Genetics Notes

Recessive Alleles

Recessive alleles are only expressed if an individual is homozygous.
The expression of a recessive allele can be masked.
Carriers of a recessive allele are heterozygous, meaning they have the allele but don't express it.
Alleles can be found on autosomes and can be either recessive or dominant.

A homozygous recessive individual has two recessive alleles.
The genotype for a homozygous recessive individual, using the letter 'a', is written as little a, little a (aa).

A heterozygous individual has a genotype of big A, little a (Aa).
Heterozygous and homozygous recessive individuals do not have the same phenotype because of the law of dominance.
The law of dominance states that when alleles are different, the dominant allele is expressed.
A heterozygous phenotype will look like the homozygous dominant phenotype.

TAC is a lysosomal storage disease.
Lysosomes are organelles within cells that function in digestion and recycling.
Lysosomal enzymes break down large molecules.
In lysosomal storage diseases, a missing enzyme causes accumulation of compounds in lysosomes, leading to cell swelling and death.
Tay-Sachs disease (TSD) is a lysosomal storage disease where the enzyme to break down a specific compound isn't produced, leading to accumulation in brain tissue.
TSD is fatal, usually by age three, and affects brain tissue, leading to respiratory failure.

Autosomal means non-sex chromosomes. Humans have 22 pairs of autosomes.
TSD is an autosomal recessive trait.
Carriers are heterozygous, so parents who are carriers have a heterozygous genotype.
Individuals with TSD have a homozygous recessive genotype (aa).

The probability of two carrier parents having a child with TSD is 25%.
Using a Punnett square, you can determine the genotypes of offspring.
Homozygous dominant individuals (AA) do not carry the allele and cannot pass it to offspring.
Heterozygous individuals are carriers.
Genotypic ratio: 1:4 homozygous dominant, 2:4 heterozygote, 1:4 homozygous recessive (1:2:1 ratio).
Phenotypic ratio: 3:1 (three without Tay-Sachs, one with Tay-Sachs).

Polydactyly (more than five fingers or toes) is an autosomal dominant trait.
If you have the allele for a dominant trait, you express the allele.
There are no carriers for a dominant trait because if you have the allele, you express it.

If one parent has polydactyly and the other does not, the probability of their child having polydactyly depends on the genotype of the affected parent.
If the affected parent is homozygous dominant, 100% of offspring will have polydactyly.
If the affected parent is heterozygous, there is a 50% chance the offspring will have polydactyly.
If a parent with a dominant trait has a child without the trait, the affected parent must be heterozygous.

Examples of single gene traits in humans: freckles, widow's peak, unattached earlobes (dominant traits).
Hypermobility of the thumb might also be a dominant trait.

A monohybrid cross looks at a single gene, while a dihybrid cross looks at two genes at the same time.
A dihybrid cross involves individuals heterozygous for both traits.
When writing the phenotype for a dihybrid cross, alleles for each trait should be kept together (e.g., FfHh).
During egg and sperm formation, each gamete gets one version of every allele.
Alleles can segregate together in different combinations (e.g., dominant alleles together, recessive alleles together).
Genotypes are determined by combining the alleles from each parent in a Punnett square.
Predictable phenotypic ratio for a dihybrid cross: 9:3:3:1.

Non-sex chromosomes are autosomes, while sex chromosomes are XX (female) or XY (male).
The X and Y chromosomes segregate during meiosis.
The X chromosome has over a thousand genes, while the Y has only a few hundred.
Males determine the sex of the child by contributing either an X or Y chromosome.
Traits found on the X chromosome are X-linked traits.
Y chromosome traits are easily tracked because the dad has the gene, the son will have the gene.

Females have two copies of every X-linked gene, while males have only one.
Females can be homozygous dominant, homozygous recessive, or heterozygous for X-linked traits, while males are either dominant or recessive.
Males with one copy of the X-linked trait will express it.
For a recessive X-linked trait, only homozygous recessive females will express it, while men will express it 100% of the time.
If a dad has an X-linked trait, daughters will only express it if the mom is also a carrier or has the trait.
Daughters of affected fathers are always carriers.
If a mother is affected with an X-linked trait, all sons will express the trait.

Red-green color blindness is the most common form.
About 8% of males in the US are affected.
Genotypes for color blindness: Females can be X^VX^V (non-colorblind), X^VX^v (carrier), or X^vX^v (colorblind). Males are either X^VY (non-colorblind) or X^vY (colorblind).
When drawing a Punnett square for X-linked traits, track the X and Y chromosomes.
If asked what is the probability of a son or daughter having a gene make sure to look at that side of the Punnett square only.

Snapdragons: a red flower crossed with a white flower results in pink flowers.
Incomplete dominance occurs because there is about half of the product with what would be a normal homozygous expression.
When tracking the incomplete dominance, use different variables example: 'r' and 'w' instead of capital and recessive.