Human Genetics Lecture 9

Human Genetics

• The cell is the smallest functional unit of the human body that can be divided and retain the characteristics necessary for life.
• Genetic information is stored in the structure of deoxyribonucleic acid (DNA).
• Genes, the basic unit of inheritance, are contained in chromosomes and consist of DNA.
• Humans are estimated to have 20,000 to 25,000 genes.

DNA Structure

• DNA is a long, double-stranded, helical molecule composed of nucleotides, which consist of phosphoric acid, a five-carbon sugar called deoxyribose, and one of four nitrogenous bases.
• The nitrogenous bases carry the genetic information and are divided into two groups: purine bases (adenine and guanine) and pyrimidine bases (thymine and cytosine).
• The backbone of DNA consists of alternating groups of sugar and phosphoric acid, with the paired bases projecting inward from the sides of the sugar molecule.

RNA

• RNA is a second type of nucleic acid involved in the actual synthesis of cellular enzymes and proteins.
• There are several types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
• Messenger RNA (mRNA) contains the transcribed instructions for protein synthesis obtained from the DNA molecule and carries them into the cytoplasm.
• Transcription is the process by which an RNA sequence is formed from a DNA template, resulting in mRNA.
• Translation is the synthesis of proteins according to the instructions carried by mRNA.

Chromosomes

• A chromosome is a single piece of coiled DNA and protein found in the cell nucleus.
• Chromosomes are packaged by proteins into a condensed structure called chromatin.
• Chromosomes form a classic four-arm structure, with a pair of sister chromatids attached to each other at the centromere.
• The centromere is a narrow region that divides the chromosome into short and long arms.
• The short arms are called p arms (from the French petit, small), and the long arms are called q arms (q follows p in the Latin alphabet).
• Chromosomes are arranged in pairs, with one member of the pair inherited from the father and the other from the mother.
• In humans, there are 46 single or 23 pairs of chromosomes.
• The sex chromosomes make up the 23rd pair of chromosomes, with two sex chromosomes determining the sex of a person (XX for females, XY for males).

Human Karyotype

• Cytogenetic is the study of the structure and numeric characteristics of the cell's chromosomes.
• A karyotype is a photographic representation of a stained metaphase spread in which the chromosomes are arranged in order of decreasing length.
• Karyotypes describe the number of chromosomes and their physical characteristics, including length, centromere position, banding pattern, and any differences between the sex chromosomes.

Patterns of Inheritance

• The characteristics inherited from a person's parents are inscribed in gene pairs located along the length of the chromosomes.
• Alternate forms of the same gene are possible (i.e., one inherited from the mother and the other from the father), and each may produce a different aspect of a trait.
• The genotype of a person refers to the genetic composition of a person, which is the genetic information stored in the base sequence triplet code.
• The phenotype refers to the observable expression of a genotype in terms of morphologic, biochemical, or molecular traits.
• More than one genotype may have the same phenotype.
• Expressivity refers to the manner in which the gene is expressed in the phenotype, which can range from mild to severe.
• The position of a gene on a chromosome is called its locus, and alternate forms of a gene at the same locus are called alleles.
• Alleles can be the same (homozygous) or different (heterozygous).
• When only one pair of genes is involved in the transmission of information, the term single-gene trait is used.
• Single-gene traits follow the Mendelian laws of inheritance.