Definition: During gamete formation, the two alleles for a heritable trait segregate (separate) so that each gamete carries only one allele for each gene.
Key Terminology:
Gene: Basic unit of inheritance located at a specific locus on a chromosome.
Allele: Alternative form of a specific gene (e.g., dominant allele $T$ for tallness, recessive allele $t$ for dwarfness).
Phenotype: The observable physical characteristics of an organism.
Genotype: The genetic constitution of an organism (e.g., $TT$, $Tt$, $tt$).
Homozygous: Having two identical alleles for a gene ($TT$ - homozygous dominant, $tt$ - homozygous recessive).
Heterozygous: Having two different alleles for a gene ($Tt$).
Mendel’s Second Law (Law of Independent Assortment)
Definition: During gamete formation, alleles for different traits segregate independently of one another, provided the genes are located on different non-homologous chromosomes.
Dihybrid Cross ($RrYy \times RrYy$):
Cross involving two distinct traits (e.g., seed shape: $R$/$r$ and seed color: $Y$/$y$).
Phenotypic ratio of $F_2$ generation: $9 : 3 : 3 : 1$
$9$ Round Yellow ($R\_Y\_$)
$3$ Round Green ($R\_yy$)
$3$ Wrinkled Yellow ($rrY\_$)
$1$ Wrinkled Green ($rryy$)
11.3 Genes and Alleles
ABO Blood Group System & Sex-Linked Inheritance
Multiple Alleles: A single gene locus has three or more alternative alleles within a population (e.g., ABO blood group gene $I$ with alleles $I^A$, $I^B$, and $i$).
$I^A$ and $I^B$ are codominant to each other, and both are dominant over recessive allele $i$.
Blood Type genotypes: Type A ($I^AI^A$ or $I^Ai$), Type B ($I^BI^B$ or $I^Bi$), Type AB ($I^AI^B$), Type O ($ii$).
Sex Determination: Humans have 23 pairs of chromosomes (22 pairs of autosomes, 1 pair of sex chromosomes).
Females: Homogametic sex ($44 + XX$).
Males: Heterogametic sex ($44 + XY$).
Sex-Linked Inheritance: Genes located on the $X$ chromosome that have no corresponding locus on the $Y$ chromosome.
Examples: Haemophilia and Red-Green Color Blindness (both caused by recessive alleles on the $X$ chromosome, $X^h$ and $X^c$).
Males are more frequently affected because they have only one $X$ chromosome ($X^hY$ or $X^cY$).
Pedigree Analysis: A family tree chart showing the inheritance patterns of specific traits or genetic disorders across multiple generations.