Inheritance and Genetic Principles (KAAP 221)

Genotype: Described as being analogous to a "set of plans" for an organism.
Phenotype: Described as the "detailed structure," representing the expressed physical characteristics resulting from the genotype.

Chromosomal Count: Humans possess $23$ pairs of chromosomes that provide the "blueprint" information for the organism.
Autosomal Chromosomes:
- There are $22$ autosomal pairs.
- These determine somatic (body) characteristics.
Sex Determining Chromosomes:
- There is $1$ sex determining pair.
- Females: Characterized as $XX$ . The oocyte always supplies an $X$ chromosome.
- Males: Characterized as $XY$ . The spermatid is listed as containing $X, X, Y, Y$ .

Homologous Chromosomes: These are pairs of chromosomes with the same structure (pairs $1$ through $22$ ) that affect the same traits.
Locus: The specific position on a chromosome where a gene for a particular trait is located.
Allele: A specific form or variant of a trait.
Homozygous: A condition denoted by the prefix "Homo-" meaning "same," where both alleles for a trait are identical.
Heterozygous: A condition denoted by the prefix "Hetero-" meaning "different," where the two alleles for a trait are distinct.
Trait Expression: The expressed trait in an individual depends on the dominance of the alleles possessed.

Genetic Contribution: Maternal alleles are contributed by the ovum; paternal alleles are contributed by the sperm.
Albinism Example 1:
- Cross results in offspring that are all heterozygous ( $Aa$ ).
- Outcome: All children have normal skin pigmentation.
Albinism Example 2:
- Cross involving a maternal allele ( $a$ ) and paternal alleles ( $A$ ) and ( $a$ ).
- Heterozygous ( $Aa$ ): $50\%$ of the children are heterozygous and have normal pigmentation.
- Homozygous Recessive ( $aa$ ): $50\%$ of the children are homozygous recessive and exhibit albinism.

Inheritance Involving Autosomes:
- Simple Inheritance: The phenotype is determined by a single pair of alleles. Approximately $80\%$ of the human genotype falls within this category.
- Polygenic Inheritance: The phenotype is determined by interactions among the alleles of several different genes.
- Strict Dominance: A pattern where one allele dominates the other allele and solely determines the phenotype.
- Incomplete Dominance: Heterozygous alleles produce a phenotype that is intermediate to the phenotypes of individuals who are homozygous for one allele or the other; it is not completely dominant.
- Codominance: An individual who is heterozygous for a given trait exhibits both of the phenotypes for that trait simultaneously.
Inheritance Involving Sex Chromosomes:
- Sex-linked Inheritance: Involves genes located specifically on the sex chromosomes. Most documented examples involve the $X$ chromosome.

Male Expression: Because a man has only one $X$ chromosome ( $XY$ ), whichever allele that chromosome carries determines his vision. If he carries the recessive allele, he will be red-green color blind.
Female Expression: A woman has two $X$ chromosomes and can be:
- Homozygous Dominant ( $X^C X^C$ ): Normal color vision.
- Heterozygous ( $X^C X^c$ ): Normal color vision, but acts as a carrier.
- Homozygous Recessive ( $X^c X^c$ ): red-green color blind; she must carry two recessive alleles to express the trait.
Colorblindness Cross Example:
- Parents: Carrier female ( $X^C X^c$ ) and Normal male ( $X^C Y$ ).
- Offspring Outcomes:
  - Normal daughter ( $X^C X^C$ ).
  - Normal daughter/Carrier ( $X^C X^c$ ).
  - Normal son ( $X^C Y$ ).
  - Color-blind son ( $X^c Y$ ).

Trisomy 21 (Down Syndrome): A chromosomal abnormality characterized by the presence of an extra chromosome $21$ .
Klinefelter Syndrome ( $XXY$ ): A condition characterized by the presence of two $X$ chromosomes and one $Y$ chromosome.
Turner Syndrome ( $XO$ ): A condition characterized by the presence of only one $X$ chromosome (monosomy $X$ ).
Chromosomal Links to Disease/Disorder:
- Mutation: Defined as a change in DNA nucleotides.
- Effect: Mutations lead to altered protein structure and function.