Topic 4.3 Theoretical genetics

Question 1

4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

Answer 1

Genotype: the genetic make-up of an individual

Phenotype: the expression of genes

Dominant allele: dominant trait fully expressed when heterozygous

Recessive allele: trait only seen if both chromosomes possess this allele, otherwise it is masked

Codominant alleles: two traits are both expressed when heterozygous

Locus: position of a gene on a chromosome

Homozygous: both alleles the same

Heterozygous: both alleles different

Carrier: when an individual is heterozygous and has an allele, but does not express it

Test Cross: cross performed when you think you know the genotype of an individual and you want to see what phenotypes are produced to confirm your hypothesis

Question 2

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

Answer 2

P: GG & gg

F₁: Punnett square w/ G, G vs. g, g

results in 4 Gg

F₁ = genotype 4:0 & phenotype 4:0

F₂: Punett square w/ G, g vs. G, g

results in 1 GG, 2 Gg, 1 gg

F₂ = genotype 1:2:1 & phenotype 3:1

Question 3

4.3.3 State that some genes have more than two ________ (multiple ________).

Answer 3

Some genes have more than two alleles (multiple alleles).

Question 4

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

Answer 4

• I^A and I^B are both dominant - therefore called codominant
  
  • codominant alleles are pairs of alleles that affect the phenotype when present together in a heterozygote
  • I^AI^A = group A blood
  • I^BI^B = group B blood
  • I^AI^B = group AB blood
• i is the third allele controlling ABO blood groups. If more than 2 alleles of a gene, they are called multiple alleles.
  
  • recessive to I^A and I^B
  • I^Ai = group A blood
  • I^Bi = group B blood
  • ii = group O blood

Question 5

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

Answer 5

• 2 chromosomes - X & Y - determine gender of child → called sex chromosomes
• X chromosome = relatively large, carries many genes
• Y chromosome = much smaller, carries only a few genes
• XX = female
• XY = male
  
  • b/c when women reproduce, pass on 1 X; when men reproduce, pass on X or Y
    
    • therefore, gender of child depends on what chromosome sperm fertilizing egg carries

Question 6

4.3.6 State that some genes are _______ on the X chromosome and ______ from the shorter Y chromosome in humans.

Answer 6

Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

Question 7

4.3.7 Define sex linkage.

Answer 7

Sex linkage: the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome.

Sex-linked genes are almost always located on the X chromosome. e.g. hemophilia & red-green colour blindness.

Question 8

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

Answer 8

• 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 the condition
• This means they have a higher frequency of being recessive and expressing the trait
• 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 9

4.3.9 State that a human female can be __________ or ___________ with respect to sex-linked genes.

Answer 9

A human female can be homozygous or heterozygous with respect to sex-linked genes.

e.g. X^HX^H or X^HX^h

Question 10

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

Answer 10

• 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 11

4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

Answer 11

Autosomal Dominance/Recessive

G = 1:2:1

P = 3:1

Codominance

G = 1:2:1

P = 1:2:1

if multiple alleles**, e.g. i in ABO group determining alleles

G = 1:1:1:1

P = 1:1:1:1

X-linked Recessive

G = 1:1:1:1

P = 2 (unaffected females): 1 (unaffected male): 1 (affected male)

Question 12

4.3.12 Deduce the genotypes and phenotypes of individuals in pedigree charts.

Answer 12

In pedigree charts…

• Squares represent males
• Circles represent females
• Shaded = affected
• Unshaded = unaffected