Genotypes and Phenotypes

What does the overall appearance or phenotype of an organism depend on? Hence, what is the phenotype?

1. It's genes (genotype)

2. The effects of the environment in which it lives

Phenotype: The observable characteristics of an organism

• Determined by genotype + environmental effects

How do you define genotype? What are the two main forms of genotype?

Genotype: The combination of alleles an individual possesses for a particular gene 

• Two main forms of genotypes:

• Homozygous: Two identical allele

• Heterozygous: Two different alleles

What are the specific types of genotypes?

• Homozygous dominant: Two alleles for the dominant trait

• Homozygous recessive: Two alleles for the recessive trait

• Heterozygous: One dominant and one recessive allele, 2 different

Hemizygous: Having only one allele for a particular gene instead of two

What are some examples of hemizygosity?

• Human males (XY) are hemizygous for most genes on the X chromosome the case for sex-linked traits

• Female birds (ZW) are hemizygous for many genes on the Z chromosome

• Can also occur due to chromosomal abnormalities such as aneuploidy or chromosome deletions, where one copy of a gene or chromosome region is missing

How do dominant and recessive traits differ?

Feature	Dominant Allele/Trait	Recessive Allele/Trait
Expression in phenotype	Always expressed in the phenotype	Only expressed when two recessive alleles are present
Number of copies needed	One copy of the allele inherited to be expressed	Two copies of the allele
Effect of another allele	Masks the effect of a recessive allele in a heterozygous individual.	Its effect is masked if a dominant allele is present
Symbol used	Represented by an uppercase letter	Represented by a lowercase letter

What are the defining features of dominant traits?

• Always expressed in the phenotype

• Only one copy of the allele needs to be inherited for the trait to be expressed

• They mask the effect of recessive alleles in heterzygous individuals

• Represented by an uppercase letter

What are the defining features of recessive traits?

• Only expressed in homozygous recessive individuals (when two recessive alleles are present)

• Two copies of the allele need to be inherited for the trait to be expressed

• Its effect is masked if a dominant allele is present

• Represented by a lowercase

Describe the 3 rules of assigning allele symbols. Why are they important?

1. Use a single letter relevant to the gene

• Taken from the dominant traits’ name

2. Avoid letters that are hard to tell upper and lowercase apart (C/c, S/s, O/o)

• Underline lowercase letters if using letters hard to tell apart

3. Never use two unrelated letters

• Keeps it clear they are alleles of the same gene

How does assigning allele symbols differ between autosomal traits and sex-linked traits?

Autosomal traits: The allele letter alone is the symbol

• Dominant trait: Uppercase and Recessive trait: Lowercase of same letter

Sex-linked traits: The allele letter is superscript on the relevant sex chromosome

• Shows that the gene is carried on that chromosome and not separate from it

What are the possible genotype types for both genders in sex-linked traits?

• Females (XX): Can be homozygous dominant, heterozygous (carrier), or homozygous recessive for X-linked traits

• Males (XY): Are hemizygous for X-linked traits because they only have one X chromosome and therefore only one allele for the gene

Why are males more likely to experience sex-linked disorders?

• Males only have one X chromosome while females have two (XX)

• Any faulty genes/mutations on his X chromosome will be expressed in the phenotype

• Whereas females generally require 2 copies of the alleles to be affected (as most sex-linked disorders are recessive)

What is an example of a recessive x-linked disorder? What are the genotypes for each gender?

Haemophilia: Condition where blood lacks enough clotting factors
3 possible genotypes for females (XX):
1. XᴴXᴴ = unaffected (healthy)
2. XᴴXʰ = unaffected (healthy) carrier
3. Xʰxʰ affected (disease)

2 possible genotypes for males (XY):
1. XᴴY= unaffected (healthy)
2. XʰY = affected (disease)

What is an example of a dominant x-linked disorder? What are the genotypes for each gender?

Rett syndrome: A rare genetic neurological disorder primarily affecting girls
3 possible genotypes for females (XX):
XᴿXᴿ-affected (disease)
XᴿXʳ-affected (disease)
XʳXʳ- unaffected (healthy)
2 possible genotypes for males (XY):
XᴿY-affected (disease)
XʳY-unaffected (healthy)

What are the features of recessive X-linked disrders? Who do they predominatly affect?

• Individuals require 2 copies of the alleles in order to be affected

• Predominantly affect men as possessing the allele for the gene will cause the disease despite being recessive

• Women with turner syndrome experience these disorders at similar rates to men due to their single X chromosome

What are the features of dominant X-linked disrders? Who do they predominatly affect?

• Individuals only require one copy of the allele to be affected

• Much less common than X-linked recessive

• Predominatly in females as males with X-linked dominant traits experience more severe/lethal symptoms without the compensation of another X chromosome

• There are no carriers as the heterozygote female expresses the trait (and men can't be carriers)