question 4

Define recessive allele

Answer 4

an allele which only has an effect on the phenotype when present in the homozygous state

Question 5

Define codominant alleles

Answer 5

pairs of alleles that both affect the phenotype when present in a heterozygote

Question 6

Define locus

Answer 6

the particular position on homologous chromosomes of a gene

Question 7

Define homozygous

Answer 7

having the two identical alleles of a gene

Question 8

Define heterozygous

Answer 8

having two different alleles of a gene

Question 9

Define carrier:

Answer 9

an individual that has a recessive allele of a gene that does not have an effect on their phenotype

Question 10

Define test cross

Answer 10

testing a suspected heterozygote by crossing with a known homozygous recessive

Question 11

Can genes have more than 2 allele ?

Answer 11

Yes. Multiple alleles

Question 12

Describe ABO blood groups as an example of codominance and multiple alleles :

Answer 12

• multiple alleles: 3 alleles, IA, IB, i

* codominance/dominance: (IA = IB) > i

Question 13

What are the sex chromosomes for a male ?

Answer 13

XY

Question 14

What are the sex chromosomes for a female ?

Answer 14

XX (double seinG)

Question 15

Can some genes be present on the X chromosome and absent from the shorter Y chromosome in humans ?

Answer 15

YES

Question 16

Define: sex linkage

Answer 16

Sex linkage = genes carried on the sex chromosomes, most often on the X chromosome

Question 17

Describe the inheritance of colour blindness and hemophilia as examples of sex linkage.

Answer 17

• red/green color blindness: XB, Xb, Y

* hemophilia: XH, Xh, Y

Question 18

Can a human female can be homozygous or heterozygous with respect to sex-linked genes ?

Answer 18

Yes.

Question 19

Explain that female carriers are heterozygous for X-linked alleles.

Answer 19

• heterozygous female carriers do not show the disease

* but can pass it on to half of their male offspring

Question 20

State the 4 rules of pedigree charts :

Answer 20

• Roman numeral identifies generation number
• Arabic numeral identifies individual within a generation, numbering from
left to right
• square = male; circle = female
• shaded = affected by genetic disorder or trait