Chapter 4 - Modification of Mendelian Ratios

Flashcards covering modified Mendelian ratios, including incomplete dominance, codominance, multiple alleles, lethal alleles, gene interaction, epistasis, pleiotropy, sex-linked inheritance, sex-limited/influenced traits, and environmental effects on phenotypic expression.

What are some reasons the classic Mendelian ratios (3:1, 9:3:3:1) are modified in inheritance?

Traits do not demonstrate complete dominance, are not autosomally inherited, or are not inherited independently; or when more than one gene pair influences a single trait.

What type of dominance results in offspring with an an intermediate phenotype in heterozygotes?

Incomplete (or partial) dominance.

When crossing pink snapdragons (F1 from red
x white parents), what is the phenotypic ratio in the F2 generation, and how does it relate to the genotypic ratio?

1 Red: 2 Pink: 1 White; the phenotypic ratio is identical to the genotypic ratio.

What term describes the joint expression of both alleles in a heterozygote, where the influence of both alleles is clearly evident without blending?

Codominance.

Which human blood group serves as an example of codominance, characterized by antigens on the surface of red blood cells?

The MN blood group.

What is an example of multiple alleles existing in a human population, demonstrating both dominant and codominant inheritance?

The human ABO blood groups.

What term describes alleles that, when homozygous, may result in death before birth or cause distinctive mutant phenotypes?

Lethal alleles.

What human genetic disorder is caused by an autosomal dominant lethal allele, where the onset is delayed until adulthood?

Huntington Disease.

What happens when the cellular functions of numerous gene products contribute to the development of a common phenotype?

Gene interaction.

What genetic phenomenon occurs when the expression of one gene or gene pair masks or modifies the expression of another gene or gene pair?

Epistasis.

What genetic concept describes the expression of a single gene having multiple phenotypic effects?

Pleiotropy.

What autosomal dominant disorder, characterized by defects in connective tissue due to fibrillin, is an example of pleiotropy?

Marfan Syndrome.

What describes genes located on the X chromosome, leading to differences in inheritance patterns between sexes?

X-linkage (or sex linkage).

Why are X-linked recessive traits, such as hemophilia and red-green color blindness, more common in males than in females?

Males have only one X chromosome, so a single recessive allele on the X chromosome will be expressed, whereas females need two copies to express the trait.

What type of autosomal trait is expressed in only one of the sexes, impacting primary or secondary sexual characteristics?

Sex-limited traits.

What type of autosomal trait's expression is influenced by an individual's gender, where the trait may be dominant in one sex and recessive in the other (e.g., pattern baldness)?

Sex-influenced traits.

What kind of mutations exhibit environment-dependent expression of a genotype, often influenced by factors like temperature or nutrition?

Conditional mutations.

Why do Siamese cats have darker fur on cooler parts of their bodies (e.g., ears, paws, tail)?

They have a temperature-sensitive defect in tyrosinase, with the allele only functional at cool temperatures, leading to pigment production in those areas.