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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.

Homozygous Recessive

  • 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).

Heterozygous Phenotype

  • 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.

Lysosomal Storage Diseases

  • 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 Recessive Traits

  • 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).

Probability of Inheritance

  • 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).

Autosomal Dominant Traits

  • 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.

Probability with Dominant Traits

  • 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.

Single Gene Traits

  • 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.

Dihybrid Cross

  • 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.

Sex-Linked Traits

  • 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.

Heritability of X-linked Traits

  • 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.

Examples of X-linked Traits

  • Duchenne muscular dystrophy is an X-linked muscle wasting disorder.
  • If the mom carries muscular dystrophy, has a 50% of passing it on the to son.
  • Color blindness is another common X-linked trait.

Color Blindness

  • 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.

Non-Mendelian Traits

  • Some traits are inherited via incomplete dominance or codominance.
  • Pleiotropic, polygenic, and epistatic traits also exist.

Degrees of Dominance

  • Complete dominance: one allele clearly determines the phenotype.
  • Incomplete dominance: heterozygotes have an intermediate phenotype.
  • Codominance: two alleles both contribute equally to the phenotype.

Examples of Incomplete Dominance

  • 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.