4.3 Theoretical Genetics

Question 1

Define genotype

Answer 1

The complete set of genetic information of an organism; the allele combination of an organism

Question 2

Define phenotype

Answer 2

The set of characteristics of an organism resulting from its genotype and environmental factors

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 that 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 of a gene on homologous chromosomes (plural: loci)

Question 7

Define homozygous

Answer 7

Having two identical alleles of a gene

Question 8

Define hetrozygous

Answer 8

Having two different alleles of a gene

Question 9

Define carrier

Answer 9

An individual that has one copy of a recessive disease causing allele in their genotype which is not expressed in their phenotype. Carriers can pass this allele onto offspring.

Question 10

Define test cross

Answer 10

Testing a suspected heterozygote by crossing it with a known homozygous recessive

Question 11

Do you know how to do Punnett grids?

Answer 11

You better

Question 12

Describe how pairs of non-recessive alleles can affect the genotype

Answer 12

• Share codominance (be expressed equally in the phenotype)
  
  • Share incomplete dominance (neither is fully expressed in the phenotype, resulting in blending)
  • Demonstrate a dominance order (e.g. allele A > allele B > allele C)

Question 13

Explain how sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans

Answer 13

• Humans have 23 pairs of chromosomes.
• First 22 are autosomes - each chromosome pair has the same genes and structure.
• The 23rd pair are sex chromosomes which determine gender.
– XY=Male
– XX=Female
– Y chromosomes shorter than X chromosome
• Gender is determined by sperm, as female eggs will always contain X, but male sperm could contain X or Y

Question 14

Define sex-linkage

Answer 14

Sex linkage refers to when a gene controlling a characteristic is found on only one of the sex chromosomes.
• Sex linked genes are usually X-linked, as very few genes exist on the shorter Y chromosome
• If the gene is h, represented as Xh because it’s on the X chromosome (XH for dominant)

Question 15

Explain the presence of sex-linked genes

Answer 15

• The Y chromosome is much shorter than the Y chromosome, and contains less genes.
• X and Y chromosomes have a homologous section where you will find the same genes (alleles).
• A male will only have one allele for chromosomes which appear on the non-homologous section of the X chromosome.
Males more likely to display recessive x-linked genetic abnormalities.
• The genes for red-green colour blindness and haemophilia are x-linked recessive.
• If a female has this gene on only one of her chromosomes, the other allele will dominate it.
• If a male has this gene, he has no alternative allele.

Question 16

Describe the inheritance of colour blindness and haemophilia as examples of sex linkage

Answer 16

• Colour blindness and haemophilia are both examples of X-linked recessive conditions.
  
  • The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome).
  • As males only have one allele for this gene they cannot be a carrier for these conditions.
  • This also means they have a higher frequency of displaying these conditions.
  • Males will always inherit an X-linked recessive condition from their mother.
  • Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents.

Question 17

Explain how human females can be heterozygous or homozygous with regard to x-linked genes

Answer 17

As human females have two X chromosomes (and therefore two alleles for any given X-linked gene), they can be either homozygous or heterozygous with regard to sex-linked genes.

Question 18

Explain how females can be carriers for X-linked recessive alleles

Answer 18

• An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier.
  
  • Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves.
  • Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers.
  • Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater.

Question 19

Outline the rules of a pedigree chart

Answer 19

A pedigree is a chart of the genetic history of a family over several generations
• Males are represented as squares, while females are represented as circles
• Shaded symbols means an individual is affected by a condition, while an unshaded symbol means they are unaffected
• A horizontal line between a man and woman represents mating and resulting children are shown as offshoots to this line

Question 20

State the genotype of blood type A

Answer 20

I^A I^A or I^A i

Question 21

State the genotype of blood type B

Answer 21

I^B I^B or I^B i

Question 22

State the genotype of blood type AB

Answer 22

I^A I^B

Question 23

State the genotype of blood type O

Answer 23

i i