2.2 Complex inheritance patterns and dihybrid crosses

Incomplete dominance

• The heterozygous genotype will have its own phenotype

• Ex: Straight hair - SS; Wavy Hair SC ; Curly Hair: CC

Codominance

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

• Ex: Red Blood Cell Surface Gene

  • A and B are both dominant to O

  • A and B are codominant with each other

X chromosome

Large and contains many genes needed by both males and females

Y chromosome

Small and only important gene is sox-related protein Y (sry), which promotes male development in most mammals

Males have only one copy of each gene on the…

X chromosome (X-linked genes), so phenotypes from recessive alleles are seen more in men than women

Males are referred to as…

hemizygous for X-linked genes

Penetrance

How likely a genotype is to create a phenotype

Complete penetrance

classic dominance type relationships

Incomplete penetrance

same genotype does not always show the most common phenotype

What’s an example of incomplete Penetrance?

BRCA

How is BRCa an example of incomplete penetrance?

• Inherited BRCA1 mutation carries 87% risk of breast cancer

• Deficient alleles are recessive

• Requires somatic mutation to cause loss of heterozygosity

Epistasis

Multiple genes control same trait, and if one gene overrides another that is an epistatic interaction

What’s an example of epistasis?

• Dog fur color is controlled by 2 genes , TYRP1 (B) and MCR1 (E)

• Homozygous recessive ee prevents expression of either B allele

Complementation tests are used to

find out if two similar phenotypes come from mutations in the same genes

Epigenetics

literally any change in gene expression

True or False: Changing gene expression can have as large as an effect on phenotype as turning off a gene and these effects are inheritable

True

Human mitochondrial DNA is

haploid and mostly inherited from mother

How is nuclear DNA different than Mitochondrial DNA?

Inherited from all ancestors vs from a maternal lineage

Mitochondrial genes are functionally…

hemizygous in both sexes

Mitochondrial genome codes for…

metabolic genes and gene expression necessities

Dihybrid cross

when both parents are heterozygous at 2 loci