L3 - Mutations and the Phenotype I

Genome

All of the genetic code in a single organism. The genome comprises both coding and noncoding DNA

Coding DNA

DNA translated into proteins

Non-coding DNA

Not translated into proteins

Allele

Variants of the same gene on homologous chromosomes

Homologous chromosomes

Two complete sets of the same genes that are inherited from the parents. One set from the sperm, and the other from the egg

Dominant allele

Expressed in the phenotype if present

Recessive allele

Expressed in the phenotype if no dominant allele is present

Homozygous

Two copies of the same allele for one gene

Heterozygous

Two different alleles for one gene

Somatic cells

Not involved in reprduction, most common cell in a eukaryotic organism

Germ cells

Involved in sexual reproduction, there are sperm or egg cell

Phenotype

Physical expression of traits measured externally, both necessary for survival and those with no impact on survival

Genotype

Comprised of the genes responsible for the phenotype

Types of mutations

Substitution, deletion, duplication, and insertion

Substitution

One nucleotide is switched for a different one.

Ex: Thymine replaced by cytosine

Insertion/Addition

Extra nucleotides are added to a strand of DNA

Deletion

Sections of DNA are removed or lost.

What is Huntington’s disease

Progressive and fatal, caused by duplication of huntingtin gene. CAG regions duplicated, messing up 3D conformation of proteins.

Leads to neuron degeneration and cognitive disabilities

How does Huntington’s disease manifest

Errors in duplicating CAG cause symptoms.

The longer the disease has been in the family, the earlier the symptoms appear, due to build up of duplication errors

Mutations in non-coding region

No change in phenotype since non-coding regions aren’t translated

Mutations in coding region but with no change to protein structure

The mutation results in a different codon for the same amino acid

Mutations in coding region with change in protein structure

Changes the phenotype when protein function is altered or when the protein plays a critical role which can be seen in the phenotype.

what are gene-level mutations and its types

Most common, survivable, and easily passed on to offspring.

Somatic, germline, and de novo mutations

Somatic mutations

Occur during an individual’s lifetime. Triggered by environmental effects and occur to somatic cells only.

Not passed on to offspring. (e.g., Skin cancer)

Germ-line mutations

Occur in germ cells and therefore passed on to offspring.

Allows the organism to reproduce before killing, in order to pass on.

(e.g., BRCA I gene, increasing risk Breast cancer)

de novo (new) mutations

Occur either in germ cells or in developing embryo.

Can be considered both somatic or germ-line mutations but aren’t present in the parent (de novo)

Crucial for evolution as they create much of the genotypic/phenotypic variation needed for natural selection