Genetics - Inheritance

Mendel's Law of Segregation: During meiosis, the two alleles of a gene separate so that each gamete receives only one allele. This results in a $3:1$ phenotypic ratio in a monohybrid cross of $F_1$ heterozygotes ($Aa \times Aa$).

Mendel's Law of Independent Assortment: The alleles of two different genes get sorted into gametes independently of one another. This applies to genes on different chromosomes and results in a $9:3:3:1$ ratio in a dihybrid cross ($AaBb \times AaBb$).

Genotype vs. Phenotype: The genotype is the symbolic representation of the pair of alleles (e.g., $BB$, $Bb$, $bb$), while the phenotype is the physical characteristic (e.g., Brown eyes).

Codominance and Multiple Alleles: Some genes have more than two alleles (multiple alleles), such as ABO blood groups where $I^A$ and $I^B$ are codominant and $i$ is recessive. Genotypes include $I^A I^A$, $I^A i$, $I^B I^B$, $I^B i$, $I^A I^B$, and $ii$.

Sex Linkage: Genes located on the sex chromosomes ($X$ or $Y$) show different inheritance patterns. Hemophilia and color blindness are $X$-linked recessive traits, denoted as $X^H$, $X^h$, or $X^Y$. Males ($XY$) are more likely to express recessive $X$-linked traits as they only have one $X$ chromosome.

Linked Genes: Genes located on the same chromosome do not assort independently unless crossing over occurs. The closer two genes are, the lower the probability of a chiasma forming between them.

Polygenic Inheritance: Characteristics like skin color or height are determined by the additive effect of two or more genes, leading to continuous variation represented by a normal distribution curve.

Chi-squared ($\chi^2$) Test: A statistical test used to determine if there is a significant difference between the observed ($O$) and expected ($E$) frequencies in a genetic cross.