Mendelian Genetics and Inheritance Patterns (1C) - MCAT Biological and Biochemical Foundations of Living Systems

Having two identical alleles (e.g., $AA$ or $aa$). Homozygosity occurs when both alleles at a locus are the same, resulting in consistent expression regardless of dominance.

$\frac{1}{2}$. Heterozygous offspring require one dominant and one recessive allele, occurring in two of four possible combinations.

Cross with homozygous recessive: $A_ \times aa$. A testcross reveals unknown genotypes by crossing with homozygous recessive, exposing hidden recessive alleles in offspring.

Not all individuals with the genotype show the phenotype. Incomplete penetrance means environmental or genetic factors prevent phenotype manifestation in some genotype carriers.

X-linked recessive inheritance. X-linked recessive traits skip generations and affect males more due to their single X chromosome lacking a compensating allele.

An alternative version of a gene at a locus. Alleles represent variations of a gene occupying the same chromosomal locus, leading to potential differences in trait expression.

Genotype = alleles; phenotype = expressed traits. Genotype refers to the genetic makeup, while phenotype is the observable outcome influenced by both genetics and environment.

$\frac{1}{4}$. With independent assortment, each allele pair segregates separately, so the chance of both recessive alleles in a gamete is $\frac{1}{2} \times \frac{1}{2}$.

Having two different alleles (e.g., $Aa$). Heterozygosity arises from differing alleles at a locus, allowing for interactions like dominance or codominance in phenotype.

An allele expressed in the phenotype of $Aa$. Dominant alleles override recessive ones in heterozygotes, determining the observed trait in Mendelian inheritance.

An allele masked in $Aa$; expressed only in $aa$. Recessive alleles require homozygosity for expression, as they are suppressed by dominant alleles in heterozygotes.

Alleles separate into different gametes during meiosis. Mendel's law of segregation explains how paired alleles divide during gamete formation, ensuring each gamete receives one allele.

Alleles of different genes assort independently into gametes. Independent assortment occurs when genes on different chromosomes segregate randomly, producing varied gamete combinations.

$3:1$ (dominant:recessive). In complete dominance, the monohybrid cross yields three dominant phenotypes to one recessive due to heterozygote masking.

$1:2:1$ ($AA:Aa:aa$). The genotypic ratio reflects the probabilities of homozygous dominant, heterozygous, and homozygous recessive outcomes from segregation.

$\frac{1}{4}$. The probability of homozygous recessive offspring is $\frac{1}{2} \times \frac{1}{2}$ from each parent's contribution of the recessive allele.

$1:1$ (dominant:recessive). The 1:1 ratio in a testcross confirms heterozygosity, as half the offspring inherit the recessive allele from the tested parent.

An allele at one gene masks/modifies another gene’s phenotype. Epistasis involves gene interactions where one gene's product affects another's expression, altering expected dihybrid ratios.

A heritable DNA unit that affects a trait. Genes serve as the fundamental units of heredity, encoding information in DNA that determines specific traits passed from parents to offspring.

Phenotype severity varies among individuals with the genotype. Variable expressivity results from modifiers causing differing degrees of trait severity among individuals with identical genotypes.

One gene influences multiple phenotypic traits. Pleiotropy occurs when a single gene mutation affects multiple systems, as in sickle cell anemia impacting blood and organs.

Heterozygote shows an intermediate phenotype. Incomplete dominance results from neither allele fully dominating, blending traits in heterozygotes like in snapdragon flower color.

Heterozygote fully expresses both alleles. Codominance allows both alleles to contribute equally to the phenotype, as seen in ABO blood types with distinct expressions.

$1:2:1$ (three distinct phenotypes). Incomplete dominance produces three phenotypes in the ratio due to the intermediate heterozygote differing from both homozygotes.