Chromosome Theory of Inheritance, Sex-Linked Genes, and Nucleic Acids

Chromosome Theory of Inheritance

Definition and Key Concepts

The chromosome theory of inheritance states that genes are located at specific positions (loci) on chromosomes, and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns. Chromosomes undergo segregation and independent assortment during meiosis, which explains Mendel's laws.

• Genes are segments of DNA located on chromosomes.

• Linked genes are genes located close to each other on the same chromosome and tend to be inherited together.

• Non-linked genes are located on different chromosomes or far apart on the same chromosome and assort independently.

Linked Genes and Mendel's Law of Independent Assortment

• Linked genes do not always follow Mendel's law of independent assortment because they tend to travel together during meiosis and fertilization.

• These genes are often inherited as a set and do not follow the expected 9:3:3:1 ratio for dihybrid crosses.

Genetic Crosses: Linked vs. Non-linked Genes

When two characters are controlled by genes (A and B), their inheritance patterns differ depending on whether the genes are linked or on different chromosomes.

A & B are linked genes	A & B are on different chromosomes
Expected ratio of phenotypes from AaBb x AaBb: 3:1	Expected ratio: 9:3:3:1
Law of independent assortment applies? No	Law of independent assortment applies? Yes
Gametes produced: AB and ab by both individuals	Gametes produced: AB, Ab, aB, ab by both individuals

Sex-Linked Genes and Inheritance

Definition and Characteristics

A sex-linked gene is a gene located on a sex chromosome (X or Y). The X chromosome contains many more genes than the Y chromosome. Most sex-linked genes are found on the X chromosome.

• Male members of an XY species cannot be carriers of X-linked traits because they have only one X chromosome.

• Females (XX) can be carriers if they have one normal and one mutated allele.

Examples: X-Linked Traits

• White eye color in fruit flies is a recessive X-linked trait. If a white-eyed female (XwXw) is crossed with a red-eyed male (XWY), all male offspring will be white-eyed, and all female offspring will be carriers.

	Male Offspring	Female Offspring
Genotype	XwY	XWXw
Phenotype	White-eyed, 100%	Carriers, red-eyed, 100%

Color Deficiency Example

Genotype	Male/Female	Outcome
XX	Female	Unaffected
XY	Male	Color deficient
XXc	Female	Carrier

Nucleic Acids: DNA and RNA

Types and Monomers

• DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are the two main types of nucleic acids.

• The monomers of nucleic acids are nucleotides.

Structure of a Nucleotide

• Each nucleotide consists of three components:

  1. Phosphate group

  2. Nitrogenous base

  3. Sugar (deoxyribose in DNA, ribose in RNA)

• The phosphate group gives DNA its negative charge.

Sugar-Phosphate Backbone

• Nucleotides are joined by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a repeating sugar-phosphate backbone.

DNA vs. RNA

• DNA contains deoxyribose sugar; RNA contains ribose sugar.

• DNA is double-stranded; RNA is usually single-stranded.

Nitrogenous Bases

DNA	RNA
Adenine	Adenine
Thymine	Uracil
Guanine	Guanine
Cytosine	Cytosine

DNA Structure and Replication

Complementary Base Pairing

• Adenine (A) pairs with Thymine (T) in DNA; Guanine (G) pairs with Cytosine (C).

• In RNA, Adenine pairs with Uracil (U).

• The bases are held together by hydrogen bonds.

Base Composition Example

• If a DNA molecule is 20% Adenine, it will also be 20% Thymine. The remaining 60% is split equally between Guanine and Cytosine (30% each).

Base	% of DNA molecule
Adenine	20%
Thymine	20%
Guanine	30%
Cytosine	30%

DNA Replication

• Cells replicate their DNA to pass genetic information to offspring and maintain genetic continuity.

• Complementary base pairing allows each strand to serve as a template for a new strand.

• DNA polymerase is the enzyme that connects nucleotides during DNA replication.

Summary Table: DNA Replication

Location	Nucleus
Key Players	DNA, DNA Polymerase
Start	Origins of Replication
Stop	End of the DNA
Result	Two DNA molecules, each consisting of one old and one new strand (semi-conservative replication)

Key Equations and Concepts

• Base Pairing Rule:

$\text{A} = \text{T}$ $\text{G} = \text{C}$

• Chargaff's Rule: The amount of Adenine equals Thymine, and the amount of Guanine equals Cytosine in DNA.

Example: If a DNA sample contains 30% Cytosine, it must also contain 30% Guanine, and the remaining 40% is split equally between Adenine and Thymine (20% each).