Gene Mutations & Patterns of Inheritance

Gene Mutations and Patterns of Inheritance

Gene Mutations

• Mutations are any mistake or change in the DNA sequence that alter the nucleotide sequence.
• Mutations can result from mutagens like radiation and chemicals.
• Mutations can produce incorrect codons in mRNA, leading to proteins with incorrect amino acids and genetic diseases.
• Mutations occur during DNA replication and can be inherited if in sex cells (gametes).
• Mutation is a sudden change in the physical and chemical properties of an organism's genetic material.
• A mutation in a gene can result in a loss or gain of function; truncated proteins disrupt gene expression, or increase the affinity of an enzyme for its substrate, up-regulating gene expression.
• Point mutation record dates back to 1791, when Seth Wright noticed a lamb with short legs.
• The first scientific study of mutation started in 1910, when Morgan studied fruit flies.
• DNA can become mutated by inheritance or acquired via random DNA replication errors or environmental damage.
• Mutation can be bad, leading to cancer, aging, birth defects, or good, helping an organism survive better, like antibiotic resistance.

Mutagens

• Mutagens are substances that cause mutations.
• Physical mutagens include ionizing radiation (X-rays, gamma, alpha) and non-ionizing radiation (temperature).
• Chemical mutagens include arsenic, nickel, chromium, and aromatic hydrocarbons found in cigarette smoke.
• Biological mutagens include viruses/bacteria.
• Cellular metabolism, UV light exposure, ionizing radiation, chemical exposure, and replication errors can cause DNA damage.

Types of Mutation

• Mutations can be grouped according to their size, the type of cell involved, mode of origin, direction, and phenotypic effects.
• Gene mutations change the nucleotide sequence of a gene,types include point mutations, insertions, and deletions.
• Point mutations (or substitutions) change a single nucleotide base.

Point Mutations

• Missense mutations change a DNA base pair, resulting in the substitution of one amino acid for another.
• Sickle cell anemia, where glutamic acid is replaced by valine, is an example of a missense mutation.
• Nonsense mutations change a DNA base pair, which prematurely signals the cell to stop building a protein, creating a shortened protein (ex. duchenne muscular dystrophy, thalassemia, cystic fibrosis)
• Silent mutations are in DNA that have no observable effect on the organism's phenotype.
• Frameshift mutations, caused by addition or deletion of a nitrogen base, changes the reading frame.
• Frameshift mutations alter a protein, resulting in a completely different protein.

Insertion Mutation

• An insertion mutation changes the number of DNA bases and the protein may not function properly, cystic fibrosis is an example.

Deletion Mutation

• A deletion mutation changes the number of DNA bases.

Mutations and Cells

• Somatic mutations occur in somatic tissues and are not transmitted to progeny, it can be caused by ultraviolet radiation.
• Germinal mutations occur in germ tissues and can pass to progeny, it can be caused by radiation and chemical mutagens.

Mode of Mutation Origin

• Spontaneous mutations occur suddenly and are of unknown origin.
• Induced mutations can be artificially induced by abnormal environment exposures.

Direction of Mutation

• Forward mutations create a change from wild type to abnormal type and they inactivate the gene and makes it unable to grow in lactose.
• Reverse/back mutations are corrected by error mechanisms, abnormal phenotype goes to wild phenotype.

Phenotypic Effects of Mutation

• Morphological mutations change the size/shape.
• Lethal mutations affect organism viability.
• Conditional mutations cause mutant phenotype only in certain environments.
• Biochemical mutations affect the ability to grow or proliferate.

Patterns of Inheritance.

Terminologies

• Gene: A stretch of DNA or RNA that determines a certain trait.
• Phenotype: Organisms observable characteristics.
• Chromosome: A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
• Dominant: Always produces a particular characteristic.
• Recessive: Can be masked by a dominant gene.
• Homozygous: Having two identical alleles of a gene.
• Heterozygous: Having two different alleles of a gene.
• A dominant allele is expressed as a phenotype with just one allele.
• A recessive allele expresses as a phenotype only with two copies.
• Homozygous has the same alleles.
• Heterozygous has different alleles.
• Inheritance is when genetic traits/characteristics passed on from a parent to offspring.
• Mode of inheritance is defined as the manner in which a genetical trait or disorder passed from one generation to the next.
• Mendel is considered the father of genetics.

Mendelian Laws of Inheritance

• Mendel's laws of inheritance include the law of dominance, segregation, and independent assortment.
• The Law of Dominance one is able to express itself while the other remained suppressed.
• The Law of Segregation two chromosomes from the parents will segregate during Meiosis.
• Law of independent assortment, states that characteristics are inherited independently

Types of Inheritance

• Types of inheritance include dominant-recessive, incomplete dominance, co-dominance, sex-limited, and sex-linked.
• For dominant-recessive, if the gametes are dominant, the trait is inherited by offspring.
• For incomplete dominance, heterozygous phenotype displays a trait that is a blend.
• Codominance is when both alleles contribute to the phenotype and both are expressed.
• Punnett Squares can predict the genotypes of offspring.
• Single gene inheritance/Mendelian disorder is caused by a single mutation.
  • This can be autosomal dominant, autosomal recessive, or sex-linked.
• Autosomal inheritance can be dominant or recessive.
• Sex linked will be carried on either the X or Y chromosome.

Autosomal Dominant Inheritance

• Autosomal dominant expresses in heterozygous.
• Both sexes transmit the trait.
• There is 50% chance of transmit.

Autosomal Recessive Inheritance

• Recessive only expresses in homozygote
• Need two mutated
• two carriers - 25%

X Linked Recessive Inheritance

• refers to genetic conditions associated with mutations in genes on the X chromosomes.

• X Linked inheritance, the female is homogametic with two X chromosomes, and the male is the heterogametic with XY.

• Genes on the X or Y chromosome is sex-linked- In humans it is call X-Linked or Y-Linked inheritance

• Females have two X chromosomes so 2 copies of X

• Males only have one copy.

• Alleles in humans for some forms of colour Blindness (Dys), Hemophilia, & Muscular Dystrophy ARE X-Linked (Females pass on to sons).

• Y-Linked disorders are very rare*

• X linked Dominant Inheritance/ X linked dominance is a mode of genetic inheritance by which a dominant gene is carried on the X chromosomes.

• One cause is mutations on genes for the X Chromosomes - No Male to Male Transmission

Y Linked Inheritance

• Y chromosomes are passed through males only (father to son) and are Rare.
• The trait are are very few because gene resides on the human Y Chromosome.