Sex Linkage and Gene Linkage

Sex Linkage

• Specifies association of traits with sex chromosomes (X and Y).
• In many organisms, the X chromosome contains more genes than the Y chromosome.

Gene Linkage

• Represents the tendency of genes close together on a chromosome to be inherited together during meiosis.
• Physically close genes are less likely to be separated by recombination.

Sex-Linked Genes

• Genes located on sex chromosomes (X and Y) are termed sex-linked genes.
• X-linked genes exist on the X chromosome.
• Y-linked genes are found on the Y chromosome.

Sex-Linked Traits

• These are determined by genes on sex chromosomes, mostly the X chromosome
• Males (XY) more likely to express X-linked recessive traits due to having one X chromosome.
• Females (XX) may carry X-linked recessive traits without expression because of a second X chromosome.

What are Sex-linked Traits?

• Traits whose genes are on the sex chromosomes include color blindness, hemophilia which are X-linked recessive traits.
• Y-linked traits, like Y-linked infertility, are rare.

How are Sex-linked Genes Inherited?

• Inheritance differs between males and females.
• Males inherit X-linked recessive traits from their mother; only one recessive allele needed for expression.
• Females must inherit two copies (one from each parent) to express the trait.
• Y-linked traits are passed from father to son.

Recombination

• Involves the shuffling of genetic material during meiosis (prophase I) through crossing over between homologous chromosomes.
• New allele combinations result from recombination and contributes to genetic diversity.

Factors Affecting Recombination Frequency

• Heterogametic sex (e.g., male Drosophila melanogaster) has lower crossover frequency, it is suppressed completely in males.
• As maternal age increases, crossing-over decreases.
• Temperature affects it with cooler or warmer temperatures than 22°C increases the rate of crossing-over in Drosophila. Females of Drosophila with reduced recombination frequencies can pass traits.
• Young Drosophila females with high calcium intake has decreased crossing-over.
• Larval starvation increases crossing-over.
• Certain antibiotics, like mitomycin C and actinomycin D, increase crossing-over.
• X-ray irradiation increases it in Drosophila.
• Some genes affect preconditions for chromosomal exchange, while others affect after pairing.

Chromosome Stucture

• Crossing-over reduces in heterozygotes with structural rearrangements of chromosomes.
• Chromosomal aberrations in one pair can increase crossing-over in other pairs.
• Genes near the centromere has reduced crossing-over.

Chemical Basis of Heredity

• Involves molecular structures and mechanisms that pass on genetic information.
• Genetic material is stored in DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).

DNA: The Genetic Material

• Primary carrier of genetic information.
• Stores genetic instructions for development and reproduction.
• It Contains nucleotides, consisting of a phosphate group, deoxyribose sugar, and nitrogenous base.
• Four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
• It follows Chargaff's Rule, Adenine pairs with Thymine (A=T) and Cytosine pairs with Guanine (C=G).

DNA Structure

• It is a double helix and was discovered by James Watson and Francis Crick in 1953.
• The two strands run in antiparallel directions.
• Alternating phosphate and sugar molecules form the backbone.
• Hydrogen bonds hold the nitrogenous base pairs together.

RNA

• Crucial for protein synthesis and gene expression.
• While sugar molecule is Deoxyribose for DNA, RNA contains ribose.
• DNA is double-stranded, RNA is single-stranded.
• RNA has Uracil (U) instead of Thymine (T).

Types of RNA

• Messenger(mRNA) carries genetic code from DNA to ribosomes.
• Transfer(tRNA) transfers amino acids to the ribosome for protein synthesis.
• Ribosomal(rRNA) is a structural component of ribosomes.

DNA Replication

• Copies DNA before cell division occurs
• It Starts with Helicase unwinds the DNA double helix while Single-strand binding proteins prevent reannealing for initiation.
• Elongation occurs when Primase synthesizes RNA primers, DNA Polymerase adds nucleotides in a 5' to 3' direction, Leading strand is synthesized continuously, Lagging strand is synthesized in short fragments (Okazaki fragments).
• Lagging strand is synthesized in short fragments (Okazaki fragments).
• Ligase joins Okazaki fragments in the Termination phase.

Transcription (DNA → RNA)

• RNA is synthesized from DNA.
• Initiation - RNA Polymerase binds to the promoter.
• Elongation - RNA Polymerase adds complementary RNA nucleotides.
• Termination - RNA Polymerase reaches the terminator sequence.

Translation (RNA → Protein)

• mRNA is converted into a protein.
• Initiation - Ribosome binds to mRNA.
• Elongation – tRNA brings amino acids and forms a polypeptide chain.
• Termination - Stop codon signals the end of translation.

Mutations

• Permanent changes occur in the DNA sequence.
• Point Mutation involves a single base change.
• Frameshift Mutation involves Insertion or deletion of bases, shifting the reading frame.
• Silent Mutation has No effect on protein function.
• Missense Mutation Changes one amino acid while a Nonsense Mutation Introduces a premature stop codon.