Complex inheritance patterns and dihybrid crosses

Incomplete Dominance

If a gene exhibits incomplete dominance, the heterozygous genotype will have its own phenotype

Codominance

the heterozygous genotype will have the phenotype of BOTH dominant alleles at the same time

If a human with AB genotype experiences a null mutation (one that disrupts function) in
their B allele, what proteins will be present on the surface of their red blood cells?
A) A
B) B
C) AB
D) None

A) A

X-linked genes

• Males have only one copy of each gene on the X chromosome so phenotypes from recessive alleles are seen more in men than women

• Referred to for males

Homo- vs hetero- gametic

refers to which sex has same chromosomes (XX or ZZ) or different

You are studying an X-linked gene in human that has two codominant alleles, referred to as M and N. How will the phenotype of a hemizygous M male relate to the phenotypes of homozygous MM and NN females?
A) M males will be identical to MM females
B) M males will be identical to NN females
C) M males will be identical to MN females
D) M males will have a different phenotype compared to any female

A) M males will be identical to MM females

Consider a bird with a ZW sex determination system. Which sex can be hemizygous?
A) Males
B) Females
C) Neither
D) Either

B) Females

Which chromosomes cannot have their structural mutations (indels or CNVs) fixed by homology directed repair?
A) X and Y
B) Z and W
C) X and Z
D) Y and W

D) Y and W

Which mammalian sex is incapable of fixing structural mutations to the X chromosome?
A) Males
B) Females
C) Neither
D) Either

A) Males

Penetrance

• How likely a genotype is to create a phenotype

• Can be caused by somatic mutation, interactions with other genes or the environment

Complete penetrance

classic dominance type relationships

Incomplete penetrance

same genotype does not always show the most common phenotype

Epistasis

• Multiple genes control same trait

• often linked to signal transduction or metabolic pathways

You find a new striped coat phenotype in dogs that is caused by a mutation in the MCR1 gene. You call this allele, e*. You find that only 80% of e*e* dogs have striped coat as long as they have the wildtype TYRP1 allele (B). What type of inheritance pattern is this?
A) Incomplete dominance
B) Incomplete penetrance
C) Epistasis
D) A&C
E) B&C

E) B&C

Epigenetics

• Change in gene expression

• can have as large as an effect on phenotype as turning off a gene and these effects are inheritable

Gene Imprinting

• Certain genes are methylated and silenced in a sex-specific way in gametes

• Ensures proper gene expression dosage

• persists in somatic cells, but is reset in germline

• Happens in all mammals, and a few other species

Mitochondrial inheritance

• is strictly maternal only inherited from the mother

• Mitochondria are loaded into the egg, which is fertilized, no mitochondrial contribution from sperm (sperm have mitochondria still)

• Mitochondrial genes are functionally hemizygous in both sexes

• Allows for specialized tracking of maternal ancestry

Which statement is true about mitochondrial inheritance and X chromosome inheritance?
A) Mitochondrial and X chromosome inheritance are both maternal
B) X chromosomes can be inherited from either parent, but mitochondria are only inherited
maternally
C) Mitochondrial inheritance matches the sex of the offspring, while X chromosomes are inherited maternally in both sexes and paternally in females

B) X chromosomes can be inherited from either parent, but mitochondria are only inherited
maternally

What is the likelihood that an offspring from Mendel’s dihybrid cross would be yellow and wrinkled?
A) 1/2
B) 9/16
C) 3/16
D) 6/16
E) 1/4

C) 3/16